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Using ODBC with Microsoft SQL Server31 out of 53 rated this helpful

Amrish Kumar and Alan Brewer

Microsoft Corporation

September 1997

Updated May 5, 2010

IntroductionAn Application Programming Interface (API) is a definition of the set of functions an application can use to

access a system resource. ODBC is a database API based on the Call Level Interface (CLI) API definition

published by the standards organizations X/Open and ISO/CAE. ODBC applications can access data in many

of today's leading database management systems (DBMSs) by using ODBC drivers written to access thoseDBMSs. The application calls the ODBC API functions using a standard ODBC SQL syntax, then the ODBC

driver makes any necessary translations to send the statement to the DBMS and presents the results back

to the application.

This paper describes how application programmers using the ODBC API can optimize access to Microsoft

SQL Server when using the Microsoft SQL Server ODBC driver. The paper also discusses issues commonly

raised by customers who have called Microsoft Support for help with the SQL Server ODBC driver. This

paper is not a tutorial on ODBC programming in general, nor is it a comprehensive discussion about

performance tuning on SQL Server. It assumes the reader is already familiar with ODBC programming and

the use of SQL Server. For more information about ODBC, see the Microsoft ODBC 2.0 Programmer'sReference and SDK Guide available on MSDN and from Microsoft Press, and Inside ODBCby Kyle Geiger,

also available from Microsoft Press. For more information about SQL Server, see the SQL Server

documentation.

Except where noted, users should assume that this paper is discussing the operation of Microsoft

SQL Server version 6.5 and its associated version 2.65 ODBC driver. This paper uses the ODBC version 2.5

API functions because version 2.5 is the version used by most existing applications and is also the version

of the ODBC SDK that ships with Microsoft SQL Server Workstation version 6.5. Programmers writing ODBC

3.0 applications should refer to the Microsoft ODBC 3.0 Software Development Kit and Programmer's

Reference.

Readers primarily interested in performance-related issues will find most of the useful information in the

following sections of this paper:

"General Good Practices"

"Choosing a Cursor Option"

"SQLExecDirect vs. SQLPrepare/SQLExecute"

"Batching Procedure Calls"

"Text and Image Data"

ArchitectureThe Microsoft SQL Server ODBC driver uses the standard SQL Server components for communicating from

a client application to the database server. Rather than being implemented as a new layer over

SQL Server's older native API, DB-Library, the ODBC driver writes directly to the same Network-Library (Net-


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rary ayer use y - rary. e r ver s mp emen e as a na ve o erver an s a

functional replacement of the DB-Library DLL. The components involved in accessing a SQL Server from an

ODBC application are described in the following sections.

Application

The application makes calls to the ODBC API using SQL statements written in either ODBC SQL syntax or

SQL Server Transact-SQL syntax.

ODBC Driver Manager

The ODBC driver manager is a very thin layer that manages the communications between the application

and any ODBC drivers with which the application works. The driver manager primarily loads the modules

comprising the driver and then passes all ODBC requests to the driver. There are Win32 and Win16

application programming interface vers ions of the driver manager. The Win32 driver manager is

Odbc32.dll; the Win16 driver manager is Odbc.dll.

SQL Server ODBC Driver

The SQL Server ODBC driver is a single DLL that responds to all calls the application makes to the ODBC

API. If the SQL statements from the application contain ANSI or ODBC SQL syntax that is not supported bySQL Server, the driver translates the statements into Transact-SQL syntax (the amount of translation is

usually minimal) and then passes the statement to the server. The driver also presents all results back to the

application. The Win32 SQL Server ODBC driver is Sqlsrv32.dll; the Win16 driver is Sqlsrvr.dll.

SQL Server Client Network Library

The driver communicates with the server through the SQL Server Net-Libraries using the SQL Server

application-level protocol called Tabular Data Stream (TDS). The SQL Server TDS protocol is a half-duplex

protocol with self-contained result sets (that contain both metadata and data) optimized for database

access.

There is a different Net-Library for each protocol SQL Server supports. The job of the Net-Library is to

process TDS packets from the driver while insulating the driver from details of the underlying protocol

stack. A SQL Server Net-Library accesses a network protocol by calling a network API supported by the

protocol stack. The Net-Libraries supplied by SQL Server for use by SQL Server client applications are listed

in the following table.

Net-Library Win32 DLL Win16 DLL

TCP/IP Windows Sockets Dbmssocn.dll Dbmssoc3.dll

Named pipes Dbnmpntw.dll Dbnmp3.dll

Multiprotocol Dbmsrpcn.dll Dbmsrpc3.dll

Novell SPX/IPX Dbmsspxn.dll Dbmsspx3.dll

Banyan Vines Dbmsvinn.dll Dbmsvin3.dll

DECNet Dbmsdecn.dll N/A

AppleTalk Dbmsadsn.dll N/A

Network Protocol Stack


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e networ protoco stac transports t e pac ets etween t e c ent an t e server. e protoco

stack has components on both the client and the server.

Server Net-Library

The server Net-Libraries work on the server, passing TDS packets back and forth between SQL Server and

its clients. Each SQL Server can work simultaneously with any of the server Net-Libraries installed on the

server.

Open Data Services

Open Data Services supports an API defined for writing server applications. An Open Data Services

application can either be a server that accepts connections and processes queries (such as SQL Server or a

gateway to another DBMS), or it can be an extended stored procedure that allows DLLs written to the Open

Data Services API to be run as stored procedures within SQL Server. Open Data Services receives the TDS

packets from the underlying Net-Libraries and then passes the information to SQL Server by calling specific

Open Data Services callback functions implemented in the SQL Server code. It also encapsulates the results

coming back from the server in TDS packets that the Net-Library then sends back to the client.

SQL ServerSQL Server is the server engine that processes all queries from SQL Server clients.

Overall ODBC and SQL Server Architecture

The following illustration shows the overall ODBC and SQL Server architecture. It shows both a Win16 client

using TCP/IP and a Win32 client using Novell connecting to the same server.

Performance of ODBC as a Native API

One of the persistent rumors about ODBC is that it is inherently slower than a native DBMS API. This

reasoning is based on the assumption that ODBC drivers must be implemented as an extra layer over a


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,

functions and SQL syntax. This translation effort adds extra processing compared with having the

application call directly to the native API. This assumption is true for some ODBC drivers implemented over

a native DBMS API, but the Microsoft SQL Server ODBC driver is not implemented this way.

The Microsoft SQL Server ODBC driver is a functional replacement of DB-Library. The SQL Server ODBC

driver works with the underlying Net-Libraries in exactly the same manner as the DB-Library DLL. The

Microsoft SQL Server ODBC driver has no dependence on the DB-Library DLL, and the driver will function

correctly if DB-Library is not even present on the client.

Microsoft's testing has shown that the performance of ODBC-based and DB-Librarybased SQL Server

applications is roughly equal.

The following illustration compares the ODBC and DB-Library implementations.

Driver and SQL Server Versions

The following table shows which versions of the Microsoft SQL Server ODBC driver shipped with recent

versions and service packs (SP) of Microsoft SQL Server. It also lists the operating system versions under

which the drivers are certified to run and the versions of SQL Server against which they are certified to

work.

Newer drivers recognize the capabilities of older databases and adjust to work with the features that exist

in the older server. For example, if a user connects a version 2.65 driver to a version 4.21a server, the driver

does not attempt to use ANSI or other options that did not exist in SQL Server 4.21a. Conversely, older

drivers do not use the features available in newer servers.

For example, if a version 2.50 driver connects to a version 6.5 server, the driver has no code to use any new

features or options introduced in the 6.5 server.

Driver

version

Driver

date

Shipped with SQL Server

version

SQL Server

versions

supported

Operating systems

supported

2.65.0252 06/16/97 6.5 SP3 6.5

6.0

4.21a

Windows NT3.5, 3.51, 4.0

Windows 95

Windows for Workgroups

3.11

Windows 3.1


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2.65.0240 12/30/96 6.5 SP2 6.5

6.0

4.21a

Windows NT3.5, 3.51, 4.0

Windows 95


3.11

Windows 3.1

2.65.0213 07/30/96 6.5 SP1 6.5

6.04.21a

Windows NT3.5, 3.51, 4.0

Windows 95Windows for Workgroups

3.11

Windows 3.1

2.65.0201 04/03/96 6.5 6.5

6.0

4.21a

Windows NT3.5, 3.51, 4.0

Windows 95


3.11

Windows 3.1

2.50.0126 08/17/95 6.0 SP3

6.0 SP2

6.0 SP1

6.0

4.21a

Windows NT3.5, 3.51

Windows 95


3.11

Windows 3.1

2.50.0121 06/07/95 6.0 6.0

4.21a

Windows NT3.5, 3.51

Windows 95


3.11

Windows 3.1

Note:None of the Microsoft SQL Server ODBC drivers listed is certified to work with Sybase SQL Servers .

Applications needing to connect to Sybase SQL Servers must get an ODBC driver certified for use with

Sybase from either Sybase or a third-party ODBC driver vendor.

For more information about versions and Instcat.sql, see "Instcat.sql."

Setup and ConnectingAn ODBC application has two methods of giving an ODBC driver the information the driver needs to

connect to the proper server and database. Either the application can connect using an existing ODBC datasource containing this information, or it can call either SQLDriverConnect or SQLBrowseConnect, which

provides the information in the connection string parameter.

Setting up a Data Source

ODBC data sources contain information that tells a driver how to connect to a database. ODBC data

sources can be created by using the ODBC Administrator application in Control Panel or by an application

calling the ODBC SQLConfigDataSource function.

Data source definitions are stored in C:\Windows\System\Odbc.ini for the Microsoft Windows version 3.x

and Windows for Workgroups version 3.xoperating systems.

Win32 data sources fall into one of two categories (for details, see Microsoft Knowledge Base article

Q136481):


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Windows NTuser-specific data sources and Windows 95 data sources

On the Microsoft Windows NT operating system, user data sources are specific to the

Windows NTaccount under which they were defined. User-specific data sources are not always

visible to applications running as Windows NT services. Windows 95 data sources are stored in the

following registry key:

HKEY_CURRENT_USER\Software\ODBC\Odbc.ini.

Windows NTsystem data sources

On Windows NT, system data sources are visible to all Windows NTaccounts on the computer.System data sources are always visible to applications running as Windows NTservices. The ODBC

driver manager that ships with Microsoft Office 97 also supports system data sources on

Windows 95 clients. Windows NTsystem data sources are stored in the following registry key:

HKEY_LOCAL_MACHINE\Software\ODBC\Odbc.ini.

Information about the drivers installed on a client is stored in C:\Windows\System\Odbcinst.ini in Windows

3.xor Windows for Workgroups 3.xand in HKEY_LOCAL_MACHINE\Software\ODBC\Odbcinst.ini in

Windows NTand Windows 95.

Each driver needs to store driver-specific information in its data sources. When a user adds a data sourceusing ODBC Administrator, the driver displays a dialog box, where the user specifies data source

information. When a data source is defined with SQLConfigDataSource, the function accepts an attribute

string parameter that can contain driver-specific keywords. All of the SQLConfigDataSource driver-specific

keywords for the SQL Server ODBC driver have counterparts in the dialog box that displays when using

ODBC Administrator.

Here's an example SQLConfigDataSource call that sets up a SQL Server data source referencing a server

using DHCP on TCP/IP:

Driver-specific SQLConfigDataSource Keywords

The following sections describe the driver-specific keywords supported by the Microsoft SQL Server ODBC

driver.

SERVER, NETWORK, and ADDRESS

The SERVER, NETWORK, and ADDRESS parameters associate a data source with a specific instance of

SQL Server on the network. These parameters are directly related to the advanced entries created with the

SQL Server Client Configuration Utility:

The SERVER parameter specifies a name or label for the connection entry.

The NETWORK parameter is the name of the Net-Library module to use, without the .dll suffix (for

RETCODE retcode;

UCHAR *szDriver = "SQL Server";

UCHAR *szAttributes =

"DSN=my65dsn\0DESCRIPTION=SQLConfigDSN Sample\0"

"SERVER=my65server\0ADDRESS=HRServer\0NETWORK=dbmssocn\0"

"DATABASE=pubs\0";

retcode = SQLConfigDataSource(NULL,

ODBC_ADD_DSN,

szDriver,

szAttributes);


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examp e, mssocn, no mssocn. .

The ADDRESS parameter is the network address of the Windows NTserver running SQL Server.

If ADDRESS is present, it is always used as the network address for the connection. If ADDRESS is not

present, then SERVER is used as the network address for the connection.

Here's an example entry to make a named pipes connection to a server:

The following entry evaluates to the same network address:

Here's an example entry to make a sockets connection to the same computer:

There are two special cases to consider:

Connecting to a SQL Server running on the same computer as the client.

The ODBC data source for this case is specified as:

When using this data source, the driver attempts to connect to a SQL Server on the same computer

using Windows NTlocal-named pipes instead of a network implementation of named pipes.

Setting up a data source that connects to a server using whatever Net-Library is currently set as the

default on the client.

An example of an entry for this case is:

The default Net-Library is set using the SQL Server Client Configuration Utility.

The SERVER, NETWORK, and ADDRESS parameters specified on SQL Server ODBC driver data sources

SERVER=xyz,NETWORK=dbnmpntw,ADDRESS=HRServer

SERVER=HRServer,NETWORK=dbnmpntw

SERVER=tcpxyz,NETWORK=dbmssocn,ADDRESS=123.123.123.123,1433

SERVER=(local),NETWORK=(default),ADDRESS=(default)

SERVER=HRServer,NETWORK=(default),ADDRESS=(default)


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opera e e same way as e erver , , an onnec on r ng parame ers spec e or a vance en r es

made with the SQL Server Client Configuration Utility. For more information about the advanced-entry

parameters, see the Microsoft SQL Server Administrator's Companion. The same parameters can be specified

in the data source creation dialog box displayed in ODBC Administrator.

The relationship between the parameters is illustrated in the following table.

SQLConfigDataSource ODBC Administrator SQL Client Configuration Utility

SERVER Server Server

NETWORK Network Library DLL

ADDRESS Network Address Connection String

If a data source is defined with the SERVER, NETWORK, and ADDRESS parameters, a SQL Server advanced

connection entry is made in the registry, and can be viewed using the SQL Client Configuration Utility.

DATABASE

This parameter specifies the default database for the ODBC data source.

LANGUAGE

This parameter specifies the default national language to use.

OEMTOANSI

This parameter specifies whether to convert extended characters to OEM values.

SQL Server is usually run with one of three code pages:

437 code page.The default code page for U.S. MS-DOS computers.

850 code page.

The code page typically used by UNIX systems.

ISO 8859-1 (Lantin1 or ANSI) code page.

The code page defined as a standard by the ANSI and ISO standards organizations. The default

code page for U.S. Windows computers. Sometimes called the 1252 code page.

The 437 and 850 code pages are sometimes collectively referred to as the OEM code pages.

All three code pages define 256 different values to use in representing characters. The values from 0 to128

represent the same characters in all three code pages. The values from 129 to 255, which are known as the

extended characters, represent different characters in all three code pages.

Because ODBC applications are Windows applications, they generally use ANSI code page 1252. If they are

communicating with a SQL Server also running ANSI code page 1252, there is no need for character-set

conversion. If they connect to a server running a 437 or 850 code page however, the driver must be

informed that it should convert extended characters from their 1252 values to 437 or 850 values before

sending them to the server. In this case, the data source should have OEMTOANSI=YES. For a more in-

depth discussion of SQL Server code pages, see Microsoft Knowledge Base article Q153449.

TRANSLATIONDLL

This parameter specifies the name of the ODBC translation DLL to use with the data source.


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This parameter specifies the name of the translator to use with the data source.

TRANSLATIONOPTION

This parameter specifies whether translation should be done on the data going to SQL Server. YES specifies

translation; NO specifies no translation. For more information about ODBC translation, see the ODBC 2.0

Programmer's Reference.

USEPROCFORPREPARE

This parameter specifies whether the driver generates stored procedures to support the ODBC SQLPrepare

function. For more information, see "SQLExecDirect vs. SQLPrepare/SQLExecute."

The following driver-specific SQLConfigDataSource keywords are new in SQL Server 6.5 SP2.

QuotedID

This parameter specifies whether the driver should issue a SET QUOTED IDENTIFIERS ON option when

connecting to a SQL Server version 6.0 or later database. YES specifies QUOTED_IDENTIFIERS is ON; NO

specifies the option is OFF. For more information, see "SET Options Used by the Driver."

AnsiNPWThis parameter specifies whether the driver should SET ON the ANSI_NULLS, ANSI_PADDING, and

ANSI_WARNINGS options when connecting to a SQL Server version 6.5 or later database. YES specifies the

options are ON; NO specifies they are OFF. For more information, see "SET Options Used by the Driver."

The followingdriver-specific SQLConfigDataSource keywords are new in SQL Server 6.5.

QueryLogFile

This parameter specifies the file name the driver should use to log long-running queries. Include the full

path name for the file. For more information, see "ODBC Driver Profiling Features."

QueryLog_ONThis parameter specifies whether the data source should do query profiling. 1 specifies profiling is done;

omitting the parameter specifies no profiling. For more information, see "ODBC Driver Profiling Features."

QueryLogTime

This parameter specifies the interval for long-running queries. The interval is specified in milliseconds. If a

query is outstanding for a period exceeding the QueryLogTime, it is written to the QueryLogFile. For more

information, see "ODBC Driver Profiling Features."

StatsLogFile

This parameter specifies the file name the driver should use to log long performance statistics. Include the

full path name for the file. For more information, see "ODBC Driver Profiling Features."

StatsLog_On

This parameter specifies whether the data source should log performance statistics. 1 specifies profiling is

done; omitting the parameter specifies no profiling. For more information, see "ODBC Driver Profiling

Features."

Trusted_Connection

This parameter specifies whether the data source should use trusted connections when connecting to

SQL Server. 1 specifies trusted connections; omitting the parameter specifies no trusted connections. For

more information, see "Integrated and Standard Security."

Creating Data Sources in ODBC Administrator

When you add, modify, or double-click a SQL Server data source in ODBC Administrator, the SQL Server

ODBC driver displays the ODBC SQL ServerSetup dialog box. The parameters in this dialog box control


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t e same eatures t at are contro e y t e SQLCon gDataSource eywor s ear er n t s paper,

although they have slightly different names. Many of the options are in the dialog box that displays when

you click Options. To specify the query and performance profiling options, click Options, and then click

Profiling.

Driver-specific SQLDriverConnect Keywords

An ODBC application can connect to a SQL Server without referencing a data source:

The SQL Server ODBC driver supports three classes of keywords on SQLDriverConnect:

The standard ODBC keywords

The SQL Server ODBC driver supports the four standard ODBC SQLDriverConnect keywords: DSN,

UID, PWD, and DRIVER.

The driver-specific SQLConfigDataSource keywords

On SQLDriverConnect the SQL Server ODBC driver supports all of the driver-specific keywords it

supports for SQLConfigDataSource. See the list earlier in this paper for a description of these

driver-specific keywords.

The driver-specific keywords APP and WSIDIn addition to supporting the same driver-specific keywords as SQLConfigDataSource,

SQLDriverConnect also supports the two driver-specific keywords APP and WSID.

APP

This keyword specifies the application name to be recorded in the program_name column in

master.dbo.sysprocesses . APP is equivalent to a DB-Library application calling the DBSETLAPP function in

C or the SQLSetLApp function in the Visual Basic programming system.

WSID

This keyword specifies the workstation name to be recorded in the hostname column in

master.dbo.sysprocesses . WSID is equivalent to a DB-Library application calling the DBSETLHOST function

in C or the SQLSetLHost function in Visual Basic.

Connection Messa es

RETCODE retcode;

UCHAR szDSN[MAXBUFLEN+1] =

"DRIVER={SQL Server};SERVER=MyServer;"

"UID=sa;PWD=astring;APP=Generic32;DATABASE=pubs";

UCHAR szUID[MAXUID+1] = "sa",

szAuthStr[MAXAUTHSTR+1] = "password",

szConnStrOut[MAXBUFLEN+1];

SWORD swStrLen;

retcode = SQLDriverConnect(hdbc1,NULL,

szDSN,

(SWORD)strlen(szDSN),

szConnStrOut,

MAXBUFLEN,

&swStrLen,

SQL_DRIVER_NOPROMPT);


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The SQL Server ODBC driver returns SQL_SUCCESS_WITH_INFO on a successful SQLConnect,

SQLDriverConnect, or SQLBrowseConnect. When an ODBC application calls SQLError after getting

SQL_SUCCESS_WITH_INFO, it can receive the following messages:

5701indicates SQL Server initially putting the user's context into the default database defined at the

server for the login ID used in the connection

5703indicates the language being used on the server

If either the ODBC data source has a default database specified or the application specified theDATABASE keyword on SQLDriverConnect or SQLBrowseConnect, there will be a second 5701

message that indicates the user's context has been switched to the database requested.

The following example shows these messages being returned on a successful connect by the System

Administrator (SA) login. The SA login has its default database at the server defined as the master

database, the server is running US English, and the connect used an ODBC data source that specified pubs

as the default database.

Applications can ignore these 5701 and 5703 messages; they are informational only. Applications cannot,

however, ignore a return of SQL_SUCCESS_WITH_INFO return code on the SQLConnect, SQLDriverConnect,

or SQLBrowseConnect. This is because messages other than 5701 and 5703 that do require action may be

returned. For example, if a driver connects to a SQL Server with outdated system stored procedures, one of

the messages returned through SQLError is:

An application's error handling routines for SQL Server connections should call SQLError until it returns

SQL_NO_DATA_FOUND and act on any messages other than the ones that return apfNative code of 5701 or

5703.

Integrated and Standard Security

Full Connect:szSqlState = "01000", *pfNativeError = 5701,

szErrorMsg="[Microsoft][ODBC SQL Server Driver][SQL Server]

Changed database context to 'master'."

szSqlState = "01000", *pfNativeError = 5703,


Changed language setting to 'us_english'."

szSqlState = "01000", *pfNativeError = 5701,


Changed database context to 'pubs'."

Successfully connected to DSN 'my60server'.

SqlState: 01000

pfNative: 0

szErrorMsg: "[Microsoft][ODBC SQL Server Driver]The ODBC

catalog stored procedures installed on server

my421server are version 02.00.4127; version 06.00.0115

or later is required to ensure proper operation.

Please contact your system administrator."


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Standard security

The SA defines SQL Server logins with passwords in SQL Server and then associates the logins with

users in individual databases. With older versions of SQL Server, all connection attempts must

specify a valid login and password. SQL Server version 6.0 or 6.5 also allows trusted connections to

a server running standard security. SQL Server logins are separate from Windows NTuser IDs.

Integrated securityThe SA defines logins for those Windows NTuser accounts that are allowed to connect to

SQL Server. Users do not have to specify a separate login and password when they connect to

SQL Server after logging on to the Windows NTnetwork. When they attempt to connect, the Net-

Library attempts a trusted connection to SQL Server. If the user's Windows NTaccount is one that the

SA specified to SQL Server, the connection succeeds.

Mixed security

The SA defines both SQL Server logins and Windows NTaccounts as SQL Server logins. Users with

validated Windows NTaccounts can connect using trusted connections; other users can connect using

standard security with the SQL Server logins.

The SQL Server ODBC driver always uses a trusted connection when connecting to a server running

integrated security. The driver can also be instructed to open trusted connections when connecting to a

server that is running with standard or mixed security. Only the named pipes or multiprotocol Net-Libraries

support integrated security and trusted connections.

There are two ways to tell the driver to use trusted connections:

Driver-specific data source options

When defining a data source using the ODBC Administrator, you can select Use Trusted Connection.When defining a data source using SQLConfigDataSource, an application can specify

Trusted_Connection=1.

Driver-specific connection options

Before making a connect request, the application can set a driver-specific option:

Integrated security offers several benefits:

Passwords do not need to be stored in the application.

Passwords are never present in the SQL Server TDS packets.

Integrated security is easy to administer because the SA can use the SQL Security Manager utility to

create SQL Server logins from existing Windows NTaccounts.

Protocol Considerations

Integrated security is only available when using either the named pipes or multiprotocol Net-Libraries.

When using the multiprotocol Net-Library, the SA can also configure the server to encrypt packets sent

across the network, so that even users of network sniffers cannot see the data. The named pipes and

multiprotocol Net-Libraries can also work with either a TCP/IP, SPX/IPX, or NetBEUI protocol stack. This

means a client running only a TCP/IP protocol stack can use either the Windows sockets, named pipes, or

SQLSetConnectOption(hdbc, SQL_INTEGRATED_SECURITY, SQL_IS_ON);


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multiprotocol Net-Libraries. The Windows sockets (TCP/IP), SPX/IPX, Appletalk, DECNet, and Banyan Vines

Net-Libraries only work with their single, associated, protocol stack.

Due to their added functionality, such as the encryption feature, the multiprotocol Net-Libraries are

somewhat slower than the others. Testing at Microsoft has found that the TCP/IP Net-Libraries are

somewhat faster than the other Net-Libraries. Other considerations, however, such as database design,

indexing, and the design of queries and applications, usually have a greater impact on performance than

the choice of a Net-Library.

Applications running against SQL Server 6.0 or 6.5 can sometimes improve their performance by resetting

the TDS network packet size. The default packet size is set at the server, and is 4K. 4K generally gives the

best performance. Applications can set the packet size themselves if testing shows that they perform better

with a different packet size. ODBC applications can do this by calling SQLSetConnectionOption with the

SQL_PACKET_SIZE option before connecting. Some applications may perform better with a larger packet

size, but performance improvements are generally minimal for packet sizes larger than 8K.

Verifying and Testing Data Sources

The Odbcping.exe utility can be used to check whether an ODBC connection can be made between a client

and a SQL Server. The command syntax to use the utility is:

odbcping {/Sservername |/Ddatasource}/Ulogin_id/Ppassword

where

servername

Is the network name of the server running SQL Server.

datasource

Is the name of an ODBC data source.

login_id

Is the SQL Server login ID.

password

Is the login password.

You must specify either/S or /D, but not both. (The version ofodbcping that ships with SQL Server 6.0 will

not accept the/D parameter, only/S,/U, and/P.)

When odbcping makes a successful connection, it displays a message indicating the connection was

successful and the versions of the driver and server. For example:

If the connect attempt is not successful, odbcping displays the errors it receives. (The 6.0 version of

odbcping does not display the Native Error code.) For example:

CONNECTED TO SQL SERVER

ODBC SQL Server Driver Version: 02.65.0201

SQL Server Version: SQL Server for Windows NT6.50 - 6.50.201 (Intel X86)

Apr 3 1996 02:55:53

Copyright (c) 1988-1997 Microsoft Corporation


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ThepfNative (or Native Error) code is important in diagnosing connection problems. For more information,

see "pfNative Error Codes."

Processing Queries and Results

General Good Practices

The following sections discuss general practices that will increase the performance of SQL Server ODBC

applications. Many of the concepts apply to database applications in general.

Columns in a Result Set

Applications should select only the columns needed to perform the task at hand. Not only does this reduce

the amount of data sent across the network, it also reduces the impact of database changes on the

application. If an application does not reference a column from a table, then the application is not affected

by any changes made to that column.

Stored Procedures

Sites can realize performance gains by coding most of their SQL statements into stored procedures and

having applications call the stored procedures rather than issuing the SQL statements themselves. This

offers the following benefits:

Higher performance

The SQL statements are parsed and compiled only when the procedures are created, not when the

procedures are executed by the applications.

Reduced network overhead

Having an application execute a procedure instead of sending sometimes complex queries across

the network can reduce the traffic on the network. If an ODBC application uses the ODBC { CALL

MyProcedure} syntax to execute a stored procedure, the ODBC driver makes additional

optimizations that eliminate the need to convert parameter data (for more information, see "ODBCCall vs. Transact-SQL EXECUTE").

Better consistency

The organization's business rules can be coded and debugged once in a stored procedure, and they

will then be consistently applied by all of the applications. The site does not have to depend on all

application programmers coding their SQL statements correctly in all the applications.

Better accuracy

Most sites will have their best SQL programmers developing stored procedures. This means that the

SQL statements in procedures tend to be more efficient and have fewer errors than when the code is

developed multiple times by programmers of varying skill levels.

The Enterprise versions of the Microsoft Visual C++ development system and Microsoft Visual Basic

programming system also offer a new SQL debugger tool. With SQL Debugger, programmers can use the

COULD NOT CONNECT TO SQL SERVER

SQLState: 01000 Native Error: 2

Error Message: [Microsoft][ODBC SQL Server Driver][dbnmpntw]

ConnectionOpen (CreateFile()).

SQLState: 08001 Native Error: 6

Error Message: [Microsoft][ODBC SQL Server Driver][dbnmpntw]

Specified SQL Server not found.


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,

variables, to debug their SQL Server stored procedures.

Batches

An application that builds several SQL statements to execute realizes better performance if it batches the

statements together and sends them to the server all at once. This will reduce the number of network

roundtrips the application uses to perform the same work. For example:

The application uses SQLMoreResults to be positioned on the next result set when they are finished with

the current result set.

SQLBindCol and SQLGetData

Excess use ofSQLBindCol to bind a result set column to a program variable is expensive because

SQLBindCol causes an ODBC driver to allocate memory. When an application binds a result column to a

variable, that binding remains in effect until the application either calls SQLFreeStmt with fOption set to

either SQL_DROP or SQL_UNBIND. The bindings are not automatically undone when the statement

completes.

This logic allows applications to effectively deal with situations where they may execute the same SELECT

statement several times with different parameters. Since the result set will keep the same structure, the

application can bind the result set once, process all the different SELECT statements, then do a

SQLFreeStmt with fOption set to SQL_UNBIND after the last execution. Applications should not call

SQLBindCol to bind the columns in a result set without first calling SQLFreeStmt with fOption set toSQL_UNBIND to free any previous bindings.

When using SQLBindCol, applications can either do row-wise or column-wise binding. Row-wise binding is

somewhat faster than column-wise binding.

Applications can use SQLGetData to retrieve data on a column-by-column basis, instead of binding the

result set columns using SQLBindCol. If a result set contains only a couple of rows, then using SQLGetData

instead ofSQLBindCol is faster, otherwise, SQLBindCol gives the best performance. If an application does

not always put the data in the same set of variables, it should use SQLGetData instead of constantly

rebinding. Applications can only use SQLGetData on columns that are in the select list after all columns are

bound with SQLBindCol. The column must also appear after any columns on which the application hasalready used a SQLGetData.

Data Conversion

The ODBC functions dealing with moving data into or out of program variables, such as SQLBindCol,

SQLBindParameter, and SQLGetData, allow implicit conversion of data types. For example, an application

that displays a numeric column can ask the driver to convert the data from numeric to character:

SQLExecDirect(hstmt,

"select * from authors; select * from titles",

SQL_NTS);'

retcode = SQLBindCol(hstmt1,

1, // Point to integer column

SQL_C_CHAR,

&szCharVar,

ARRAYSIZE,

&cbCharVar);


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Applications should minimize data conversions. Unless data conversion is a required part of the application,

the application should bind columns to a program variable of the same data type as the column in the

result set.

If the application needs to have the data converted, it is more efficient for the application to ask the driver

to do the data conversion than for the application to do it.

The driver normally just transfers data directly from the network buffer to the application's variables.

Requesting the driver to perform data translation forces the driver to buffer the data and use CPU cycles to

perform the conversion.

Data Truncation

If an application attempts to retrieve data into a variable that is too small to hold it, the driver generates a

warning. The driver must allocate memory for the warning messages and spend CPU resources on some

error handling. This can all be avoided if the application allocates variables large enough to hold the data

from the columns in the result set, or uses the SUBSTRING function in the select list to reduce the size of thecolumns in the result set.

Query Options

Timeout intervals can be adjusted to prevent problems. Also, having different settings for some ODBC

statement and connection options among several open connection or statement handles can generate

excess network traffic.

Calling SQLSetConnectOption with fOption set to SQL_LOGIN_TIMEOUT controls the amount of time an

application waits for a connection attempt to timeout while waiting to establish a connection (0 specifies an

infinite wait). Sites with slow response times can set this value high to ensure connections have sufficient

time to complete, but the interval should always be low enough to give the user a response in a reasonableamount of time if the driver cannot connect.

Calling SQLSetStmtOption with fOption set to SQL_QUERY_TIMEOUT sets a query timeout interval to

protect the server and the user from long running queries.

Calling SQLSetStmtOption with fOption set to SQL_MAX_LENGTH limits the amount oftext and image data

that an individual statement can retrieve. Calling SQLSetStmtOption with fOption set to SQL_MAX_ROWS

also limits a rowset to the first n rows if that is all the application needs. Note that setting SQL_MAX_ROWS

causes the driver to issue a SET ROWCOUNT statement to the server, which will affect all SQL statements,

including triggers and updates.

Care should be used when setting these options, however. It is best if all statement handles on a connection

handle have the same settings for SQL_MAX_LENGTH and SQL_MAX_ROWS. If the driver switches from a

statement handle to another with different values for these options, the driver must generate the

appropriate SET TEXTSIZE and SET ROWCOUNT statements to change the settings. The driver cannot put

these statements in the same batch as the user SQL since the user SQL can contain a statement that must

be the first statement in a batch, therefore the driver must send the SET TEXTSIZE and SET ROWCOUNT

statements in a separate batch, which automatically generates an extra roundtrip to the server.

SET NOCOUNT

Applications can execute the Transact-SQL statement SET NOCOUNT ON. When this is set on, SQL Serverdoes not return an indication of how many rows were affected by data-modification statements, or by any

statements within procedures. When SET NOCOUNT is ON, the driver does not get the information it needs

to return the number of rows affected should the application call SQLRowCount after a data-modification

statement.

printf("fetched row cola = %s\n", szCharVar);


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All statements executed in a stored procedure, including SELECT statements, generate an "xrows affected"

message. Issuing a SET NOCOUNT ON at the start of a large stored procedure can significantly reduce the

network traffic between the server and client and improve performance by eliminating these messages.

These messages are typically not needed by the application when it is executing a stored procedure.

Cursors

Starting with SQL Server 6.0, the SQL Server ODBC driver supports the ODBC cursor options by using server

cursors.

Cursor Types

The ODBC standard assumes that a cursor is automatically opened on each result set and, therefore, does

not make a distinction between a result set and a cursor. SQL Server SELECT statements, however, always

return a result set. A SQL Server cursor is a separate entity created when the application needs to perform

cursor operations such as scrolling and positioned updates.

In the ODBC model, all SQL statements return a result set within a cursor, and an application retrieves rows

through the cursor using either SQLFetch or SQLExtendedFetch. Before executing an SQL statement, an

ODBC application can call SQLSetStmtOption to set statement options that control the cursor's behavior.

These are the default settings for the cursor options.

Option Default

SQL_CURSOR_TYPE SQL_CURSOR_FORWARD_ONLY

SQL_CONCURRENCY SQL_CONCUR_READ_ONLY

SQL_ROWSET_SIZE 1

When running with these default settings, the application can only use SQLFetch to fetch through the result

set one row at a time from the start of the result set to the end. When running with these default settings,

the SQL Server ODBC driver requests a default result set from the server. In a default result set, SQL Server

sends the results back to the client in a very efficient, continuous stream. The calls to SQLFetch retrieve the

rows out of the network buffers on the client.

It is possible to execute a query with these default settings, and then change the SQL_ROWSET_SIZE after

the SQLExecDirect or SQLExecute complete. In this case, SQL Server still uses a default result set to

efficiently send the results to the client, but the application can also use SQLExtendedFetch to retrieve

multiple rows at a time from the network buffers.

An ODBC application can change the SQL_CURSOR_TYPE to request different cursor behaviors from the

result set. The types of cursors that can be set are:

Static cursors

In a static cursor, the complete result set is built when the cursor is opened. The cursor does not

reflect any changes made in the database that affect either the rows in the result set, or the values in

the columns of those rows. In other words, static cursors always display the result set as it was when

the cursor was opened. If new rows have been inserted that satisfy the conditions of the cursor's

SELECT statement, the do not a ear in the cursor. If rows in the result set have been u dated, the

SQL_CURSOR_TYPE=SQL_CURSOR_STATIC


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new data values do not appear in the cursor. Rows appear in the result set even if they have been

deleted from the database. No UPDATE, INSERT, or DELETE operations are reflected in a static

cursor (unless the cursor is closed and reopened), not even modifications made by the same user

who opened the cursor. Static cursors are read-only.

Dynamic cursors

Dynamic cursors are the opposite of static cursors; they reflect all changes made to the rows in their

result set as the user scrolls around the cursor. In other words, the data values and membership of

rows in the cursor can change dynamically on each FETCH. The cursor shows all DELETE, INSERT, and

UPDATE statements either made by the user who opened the cursor or committed by other users.

Dynamic cursors do not support FETCH ABSOLUTE because the size of the result set and the position

of rows within the result set are not constant. The row that starts out as the tenth row in the result se

may be the seventh row the next time a FETCH is performed.

Forward-only cursors

This cursor is similar to a dynamic cursor, but it only supports fetching the rows serially in sequence

from the start to the end of the cursor.

Keyset-driven cursor

With a keyset-driven cursor, the membership of rows in the result set and their order is fixed when

the cursor is opened. Keyset-driven cursors are controlled through a set of unique identifiers (keys),

known as the keyset. The keys are built from a set of columns that uniquely identify the rows. The

keyset is the set of all the key values that made up the rows in the result set when the cursor was

opened. Changes to data values in nonkeyset columns for the rows (made by the current user or

committed by other users) are reflected in the rows as the user scrolls through the cursor. Inserts

are not reflected unless the cursor is closed and reopened. Deletes generate an "invalid cursor

position" error (SQLState S1109) if the application attempts to fetch the missing row. If an update is

made to a key-column value, it operates like a delete of the old key value followed by an insert of

the new key value, and the new key value is not visible to the cursor. Attempts to fetch the old key

value generate the S1109 error, but the new key value is not visible to the cursor.

Mixed cursors

SQL_CURSOR_TYPE=SQL_CURSOR_DYNAMIC

SQL_CURSOR_TYPE=SQL_CURSOR_FORWARD_ONLY

SQL_CURSOR_TYPE=SQL_CURSOR_KEYSET_DRIVEN

SQL_CURSOR_TYPE=SQL_CURSOR_KEYSET_DRIVEN

SQL_KEYSET_SIZE=n


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SQL Server does not support mixed cursors.

All ODBC cursors support the concept of a rowset, which is the number of rows returned on an

individual SQLExtendedFetch. For example, if an application is presenting a 10-row grid to the user,

the cursor can be defined with a rowset size of 10 to simplify mapping data into the grid.

Concurrency Option Overview

In addition to the cursor types, cursor operations are also affected by the concurrency options set by theapplication:

With this option set, the cursor does not support UPDATE, INSERT, or DELETE statements.

Locks are not held on the underlying rows that make up the result set.

This option offers optimistic concurrency control. Optimistic concurrency control is a standard

part of transaction control theory and is discussed in most papers and books on the subject.

The application uses optimistic control when "optimistic" that there is a slight chance that

anyone else may have updated the row in the interval between when the row is fetched and

when the row is updated. When the cursor is opened in this mode, no locks are held on theunderlying rows to maximize throughput. If the user attempts an UPDATE, the current values in

the row are compared with the values retrieved when the row was fetched. If any of the values

have changed, SQL Server returns an error . If the values are the same, the cursor engine

performs the UPDATE.

Selecting this option means the application must deal with an occasional error indicating that

another user updated the row and changed the values. A typical action taken by an

application that receives this error would be to refresh the cursor, to get the new values, and

then let the user or application decide if the UPDATE should still be performed. Note that text

and image columns are not used for concurrency comparisons.

This optimistic concurrency control option is based on row versioning. With row versioning,

the underlying table must have a version identifier of some type that the cursor engine can

use to determine whether the row has been changed since it was read into the cursor. In

SQL Server, this is the facility offered by the timestamp data type. SQL Server timestamps are

binary numbers that indicate the relative sequence of modifications in a database. Each

database has a global current timestamp value, @@dbts, which is incremented with every

change in the database. If a table has a timestamp column, then its timestamp column is

updated with the current @@dbts value every time the row is updated. The cursor engine can

then compare a row's current timestamp value with the timestamp value that was first

retrieved into the cursor to determine whether the row has been updated. The engine does

SQL_CONCURRENCY = SQL_CONCUR_READONLY

SQL_CONCURRENCY = SQL_CONCUR_VALUES

SQL_CONCURRENCY = SQL_CONCUR_ROWVER


20/66



not ave to compare t e va ues n a co umns, on y t e t mestamp va ue. an app cat on

requests SQL_CONCUR_ROWVER on a table that does not have a timestamp column, the

cursor defaults to the values-based optimistic concurrency control, SQL_CONCUR_VALUES.

This option implements pessimistic concurrency control, in which the application attempts to

lock the underlying database rows at the time they are read into the cursor result set. For

cursors using server cursors, an update intent lock is placed on the page holding the row

when it is read into the cursor. If the cursor is opened within a transaction, these intent-to-

update locks are held until the transaction is committed or rolled back. If the cursor has been

opened outside a transaction, the lock is dropped when the next row is fetched. Thus,

applications wanting full pessimistic concurrency control would typically open the cursor within

a transaction. An update intent lock prevents any other task from acquiring an update intent

or exclusive lock, which prevents any other task from updating the row. An update intent lock,

however, will not block a shared lock, so it does not prevent other tasks from reading the

row, unless the second task is also requesting a read with an update intent lock.

In all of these concurrency options, when any row in the cursor is updated, SQL Server locks it with an

exclusive lock. If the update has been done within a transaction, the exclusive lock is held until the

transaction is terminated. If the update has been done outside of a transaction, the update is automatically

committed when it is completed and the exclusive lock is freed. Because SQL Server must acquire an

exclusive lock before it updates the row, positioned updates done through a cursor (just like standard

updates) can be blocked by other connections holding a shared lock on the row.

Isolation Levels

The full locking behavior of cursors is based on an interaction between the concurrency options discussed

above and the transaction isolation level set by the client. ODBC clients set the transaction isolation level by

setting the connection option SQL_TXN_ISOLATION. Users should combine the locking behaviors of the

concurrency and transaction isolation level options to determine the full locking behavior of a specific

cursor environment.

READ COMMITTED (The default for both SQL Server and ODBC)

SQL Server acquires a shared lock while reading a row into a cursor but frees the lock immediately

after reading the row. Because a shared lock request is blocked by an exclusive lock, a cursor is

prevented from reading a row that another task has updated but not yet committed.

READ UNCOMMITTED

SQL Server requests no locks while reading a row into a cursor and honors no exclusive locks. This

means that cursors can be populated with values that have already been updated but not yet

committed. The user is bypassing all of SQL Server's locking transaction control mechanisms.

SQL_CONCURRENCY = SQL_CONCUR_LOCK

SQL_TXN_ISOLATION = SQL_TXN_READ_COMMITED

SQL_TXN_ISOLATION = SQL_TXN_READ_UNCOMMITED


21/66



REPEATABLE READ or SERIALIZABLE

Or

SQL Server still requests a shared lock on each row as it is read into the cursor as in READ

COMMITTED, but if the cursor is opened within a transaction, the shared locks are held until the end

of the transaction instead of being freed after the row is read. This is the same effect as specifying

HOLDLOCK on a SELECT statement.

Note that the ODBC API specifies additional transaction isolation levels, but these are not supported by

SQL Server or the Microsoft SQL Server ODBC driver.

Server Cursors

Prior to version 6.0, SQL Server sent result sets back to clients using only one type of result set, the default

result set. While the default result set is efficient at sending results back to clients, it only supports the

characteristics of the default ODBC result set: forward-only, read-only, and a rowset size of one. Because of

this, the Microsoft SQL Server ODBC drivers that shipped with SQL Server version 4.2xonly supported the

default ODBC settings.

When using a default result set, there is only one roundtrip between the client and server; this occurs at the

time the SQL statement is executed. After the statement is executed, the server sends the packets containing

the results back to the client until all of the results have been sent back or the client has cancelled the

results by calling SQLMoreResults. Calls to SQLFetch or SQLExtendedFetch do not generate roundtrips to

the server, they just pull data from the client network buffers into the application.

SQL Server 6.0 introduced cursors that are implemented on the server (server cursors). There are two types

of server cursors:

Transact-SQL cursors

This type of cursor is based on the ANSI syntax for cursors and is meant to be used in Transact-SQL

batches, primarily in triggers and stored procedures. Transact-SQL cursors are not intended to be

used in client applications.

API server cursors

This type of cursor is created by either the DB-Library or ODBC APIs. The SQL Server ODBC driver

that shipped with SQL Server 6.0 uses API server cursors to support the ODBC cursor options.

Users access the functionality of API server cursors through either ODBC or DB-Library. If an ODBC

application executes a statement with the default cursor settings, the SQL Server ODBC driver requests a

default result set from SQL Server. If the application sets the ODBC cursor type options to anything other

than the defaults, however, then the SQL Server ODBC driver instead requests the server to implement a

server cursor with the same options requested by the application. Since the cursor is implemented on the

server, the driver does not have to use memor on the client to build a client-based cursor. Server cursors

SQL_TXN_ISOLATION = SQL_TXN_REPEATABLE_READ_UNCOMMITED

SQL_TXN_ISOLATION = SQL_TXN_SERIALIZABLE


22/66



can also reduce network traffic in cases where a user decides they do not need to retrieve an entire result

set. For example, if a user opens a cursor with 1,000 rows but then finds what they were looking for in the

first 100 rows they scroll through, the other 900 rows are never sent across the network.

When using server cursors, each call to SQLFetch, SQLExtendedFetch, or SQLSetPos causes a network

roundtrip from the client to the server. All cursor statements must be transmitted to the server because the

cursor is actually implemented on the server.

One potential drawback of server cursors is that they currently do not support all SQL statements. Servercursors do not support any SQL statements that generate multiple result sets, therefore they cannot be

used when the application executes either a stored procedure or a batch containing more than one select.

If the application has set options that cause the driver to request an API server cursor, and then it executes

a statement that server cursors do not support, the application gets an error:

Or

ODBC applications getting either of these errors when attempting to use server cursors with multiple

statement batches or stored procedures should switch to using the ODBC default cursor options.

Multiple Active Statements per Connection

After SQL Server has received a statement, the SQL Server TDS protocol does not allow acceptance of any

other statements from that connection until one of the following occurs:

The client application processes the entire result set.

The client sends a statement telling the server it can close the remainder of the result set.

This means that when an ODBC application is using a default result set, SQL Server does not support

multiple active statement handles on a connection handle and only one statement can be actively processed

at any point in time.

When an ODBC application is using API server cursors, however, the driver can support multiple active

statements on a connection. When the rowset for each cursor command has been received back at theclient, SQL Server considers the statement to have completed, and it accepts another statement from

another statement handle over that connection handle.

For example, an application can do the following to initiate processing on two statement handles:

SQLState: 37000

pfNative: 16937

szErrorMsg: [Microsoft][ODBC SQL Server Driver][SQL Server]

Cannot open a cursor on a stored procedure that

has anything other than a single select statement in it.

SQLState: 37000

pfNative: 16938

szErrorMsg: [Microsoft][ODBC SQL Server Driver][SQL Server]

sp_cursoropen. The statement parameter can only

be a single select or a single stored procedure.


23/66



When the SQLExecDirect on hstmt1 is executed, the SQL Server ODBC driver issues a cursor open request.

When SQL Server completes the cursor open, it considers the statement to be finished and allows the

application to then issue a statement on another hstmt:

Once again, after the server has finished with the cursor open request issued by the client, it considers the

statement to be completed. If at this point the ODBC application makes a fetch request as follows, the

SQL Server ODBC driver sends SQL Server a cursor fetch for the first five rows of the result set:

After the server has transferred the five rows to the driver, it considers the fetch processing completed and

accepts new requests. The application could then do a fetch on the cursor opened for the other statement

handle:

SQL Server accepts this second statement on the connection handle because, as far as it is concerned, it

has completed the last statement on the connection handle, which was the fetch of the first five rows of the

rows for hstmt1.

Choosing a Cursor Option

The choice of cursor type depends on several variables, including:

Size of the result set.

Percentage of the data likely to be needed.

Performance of the cursor open.Need for cursor operations like scrolling or positioned updates.

Desired level of visibility to data modifications made by other users.

The default settings would be fine for a small result set if no updating is done, while a dynamic cursor

SQLAllocEnv(&henv):

SQLAllocConnect(henv, &hdbc);

SQLAllocStmt(hdbc, &hstmt1);

SQLAllocStmt(hdbc, &hstmt2);

SQLSetConnectOption(hdbc, SQL_CURSOR_TYPE, SQL_CURSOR_DYNAMIC);

SQLSetConnectOption(hdbc, SQL_ROWSET_SIZE, 5);

SQLExecDirect(hstmt1, "select * from authors", SQL_NTS);

SQLExecDirect(hstmt2, "select * from titles", SQL_NTS);

SQLExtendedFetch(hstmt1, SQL_FETCH_NEXT, 1, ...);

SQLExtendedFetch(hstmt2, SQL_FETCH_NEXT, 1, ...);


24/66



of the rows.

Some simple rules to follow in choosing a cursor type are:

Use default settings for singleton selects (returns one row), or other small result sets. It is more

efficient to cache a small result set on the client and scroll through the cache.

Use the default settings when fetching an entire result set to the client, such as when producing a

report. After SQLExecute or SQLExecDirect, the application can increase the rowset size to retrievemultiple rows at a time using SQLExtendedFetch.

The default settings cannot be used if the application is using positioned updates.

The default settings cannot be used if the application is using multiple active statements.

The default settings must be used for any SQL statement or batch of SQL statements that will

generate multiple result sets.

Dynamic cursors open faster than static or keyset-driven cursors. Internal temporary work tables

must be built when static and keyset-driven cursors are opened but are not required for dynamic

cursors.

Use keyset-driven or static cursors if SQL_FETCH_ABSOLUTE is used.

Static and keyset-driven cursors increase the usage oftempdb. Static server cursors build the entire

cursor in tempdb; keyset-driven cursors build the keyset in tempdb.

Each call to SQLFetch or SQLExtendedFetch causes a roundtrip to the server when using server cursors.

Applications should minimize these roundtrips by using a reasonably large rowset size and by using

SQLExtendedFetch instead ofSQLFetch whenever possible.

Implicit Cursor Conversions

Applications can request a cursor type through SQLSetStmtOption and then execute an SQL statement that

is not supported by server cursors of the type requested. A call to SQLExecute or SQLExecDirect returns

SQL_SUCCESS_WITH_INFO and SQLError returns:

The application can determine what type of cursor is now being used by calling SQLGetStmtOption with

fOption set to SQL_CURSOR_TYPE. The cursor type conversion applies to only one statement. The next

SQLExecDirect or SQLExecute will be done using the original statement cursor settings.

Both SQL Server 6.0 and 6.5 have the following restrictions:

If an SQL statement contains UNION, UNION ALL, GROUP BY, an outer join, or DISTINCT, all cursor

types other than static are converted to static.

If a keyset-driven cursor is requested and there is at least one table that does not have a unique

index, the cursor is converted to a static cursor.

SQL Server 6.0 has the following additional restrictions:

If a dynamic cursor is requested and there is at least one table that does not have a unique index,

the cursor is converted to a static cursor.If a dynamic cursor is requested and the SQL statement contains an ORDER BY that does not match a

unique index or subquery, the cursor is converted to a static cursor.

SQLExecDirect vs. SQLPrepare/SQLExecute

szSqlState = "01S02", *pfNativeError = 0,

szErrorMsg="[Microsoft][ODBC SQL Server Driver]Cursor type changed"


25/66



This section discusses when SQLExecDirect or SQLPrepare/SQLExecute should be used.

Driver Implementation Overview

ODBC offers two options for executing a statement. If a statement is only executed once or twice, the

application can use SQLExecDirect to execute the statement. The ODBC definition ofSQLExecDirect states

that the database engine parses the SQL statement, compiles an execution plan, executes the plan, and then

returns results to the application.

If an application executes the same statement many times, then the overhead of having the engine compilethe plan every time degrades performance. An application in this case can call SQLPrepare once and then

call SQExecute each time it executes the statement. The ODBC definition ofSQLPrepare states that the

database engine just parses the statement and compiles an execution plan, then returns control to the

application. On SQLExecute, the engine simply executes the precompiled execution plan and returns the

results to the client, thereby saving the overhead of parsing and recompiling the execution plan.

SQL Server itself does not directly support the SQLPrepare/SQLExecute model, but the SQL Server ODBC

driver can use stored procedures to emulate this behavior. On a SQLPrepare, the driver asks the server to

create a stored procedure that contains the SQL statement from the application. On SQLExecute, the driver

executes the created stored procedure. The ODBC driver uses stored procedures to support

SQLPrepare/SQLExecute when the option is enabled either in the data source or the SQLDriverConnect

keywords. For example, if an application calls:

The driver sends a statement to the server:

When the application then does:

The driver sends a remote stored procedure call to have the server run the #ODBC#nnnnnnnn procedure.

Because a CREATE PROCEDURE statement essentially compiles an SQL statement into an execution plan,

and an EXECUTE statement simply executes the precompiled plan, this meets the criteria for the

SQLPrepare/SQLExecute mode.

Excess or inappropriate use ofSQLPrepare/SQLExecute degrades an application's performance.

SQL Server applications should only use SQLPrepare/SQLExecute if they plan to execute a statement more

than 3 to 5 times. If an application needs to execute a statement only once, using SQLPrepare/SQLExecute

generates two roundtrips to the server: one to create the stored procedure and another to execute it.

SQLExecDirect would only use one roundtrip and would also save the overhead of creating and storing a

stored procedure. Excess use ofSQLPrepare can also cause locking contention in the system tables in

SQLPrepare(hstmt, "select *from authors", SQL_NTS);

CREATE PROCEDURE #ODBC#nnnnnnnn as

SELECT * FROM authors

SQLExecute(hstmt);


26/66



tempdb as concurrent users continually try to create the stored procedures to support SQLPrepare.

You may think that applications must use SQLPrepare/SQLExecute to use parameter markers, even if the

application will only execute the statement once or twice. This is not true, applications can use parameter

markers with SQLExecDirect by calling SQLBindParameter before SQLExecDirect.

If an application will be run by many concurrent users and the users will all be using the same SQL

statement, the best approach is to create the SQL statement as a permanent, parameterized, stored

procedure and executed it with SQLExecDirect. Having many users concurrently issue SQLPreparecommands can create a concurrency problem on the system tables in tempdb. Even if each user is

executing exactly the same statement, the SQL Server ODBC driver on each client is creating its own copy of

a temporary stored procedure in tempdb. If the SQL statement is created as a parameterized stored

procedure, however, the procedure is created only once. Each ODBC application does not have to create a

new procedure for its exclusive use, it simply uses a copy of the permanent procedure's execution plan

from the procedure cache.

When used in the appropriate circumstances (to execute a single statement several times),

SQLPrepare/SQLExecute can provide significant performance savings.

Impact on TempdbSQL Server 6.0 introduced temporary stored procedures, which are identified by having a number sign (#)

as the first character in the name. These procedures operate like temporary tables and are automatically

dropped by the server if the connection is broken. The SQL Server ODBC driver now creates the procedures

that support SQLPrepare as temporary procedures. This makes it impossible for the ODBC-related stored

procedures to build up as a result of broken network connections or client computer failures. However, the

temporary stored procedures are always created in tempdb. This means that sites running SQL Server 6.0

or 6.5 with ODBC applications that use SQLPrepare must ensure that tempdb is large enough to hold the

temporary procedures generated to support SQLPrepare.

There is another factor to consider in relation to how many stored procedures exist in tempdb. ODBCapplications call SQLSetConnectoption with fOption set to the driver-specific value

SQL_USE_PROCEDURE_FOR_PREPARE and vParam set to either SQL_UP_OFF, SQL_UP_ON, or

SQL_UP_ON_DROP to control the generation of temporary procedures.

SQL_UP_OFF means that the driver does not generate stored procedures.

SQL_UP_ON_DROP means that the driver generates stored procedures, and that they are dropped

when the application does a SQLDisconnect, a SQLFreeStmt with fOption set to SQL_DROP, or the

next time the application issues SQLPrepare on the same statement handle.

SQL_UP_ON means that temporary procedures are created, but they are only dropped on a

SQLDisconnect.

SQL_UP_ON is the default setting. The driver can reuse procedures if an application re-prepares the same

SQL statement, and most applications realize a performance boost because the driver is not having to

continually drop stored procedures. This may result in a build up of #ODBC procedures in tempdb,

however, from applications that never disconnect or applications that make heavy use ofSQLPrepare.

These applications should set SQL_UP_ON_DROP by calling SQLSetConnectOption. Starting with the driver

that shipped in SQL Server 6.5 SP2, SQL_UP_ON_DROP is now an option that can be specified on data

sources for the SQL Server ODBC driver.

Other Considerations of SQLPrepare

To keep from having to hold locks on the tempdb system tables for the length of a user transaction, theSQL Server ODBC driver does not generate a stored procedure for SQLPrepare if it is called within a

transaction. The exception to this is when the SQLPrepare is the first statement in the transaction. In this

case, the driver generates a stored procedure but then immediately commits the CREATE PROCEDURE

statement.


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The driver does not generate a stored procedure for a SQLPrepare that uses the ODBC CALL escape clause

to call a stored procedure. On SQLExecute, the driver executes the called stored procedure (there is no

need to create a temporary stored procedure).

Calling either SQLDescribeCol or SQLDescribeParam before calling SQLExecute generates an extra

roundtrip to the server. On SQLDescribeCol, the driver removes the WHERE clause from the query and

sends it to the server with SET FMTONLY ON to get the description of the columns in the first result set

returned by the query. On SQLDescribeParam, the driver calls the server to get a description of the

columns in the tables referenced by the query. This method also has some restrictions, such as not being

able to resolve parameters in subqueries.

Stored Procedures

This section discusses issues related to executing stored procedures using the SQL Server ODBC driver.

ODBC Call vs. Transact-SQL EXECUTE

Applications can call SQL Server procedures using either the Transact-SQL EXECUTE statement or the ODBC

SQL CALL escape clause (the Transact-SQL statement appears first, followed by the ODBC SQL CALL):

Using the ODBC syntax is recommended. The ODBC syntax, in addition to being more portable, offers

improved features and performance over the EXECUTE statement.

The SQL Server TDS protocol provides two methods of sending a procedure to the server: the procedure

can be sent to the server as a regular SQL statement, or it can be sent as a TDS Remote Procedure Call

(RPC).

The TDS RPC syntax was originally defined for use by servers when one server is asked to execute a remote

stored procedure on another server, but it can also be used by applications. Using the TDS RPC syntax

means neither the driver nor the server need to perform any parameter conversions. This improves

performance, especially for image parameters. The SQL Server ODBC driver uses the TDS RPC syntax if the

application uses the ODBC CALL escape clause; it uses the regular SQL statement syntax if the application

uses the Transact-SQL EXECUTE statement.

Using the ODBC CALL escape clause also allows the application to retrieve output parameters and returncodes from a stored procedure. Output parameter and return code processing is discussed below.

Output Parameters and Return Codes

SQL Server stored procedures can return both output parameters and return codes to an application:

The arameters and return codes can be bound to ro ram variables in an ODBC a lication where the

SQLExecDirect(hstmt, "EXECUTE sp_helpdb 'pubs' ", SQL_NTS);

SQLExecDirect(hstmt, "{ call sp_helpdb ('pubs') }", SQL_NTS);

CREATE PROCEDURE odbcproc @oparm int OUTPUT AS

SELECT name FROM sysusers WHERE uid < 2

SELECT @oparm = 88

RETURN 99GO


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application can reference them. For example, to execute the procedure above using the ODBC CALL escape

clause and bind the return code and output parameters to program variables:

SQL Server does not send back the values for the return code or output parameters until the end of all

result sets for the procedure. The program variables ProcRet and OParm do not hold the output values of

99 and 88 until SQLMoreResults returns SQL_NO_DATA_FOUND.

Text and Image Data

The SQL Server ODBC driver has a couple of optimizations for text and image column processing that

applications can use to improve performance.

Bound vs. Unbound Text and Image Columns

When using server cursors (see "Cursors"), the driver is optimized to not transmit the data for unboundtext or image columns at the time the row is fetched. The text or image data is not actually retrieved from

the server until the application issues SQLGetData for the column.

This optimization can be applied to applications so that no text or image data is displayed while a user is

scrolling up and down a cursor. After the user selects a row, the application can call SQLGetData to

retrieve the text or image data. This saves transmitting the text or image data for any of the rows the user

does not select and can save the transmission of very large amounts of data.

Logged vs. Nonlogged

An application can request that the driver not log text and image modifications:

This option should only be used for situations where the text or image data is not critical, and the data

owners are willing to trade data recovery for higher performance.

Data-At-Execution and Text and Image Columns

ODBC Data-At-Execution allows applications to work with extremely large amounts of data on bound

columns or parameters. When retrieving very large text or image columns, an application cannot simply

allocate a huge buffer, bind the column into the buffer, and fetch the row. When updating very large text or

SQLRETURN rcd;

DWORD ProcRet = 0, OParm = 0;

long cbProcRet = 0, cbOParm = 0;

// Bind the return code.rcd = SQLBindParameter(hstmt, 1, SQL_PARAM_OUTPUT,

SQL_C_SLONG, SQL_INTEGER, 0, 0, &ProcRet, 0, &cbProcRet);

// Bind the output parameter.

rcd = SQLBindParameter(hstmt, 2, SQL_PARAM_OUTPUT,

SQL_C_SLONG, SQL_INTEGER, 0, 0, &OParm, 0, &cbOParm;

// First ? marks the return code,

// second ? marks the output parameter.

rcd = (SQLExecDirect(hstmt, "{? = call odbcproc(?)}", SQL_NTS;

SQLSetStmtOption(hstmt,

SQL_SOPT_SS_TEXTPTR_LOGGING,

SQL_TL_OFF);


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mage co umns, t e app cat on cannot s mp y a ocate a uge u er, n t to a parameter mar er n an

SQL statement, and then execute the statement. Whenever the size of the text or image data exceeds 400K

(64K with SQL Server 4.21a), the application must use SQLGetData or SQLPutData with their Data-At-

Execution options. Applications should always use Data-At-Execution if there is any possibility that the size

of the data will exceed these limits.

Data-At-Execution is described in the ODBC 2.0 Programmer's Reference; however, it remains one of the

hardest parts of the ODBC API for an application programmer to learn. The Appendix of this paper contains

the source code of two Win32 console applications, Getimage.c and Putimage.c, that illustrate using Data-At-Execution to read and write large amounts ofimage data. Text columns would use similar calls, the only

difference would be binding between SQL_C_CHAR and SQL_LONGVARCHAR instead of SQL_C_BINARY and

SQL_LONGVARBINARY. Programmers interested in working with text or image columns should look up the

Data-At-Execution index entries of the ODBC 2.0 Programmer's Reference, then search for "text" and "image"

in Microsoft SQL Server Programming ODBC for SQL Server.

Querying Metadata

This section discusses some common issues when getting metadata and catalog information from the

driver.

Instcat.sql

Both the SQL Server system catalog stored procedures and the ODBC API catalog functions address the

need of applications to retrieve catalog information from a database. Because there is a high correlation

between the ODBC catalog functions and the SQL Server catalog stored procedures, the SQL Server ODBC

driver implements many of the ODBC API catalog functions as calls to a corresponding SQL Server catalog

procedure. The driver is therefore dependent on the catalog stored procedures in any SQL Server to which

it connects.

Each version of the SQL Server ODBC driver is developed in conjunction with a specific version of

SQL Server. The proper operation of each driver version requires the versions of the catalog stored

procedures associated with the specific version of SQL Server with which the driver was developed, or a

later version of the procedures. For example, the 2.50.0121 driver was developed in conjunction with

Microsoft SQL Server version 6.0, and requires either the versions of the system catalog stored procedures

that were released with SQL Server 6.0, or with later vers ions, such as 6.5. The driver does not work

properly with older versions of the catalog stored procedures, such as those in SQL Server version 4.21a.

If a driver attempts to connect to a SQL Server running an older version of the catalog stored procedures

than those required by the driver, the connection completes with SQL_SUCCESS_WITH_INFO and a call to

SQLError returns the following message:

Although the connection is successful, the application may later encounter errors on calls to the ODBC API

catalog functions.

Sites running multiple versions of the driver against a server need to ensure that the server is running with

at least the version of Instcat.s l associated with the newest ODBC driver that will connect to it. For exam le,

SqlState: 01000

pfNative: 0

szErrorMsg: "[Microsoft][ODBC SQL Server Driver]The ODBC

catalog stored procedures installed on server

My421Server are version 02.00.4127; version 06.00.0115

or later is required to ensure proper operation.

Please contact your system administrator."


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a site running multiple version 6.0 servers could buy SQL Server version 6.5 and upgrade some clients to

use the new 2.65.0201 driver that comes with version 6.5. The site would also need to run the 6.5 version of

Instcat.sql against the 6.0 servers before the new driver can connect to them.

Installing a newer version of Instcat.sql into an older server does not break any existing applications

connecting to that server, even ones still using the old drivers. It simply allows the applications using the

new driver to operate correctly.

Sites should run the Instcat.sql script at the server command prompt by using the isql utility.

For more information about determining the version of Instcat.sql currently applied to a server, see

Microsoft Knowledge Base article Q137636. For more information about the isql utility, see the Microsoft

SQL Server Transact-SQL Reference.

Multiple Active Statements per Connection

Starting with SQL Server6.5 and its associated driver, users can have multiple outstanding calls for

metadata. In SQL Server 6.5, the catalog

Documents

Using ODBC with Microsoft SQL Server.pdf