SQLDay2013_MarcinSzeliga_StoredProcedures

NASI SPONSORZY I PARTNERZY

Stored Procedures –Facts and Myths

Marcin Szeligawww.sqlexpert.pl

http://blog.sqlexpert.pl/http://www.facebook.com/SQLExpertpl

[email protected]

Agenda

• Basic Fact - Plan Caching and Reuse is (usually) a Good Thing

• Obvious Consequence – You Should AvoidRecompilations …

• But Sometimes Plan Reusing is a Bad Thing

• Recap

Plan Caching and Reuse is a Good Thing• Stored procedures

– Are a convenient container for code reuse• They can be called many times and the query plans can be

reused, saving the time, CPU and memory

– Are optimized and compiled during their first execution• Optimization is based on parameters supplied during this

execution• Future executions will reuse the query plan stored in plan

cache• There are at most two instances of a query plan at any time in

plan cache:– One for all of the serial executions.– One for all of the parallel executions

• From a query plan, an execution context is derived– Execution contexts hold the values needed for a specific execution of

a query plan, and they are also cached and reused

Does compilation/recompilationreally matter?• In terms of time

– Compilation is really expensive and CPU-heavy process

• In terms of memory– Storing large number of execution plans the procedure cache “steals”

memory from the Buffer Pool– The currently used formula looks like this:

• 75% 0-4GB + 10% 4-64GB + 5% 64GB– On 16GB box 4.2 GB may be used for the procedure cache

• In terms of concurrency– Getaways throttle simultaneous compilation to really small number

• Three of them: – Small - 4 x number of logical CPUs allocated to SQL Server– Medium - Number of logical CPUs allocated to SQL Server– Large - 1

• Look for 'RESOURCE_SEMAPHORE_QUERY_COMPILE' wait

Demo

• Measuring compilation cost, in terms of time and memory

• Looking inside the Procedure Cache

Stored procedures are not the only option• SQL Server can cache query plans for different types of

batches beside ad-hoc queries and stored procedures:– Query preparation

• ODBC and OLE DB expose this functionality via SQLPrepare/SQLExecute and ICommandPrepare interfaces

• Very similar result can be achieved by using sp_executesql

– Simple parameterization• SQL Server automatically replaces constant literal values

with variables before the query plan will be compiled• SQL Server 2008 allows us to forced parameterization of

almost all types of queries

– Dynamic SQL• Strings submitted via EXEC for execution

Demo

• Checking two other options: query preparation and forced parameterization

Avoiding recompilations due to plan stability-related reasons

• Stored procedure (or a batch) must be recompiled if not doing so would result in incorrect results or actions

• 14 recompilation reasons

• 2 sub-categories:

– Scheme changes

– SET options changes

Recompilation due to schemachanges• Putting all DML statements at the beginning of a stored

procedure will NOT minimize recompilations– SQL Server 2005 onwards does statement level recompilations

• Deferred compilations mean that objects didn't exist during creation the stored procedure

• The only solution is to use temporary tables – SQL Server caches temporary objects but altering definitions of

those temporary objects disables this caching mechanism• In addition, objects cannot have named constraints

• Accessing objects created outside the current scope requires object id to name resolution– Recompilation due to the changed scheme occurs

Demo

• Avoiding recompilation due to schema changes

Recompilation due to SET option changes• When an execution plan is created, the SQL Server stores

the environmental setting with it– Some SET options used when the stored procedure was

created or altered will be saved as part of its metadata– Some execution plan attributes are considered cache key

ones

• Subsequent execution will not trigger recompilations– During the first execution a procedure is recompiled with

the correct values, and from now on its execution plan is valid

• On SQL Server 2005 onwards this has a much less impact than before

Demo

• Avoiding recompilation due to SET option changes

Recompilations due to plan optimality-related reasons

• SQL Server collects statistics about individual columns or sets of columns• Statistics contain:

– The average key length– A single-column histogram, including up to 200 values of a given column– The estimated number of rows matching the filter, or all rows in the table

• Statistics can be not only automatically created but also kept up to date• Statistics will be considered stale when:

– The first row is inserted into the empty table– The number of rows in the table was less or equal to 500 and the

colmodctr of the leading column of the statistics object has changed by more than 500

– The table had more than 500 and the colmodctr of the leading column of the statistics object has changed by more than 500 + 20% of the number of rows

– If the statistics object is defined on a temporary table, there is an additional threshold for recomputation at 6 rows

Recompilations due to statisticschanges• Even if a statistic becomes outdated, it will not be

automatically updated after the modification completes– It will automatically update the next time a query plan

uses it

• Default mechanism may lead to unnecessary recompilations

• Triggers are also being recompiled when the number of rows in the inserted or deleted virtual tables changes significantly from one execution to the next

• This is where hints come in– KEEP PLAN, disables the “6 rows” rule– KEEPFIXED PLAN disables recompilations due to statistics

changes completely

Demo

• Avoiding recompilation due to statistics changes

When Plan Reusing is a Bad Thing

• Optimization and execution are two completely separate processes– Sometime during an optimization SQL Server does not

know the actual values used in queries

– Incorrect estimation in one part of the execution plan can (and probably will) be spread to others part of the plan

• Parameter Sniffing is an expected SQL Server behavior– Parameter values used for the first execution of a stored

procedure will be used to compile an execution plan and this plan will be used over and over again

– The problem arises when a stored procedure is executed for the first time with unusual parameter values

Dealing with Parameter Sniffing

• You can:– Disable plan caching for the problematic stored procedure

by adding WITH RECOMPILE to the procedure header– Force a new compilation for a specific execution only by

adding WITH RECOMPILE to the EXEC statement– Force a compilation with typical parameter values

• By using variables• By using OPTIMIZE FOR UNKNOWN

• Remember, recompiling a new plan for a given parameter value might be way cheaper than reusing a plan optimized for different values– Especially for DWs

Demo

• Parameter Sniffing Revealed

• Dealing with Parameter Sniffing

Recap

• Recompilation not only takes a lot of resources (both CPU cycles and memory), but also can considerably slow down your queries

• There are two other mechanisms, beside stored procedures (queries preparation and auto-parameterization) that can be used to avoid unnecessary recompilations

• Temporary tables, not the permanent ones, should be used inside stored procedures

• Make an informed choice when to disable recompilations,without changing database options and using table variables

• Parameter Sniffing is rather a feature than a bug and now youknow how to deal with it

NASI SPONSORZY I PARTNERZY

Organizacja: Polskie Stowarzyszenie Użytkowników SQL Server - PLSSUGProdukcja: DATA MASTER Maciej Pilecki