Use EXPLAIN PLAN and TKPROF to Tune Your Applications

Summary: EXPLAIN PLAN overview TKPROF overview Why??? Reading execution plans Reading TKPROF reports

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Execution Plans and EXPLAIN PLAN An execution plan is a list of steps that Oracle will follow in order to execute a SQL statement.

Each step is one of a finite number of basic operations known to the database server. Even the most complex SQL statement can be broken down into a series of basic operations.

EXPLAIN PLAN is a statement that allows you to have Oracle generate the execution plan for any SQL statement without actually executing it. You will be able to examine the execution plan by querying the plan table.

The Plan Table A plan table holds execution plans generated by the EXPLAIN PLAN statement. The typical name for a plan table is plan_table, but you may use any name you wish. Create the plan table by running utlxplan.sql, located in $ORACLE_HOME/rdbms/admin.

Important Columns in the Plan Tablestatement_id Unique identifier for each execution plantimestamp When the execution plan was generatedoperation The operation performed in one step of the execution plan, such as “table access”options Additional information about the operation, such as “by index ROWID”object_name Name of table, index, view, etc. accessedoptimizer Optimizer goal used when creating execution planid Step number in execution planparent_id Step number of parent step

EXPLAIN PLAN Prerequisites INSERT privilege on a plan table All necessary privileges to execute the statement being explained SELECT privileges on underlying tables of views, if the statement being explained involves

views

EXPLAIN PLAN SyntaxEXPLAIN PLAN [SET STATEMENT_ID = <string in single quotes>][INTO <plan table name>]FOR <SQL statement>;

Querying an Execution Plan from the Plan Table Use a CONNECT BY clause to trace the hierarchy Use LPAD function to indent rows, making the hierarchy easier to follow

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Put statement_id in WHERE clause to retrieve only one execution plan at a time Sample script on next slide shows the most important information You can also try utlxpls.sql or utlxplp.sql in $ORACLE_HOME/rdbms/admin

A Simple Query to Display Execution Plans

SET VERIFY OFFACCEPT stmt_id CHAR PROMPT "Enter statement_id: "COL id FORMAT 999COL parent_id FORMAT 999 HEADING "PARENT"COL operation FORMAT a35 TRUNCATECOL object_name FORMAT a30SELECT id, parent_id, LPAD (' ', LEVEL - 1) || operation || ' ' || options operation, object_nameFROM plan_tableWHERE statement_id = '&stmt_id'START WITH id = 0AND statement_id = '&stmt_id'CONNECT BY PRIOR id = parent_idAND statement_id = '&stmt_id';

A Sample Execution Plan

SQL> EXPLAIN PLAN SET statement_id = 'demo' FOR 2 SELECT a.customer_name, a.customer_number, b.invoice_number, 3 b.invoice_type, b.invoice_date, b.total_amount, 4 c.line_number, c.part_number, c.quantity, c.unit_cost 5 FROM customers a, invoices b, invoice_items c 6 WHERE c.invoice_id = :b1 7 AND c.line_number = :b2 8 AND b.invoice_id = c.invoice_id 9 AND a.customer_id = b.customer_id;Explained.SQL> @explain.sqlEnter statement_id: demo ID PARENT OPERATION OBJECT_NAME---- ------ ----------------------------------- ----------------- 0 SELECT STATEMENT 1 0 NESTED LOOPS 2 1 NESTED LOOPS 3 2 TABLE ACCESS BY INDEX ROWID INVOICE_ITEMS 4 3 INDEX UNIQUE SCAN INVOICE_ITEMS_PK 5 2 TABLE ACCESS BY INDEX ROWID INVOICES 6 5 INDEX UNIQUE SCAN INVOICES_PK 7 1 TABLE ACCESS BY INDEX ROWID CUSTOMERS 8 7 INDEX UNIQUE SCAN CUSTOMERS_PK

Other Ways to View Execution Plans The autotrace feature in SQL*Plus Performance tuning tools

Sample Autotrace Output in SQL*Plus

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Execution Plan---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=4 Card=1 Bytes=39) 1 0 NESTED LOOPS (Cost=4 Card=1 Bytes=39) 2 1 NESTED LOOPS (Cost=3 Card=1 Bytes=27) 3 2 TABLE ACCESS (BY INDEX ROWID) OF 'INVOICE_ITEMS' (Cost =2 Card=1 Bytes=15) 4 3 INDEX (UNIQUE SCAN) OF 'INVOICE_ITEMS_PK' (UNIQUE) ( Cost=1 Card=2) 5 2 TABLE ACCESS (BY INDEX ROWID) OF 'INVOICES' (Cost=1 Ca rd=2 Bytes=24) 6 5 INDEX (UNIQUE SCAN) OF 'INVOICES_PK' (UNIQUE) 7 1 TABLE ACCESS (BY INDEX ROWID) OF 'CUSTOMERS' (Cost=1 Car d=100 Bytes=1200) 8 7 INDEX (UNIQUE SCAN) OF 'CUSTOMERS_PK' (UNIQUE)

EXPLAIN PLAN Limitations The EXPLAIN PLAN statement provides a good faith estimate of the execution plan that Oracle

would use. The real plan that gets used may differ from what EXPLAIN PLAN tells you for many reasons:

– Optimizer stats, cursor sharing, bind variable peeking, dynamic instance parameters make plans less stable.

– EXPLAIN PLAN does not peek at bind variables.– EXPLAIN PLAN does not check the library cache to see if the statement has already

been parsed. EXPLAIN PLAN does not work for some queries:ORA-22905: cannot access rows from a non-nested table item

Viewing Actual Execution Plans The v$sql view shows statements in the library cache. Here you can find the address, hash

value, and child number for a statement of interest. The v$sql_plan view shows the actual execution plan for each statement, given its address,

hash value, and child number. The columns are similar to the plan table. The v$sql_plan_statistics view shows actual statistics (rows, buffer gets, elapsed time, etc.) for

each operation of the execution plan. The v$sql_plan and v$sql_plan_statistics views are available starting in Oracle 9i.

v$sql_plan_statistics is not populated by default.

A Simple Query to Display Actual Execution Plans

SET VERIFY OFFCOL id FORMAT 999COL parent_id FORMAT 999 HEADING "PARENT"COL operation FORMAT a35 TRUNCATECOL object_name FORMAT a30SELECT id, parent_id, LPAD (' ', LEVEL - 1) || operation || ' ' || options operation, object_nameFROM ( SELECT id, parent_id, operation, options, object_name FROM v$sql_plan WHERE address = '&address' AND hash_value = &hash_value

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AND child_number = &child_number )START WITH id = 0CONNECT BY PRIOR id = parent_id;

Trace Files and TKPROF The Oracle server process managing a database session writes a verbose trace file when SQL

trace is enabled for the session. TKPROF is a utility provided by Oracle that formats SQL trace files into very helpful and

readable reports. TKPROF is installed automatically when the database server software is installed. You invoke TKPROF from the operating system command line; there is no graphical interface for TKPROF. Starting in Oracle 9i TKPROF can read extended SQL trace files and report on wait events statistics.

Enabling SQL Trace At the instance level: sql_trace = true timed_statistics = true (optional) In your own session:ALTER SESSION SET sql_trace = TRUE;ALTER SESSION SET timed_statistics = TRUE; (optional) In another session:SYS.dbms_system.set_sql_trace_in_session (<SID>, <serial#>, TRUE)

Finding the Trace File Look in the user dump destination. On OFA compliant systems this will be

$ORACLE_BASE/admin/$ORACLE_SID/udump Check timestamps and file contents to see which trace file is yours If non-DBAs need access to trace files, add _trace_files_public = true to the parameter file to

avoid permissions problems on Unix platforms Use a dedicated server connection when tracing, if possible.

Formatting a Trace File with TKPROFInvoke TKPROF from the operating system prompt like this:tkprof <trace file> <output file> \ [explain=<username/password>] \ [sys=n] [sort=<keyword>]

TKPROF Command-line Argumentstkprof <trace file> <output file> \ [explain=<username/password>] \ [sys=n] [sort=<keyword>]

trace file The SQL trace file to be formattedoutput file The formatted output to be written by TKPROFexplain= Database login to be used if you want the output to include execution plans sys=n Omit “recursive SQL” performed by the SYS usersort= List traced SQL statement in the output file in a specific order

TKPROF Sample Output

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SELECT a.customer_name, a.customer_number, b.invoice_number, b.invoice_type, b.invoice_date, b.total_amount, c.line_number, c.part_number, c.quantity, c.unit_costFROM customers a, invoices b, invoice_items cWHERE c.invoice_id = :b1AND c.line_number = :b2AND b.invoice_id = c.invoice_idAND a.customer_id = b.customer_id

call count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------Parse 1 0.05 0.02 0 0 0 0Execute 1 0.00 0.00 0 0 0 0Fetch 2 0.00 0.00 8 8 0 1------- ------ -------- ---------- ---------- ---------- ---------- ----------total 4 0.05 0.02 8 8 0 1

Misses in library cache during parse: 1Optimizer goal: CHOOSEParsing user id: 34 (RSCHRAG)

Why Look at Execution Plans and TKPROF Reports?These tools are critical to the application tuning process, and tuning at the application level is necessary for high performance systems. With EXPLAIN PLAN, v$sql_plan, and TKPROF, you can: Proactively tune applications that are in development Reactively tune production systems that are experiencing performance problems Estimate resource needs or feasibility of ad-hoc queries Quantify resource needs for specific applications

Reading Execution PlansAn execution plan is a hierarchical listing of steps. Each step is one of a few basic data access operations known to the database server. The most complex SQL statement can be broken down into a series of basic operations. “Read from the most indented step outward.” This is not exactly correct! Instead, take this approach:a) Start at the least indented stepb) Find the step or steps that provide direct input to the step noted in (a).c) Evaluate each of the steps found in (b). This may involve recursively finding steps that provide input and evaluating them.

Execution Plan Example #1SELECT customer_id, customer_nameFROM customersWHERE UPPER (customer_name) LIKE 'ACME%'ORDER BY customer_name;

OPERATION OBJECT_NAME------------------------------ --------------SELECT STATEMENT SORT ORDER BY TABLE ACCESS FULL CUSTOMERS

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Execution Plan OperationsTABLE ACCESS FULLPerform a full table scan of the indicated table and retrieve all rows that meet criteria from the WHERE clause. Input: no subordinate operations. Output: the necessary columns from the rows meeting all criteria. SORT ORDER BYSort the input rows for the purpose of satisfying an ORDER BY clause. Input: the rows to be sorted. Output: the rows in sorted order.

Execution Plan Example #2SELECT a.customer_name, b.invoice_number, b.invoice_dateFROM customers a, invoices bWHERE b.invoice_date > TRUNC (SYSDATE - 1)AND a.customer_id = b.customer_id;

OPERATION OBJECT_NAME------------------------------ --------------SELECT STATEMENT NESTED LOOPS TABLE ACCESS BY INDEX ROWID INVOICES INDEX RANGE SCAN INVOICES_DATE TABLE ACCESS BY INDEX ROWID CUSTOMERS INDEX UNIQUE SCAN CUSTOMERS_PK

Execution Plan OperationsINDEX UNIQUE SCANLook up a complete key in a unique index. Input: usually no subordinate operations. (Key values typically come from the original query or a parent operation.) Output: zero or one ROWIDs from the index. INDEX RANGE SCAN Look up a key in a non-unique index, or an incomplete key in a unique index. Input: usually no subordinate operations. Output: zero or more ROWIDs from the index.

Execution Plan OperationsTABLE ACCESS BY INDEX ROWIDLook up rows in a table by their ROWIDs. Input: a list of ROWIDs to look up. Output: the necessary columns from the rows with the given ROWIDs.NESTED LOOPSPerform a join between two sets of row data using the nested loops algorithm. Inputs: two separate sets of row data. Output: the results of the join. For each row Oracle reads from the first input, the operations that make up the second input are executed once and matching rows generate output.

Execution Plan Example #3SELECT a.customer_name, COUNT (DISTINCT b.invoice_id) open_invs, COUNT (c.invoice_id) open_inv_itemsFROM customers a, invoices b, invoice_items cWHERE b.invoice_status = 'OPEN'AND a.customer_id = b.customer_id

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AND c.invoice_id (+) = b.invoice_idGROUP BY a.customer_name;

OPERATION OBJECT_NAME-------------------------------- ----------------SELECT STATEMENT SORT GROUP BY NESTED LOOPS OUTER HASH JOIN TABLE ACCESS BY INDEX ROWID INVOICES INDEX RANGE SCAN INVOICES_STATUS TABLE ACCESS FULL CUSTOMERS INDEX RANGE SCAN INVOICE_ITEMS_PK

Execution Plan OperationsHASH JOINPerform a join between two sets of row data using the hash join algorithm. Inputs: two separate sets of row data. Output: the results of the join. Oracle reads all rows from the second input and builds a hash structure, before reading each row from the first input one at a time. For each row from the first input, the hash structure is probed and matching rows generate output.

Execution Plan OperationsNESTED LOOPS OUTERSame as the NESTED LOOPS operation, except that an outer join is performed. SORT GROUP BYSame as the SORT ORDER BY operation, except that the rows are sorted and grouped to satisfy a GROUP BY clause.

Execution Plan Example #4SELECT customer_nameFROM customers aWHERE EXISTS (SELECT 1 FROM invoices_view b WHERE b.customer_id = a.customer_id AND number_of_lines > 100)ORDER BY customer_name;

CREATE OR REPLACE VIEW invoices_view ASSELECT a.invoice_id, a.customer_id, COUNT(*) number_of_linesFROM invoices a, invoice_items bWHERE b.invoice_id = a.invoice_idGROUP BY a.invoice_id, a.customer_id;

OPERATION OBJECT_NAME----------------------------------- -------------SELECT STATEMENT SORT ORDER BY FILTER TABLE ACCESS FULL CUSTOMERS VIEW INVOICES_VIEW FILTER

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SORT GROUP BY NESTED LOOPS TABLE ACCESS BY INDEX ROWID INVOICES INDEX RANGE SCAN INVS_CUST_ID INDEX RANGE SCAN INV_ITEMS_PK

Execution Plan OperationsFILTERRead a set of row data and discard some rows based on various criteria. To determine the criteria, operations from a second input may need to be performed. Input: rows to be examined and, sometimes, an additional subordinate operation that must be performed for each row from the first input in order to evaluate criteria. Output: the rows from the first input that met the criteria. VIEWBuild a physical representation of a database view or subset of a database view. Input: set of row data. Output: set of row data that implements the view or subset of the view.

Notes on Execution Plan Operations The optimizer rewrites subqueries as joins and merges them into the main query whenever

possible. If a subquery is completely independent of the main query and cannot be merged into the

main query, the optimizer may treat the subquery as a separate statement and leave it out of the execution plan for the main query.

The optimizer expands view definitions and merges them into the main query wherever possible. A VIEW operation will only appear in an execution plan when the view definition could not be merged.

Execution Plan Example #5SELECT /*+ RULE */ a.cust_name, b.contact_nameFROM customers a, contacts@sales.acme.com bWHERE UPPER(b.contact_name) = UPPER(a.cust_name);

Execution Plan------------------------------------------------ 0 SELECT STATEMENT Optimizer=HINT: RULE 1 0 MERGE JOIN 2 1 SORT (JOIN) 3 2 REMOTE* SALES.ACME.COM 4 1 SORT (JOIN) 5 4 TABLE ACCESS (FULL) OF 'CUSTOMERS'

3 SERIAL_FROM_REMOTE SELECT "CONTACT_NAME" FROM "CONTACTS" "B”

Execution Plan OperationsREMOTESubmit a SQL statement to a remote database via Oracle Net. Input: typically no subordinate operations. Output: the results of the query from the remote database. Note that the database link used to access the remote database and the actual SQL submitted to the remote database will be accessible from the execution plan.

SORT JOIN

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Same as the SORT GROUP BY operation, except that the input is sorted by the join column or columns in preparation for a join using the merge join algorithm. MERGE JOINPerform a join between two sets of row data using the merge join algorithm. Inputs: two separate sets of row data. Output: the results of the join. Oracle reads rows from both inputs in an alternating fashion and merges together matching rows in order to generate output. The two inputs are assumed to be sorted on the join column or columns.

Summary of OperationsWe have not covered all of the execution plan operations, but we have covered some of the most common ones:

- TABLE ACCESS FULL- TABLE ACCESS BY INDEX ROWID- INDEX UNIQUE SCAN- INDEX RANGE SCAN- NESTED LOOPS- NESTED LOOPS OUTER- HASH JOIN- MERGE JOIN- FILTER- VIEW- REMOTE- SORT ORDER BY- SORT GROUP BY- SORT JOIN

Elements of a TKPROF Report Report heading

– TKPROF version, date run, sort option, trace file One entry for each distinct SQL statement in trace file

– Listing of SQL statement– OCI call statistics: count of parse, execute, and fetch calls, rows processed, and time

and I/O used– Parse information: parsing user, recursive depth, library cache misses, and optimizer

mode– Row source operation listing– Execution plan listing (optional)– Wait event listing (optional)

Report Summary– OCI call statistics totals– Counts of how many statements were found in the trace file, how many were distinct,

and how many were explained in the report.

Sample TKPROF Report Heading

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TKPROF: Release 8.1.6.1.0 - Production on Wed Aug 9 19:06:36 2000

(c) Copyright 1999 Oracle Corporation. All rights reserved.

Trace file: example.trcSort options: default************************************************************************count = number of times OCI procedure was executedcpu = cpu time in seconds executing elapsed = elapsed time in seconds executingdisk = number of physical reads of buffers from diskquery = number of buffers gotten for consistent readcurrent = number of buffers gotten in current mode (usually for update)rows = number of rows processed by the fetch or execute call

Sample OCI Call StatisticsSELECT table_nameFROM user_tablesORDER BY table_name

call count cpu elapsed disk query current rows------- ------ -------- ---------- --------- --------- --------- ---------Parse 1 0.01 0.02 0 0 0 0Execute 1 0.00 0.00 0 0 0 0Fetch 14 0.59 0.99 0 33633 0 194------- ------ -------- ---------- --------- --------- --------- ---------total 16 0.60 1.01 0 33633 0 194

Misses in library cache during parse: 1Optimizer goal: CHOOSEParsing user id: RSCHRAG [recursive depth: 0]

What the TKPROF Sample on the Previous Slide Tells Us The application called on Oracle to parse this statement once while SQL trace was enabled. The parse took 0.01 CPU seconds, 0.02 elapsed seconds. No disk I/Os or buffer gets took place during the parse, suggesting that no misses in the

dictionary cache. Oracle was called on to execute this statement once. The execution took under 0.01 CPU seconds. No disk I/Os or buffer gets took place during the execution. (Queries often defer the work to

the fetch phase.) Oracle was called on 14 times to perform a fetch, and a total of 194 rows were returned. Fetching took 0.59 CPU seconds, 0.99 elapsed seconds. Fetching required 33,633 buffer gets in consistent mode, but no physical reads were required. The statement was not in the library cache (shared pool) when the parse call came in. The cost-based optimizer and a goal of “choose” were used to parse the statement. The RSCHRAG user was connected to the database when the parse occurred. This statement was executed directly by the application; it was not invoked recursively by the

SYS user or a database trigger.

Sample Row Source Operation ListingRows Row Source Operation------- --------------------------------------------------- 194 SORT ORDER BY 194 NESTED LOOPS 195 NESTED LOOPS OUTER

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195 NESTED LOOPS OUTER 195 NESTED LOOPS 11146 TABLE ACCESS BY INDEX ROWID OBJ$ 11146 INDEX RANGE SCAN (object id 34) 11339 TABLE ACCESS CLUSTER TAB$ 12665 INDEX UNIQUE SCAN (object id 3) 33 INDEX UNIQUE SCAN (object id 33) 193 TABLE ACCESS CLUSTER SEG$ 387 INDEX UNIQUE SCAN (object id 9) 194 TABLE ACCESS CLUSTER TS$ 388 INDEX UNIQUE SCAN (object id 7)

Sample Execution Plan ListingRows Execution Plan------- --------------------------------------------------- 0 SELECT STATEMENT GOAL: CHOOSE 194 SORT (ORDER BY) 194 NESTED LOOPS 195 NESTED LOOPS (OUTER) 195 NESTED LOOPS (OUTER) 195 NESTED LOOPS 11146 TABLE ACCESS (BY INDEX ROWID) OF 'OBJ$' 11146 INDEX (RANGE SCAN) OF 'I_OBJ2' (UNIQUE) 11339 TABLE ACCESS (CLUSTER) OF 'TAB$' 12665 INDEX (UNIQUE SCAN) OF 'I_OBJ#' (NON-UNIQUE) 33 INDEX (UNIQUE SCAN) OF 'I_OBJ1' (UNIQUE) 193 TABLE ACCESS (CLUSTER) OF 'SEG$' 387 INDEX (UNIQUE SCAN) OF 'I_FILE#_BLOCK#' (NON-UNIQUE) 194 TABLE ACCESS (CLUSTER) OF 'TS$' 388 INDEX (UNIQUE SCAN) OF 'I_TS#' (NON-UNIQUE)

Notes About TKPROF Execution Plan Listings Execution plans are only included in TKPROF reports if the explain= parameter is specified

when TKPROF is invoked TKPROF will create and drop its own plan table if one does not already exist The row counts on each step are actuals—not estimates. This can be very helpful when

troubleshooting queries that perform poorly. When TKPROF runs the EXPLAIN PLAN statement for a query, a different execution plan could

be returned than was actually used in the traced session.

TKPROF Reports: More Than Just Execution Plans Listing of SQL statements and library cache miss information helps you determine if

applications are using Oracle’s shared SQL facility effectively. Parse, execute, and fetch call counts help you determine if applications are using Oracle APIs

effectively. CPU and I/O statistics help you zero in on resource-intensive SQL statements. Row counts on individual steps of the execution plans help you rework inefficient execution

plans.

Wrapping Up

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Use EXPLAIN PLAN, queries against v$sql_plan, the autotrace facility in SQL*Plus, or GUI tools to view execution plans.

Use TKPROF to format SQL trace files for human readability. Execution plans and TKPROF reports give the DBA and application developer a wealth of

information that can be used to make applications efficient and perform well. The catch: you need to know how to interpret execution plans and TKPROF reports in order to

get any benefit from them. You also ought to know when to use EXPLAIN PLAN versus when to query v$sql_plan.

Resources Download this slide show, the accompanying white paper, and many other useful

presentations at:

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Contact InformationRoger SchragDatabase Specialists, Inc.388 Market Street, Suite 400San Francisco, CA 94111

Tel: 415/344-0500Email: rschrag@dbspecialists.comWeb: www.dbspecialists.com

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