Execution Plans in practice - how to make SQL Server queries faster - Damian Widera

@ITCAMPRO #ITCAMP16Community Conference for IT Professionals

Execution plans in practice – how to

make SQL Server queries faster

Damian Widera

Microsoft Data Platform MVP

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@damianwidera

http://sqlblog.com/blogs/damian_widera/default.aspx


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Customers…


• What is bad in terms of query execution

• What was bad in previous examples is not always

bad…

• SQL Server 2016 – how could it help?

Today I am going to tell about….


SQL Server School of Art


• A query takes long time to execute

• A query consumes too much resources

• Then we have:

–Reduced concurency

–More locks that should be

• Locks contentions, waits, etc…

What is bad in terms of query execution


• A query (logical) is parsed and optimized based on

all available information (cost optimization)

• Estimation != Guestimation

• Plan(s) is (are) produced

How it works


Bad row estimates can impact a variety of decisions including:

• index selection seek vs. scan operations,

• parallel versus serial execution,

• join algorithm selection,

• inner vs. outer physical join selection (e.g. build vs. probe),

• spool generation,

• bookmark lookups vs. full clustered or heap table access,

• stream or hash aggregate selection

Why it is so important?


• Index or table scans (*): May indicate a need

for better or additional indexes (*)

– THIS IS NOT ALWAYS WRONG!!!!

• Bookmark Lookups: Consider changing the current

clustered index, consider using a covering index, limit the

number of columns in the SELECT statement.

• Filter: Remove any functions in the WHERE clause, don’t

include views in your Transact-SQL code, may need

additional indexes.

What to look for…


• Sort: Does the data really need to be sorted? Can an index be used to avoid sorting? Can sorting be done at the client more efficiently? „Hidden” sort. Nonlinearoperation

• Spool. Used as a cache in the QP. Reason – lack of indexesor uniqueness. Implemented as a hidden tables in tempdb

• Hash. Used for joins and aggregations. Builds a hashtablein memory (or in tempdb). Means there is no good index.

What to look for…



• When index scan is the best option

–Two examples

• High density -> Seek

• Low density -> Scan

• When cursor can be the best option (especially for

SQL Server 2008 R2 and older)

–Running total

What was bad in previous example is not always bad…



New functionality that makes DBAs’ happy - Query

Store

will be available in all editions

SQL Server 2016 – how could it help?


Query data store

Durability latency controlled by DB option

DATA_FLUSH_INTERNAL_SECONDS

Compile

Execute

Plan store

Runtime stats

Query Store

schema

Collects query texts (plus all relevant properties)

Stores all plan choices and performance metrics

Works across restarts / upgrades / recompiles

Dramatically lowers the bar for performance troubleshooting

New Views

Intuitive and easy plan forcing


Query Store schema explained


Query Store Dashboard



Q & A

Technology

Execution Plans in practice - how to make SQL Server queries faster - Damian Widera