Maciej Pilecki Consultant Project Botticelli Ltd. DAT404

SELECT About FROM Speakers WHERE Name = ’Maciej Pilecki’ Over 10 years of SQL Server experience SQL Server MVP since Jan 2006 Specializing in SQL Server database development, administration, performance tuning and troubleshooting Delivering consulting services around the world (special discounted rate for TechEd attendees) Frequent speaker at many international conferences Living in Wroclaw, Poland

Agenda Query optimization and execution Plan caching and reuse What is the Procedure Cache? Managing the Procedure Cache Controlling plan reuse

Query Execution You type your query in SSMS, hit F5 and get the results......well, it is more complicated than that...

Query Execution Every query goes through a numer of steps: Parsing Algebraization Optimization Execution

Query Optimization Process of selecting one execution plan from many possible plans Cost-based – aimed at finding execution plan with the lowest (or close to lowest) estimated execution cost Source for graphics: SQL Server BOL

Can optimization go wrong? Optimizer ESTIMATES the cost for each potential plan That estimate can be wrong (lack of stats, outdated stats, query structure etc.) A WRONG (sub-optimal) plan can be selected (but it seems best) You can outsmart the optimizer with hints! (if you have to)

Query Optimization Query Optimization is good - improves query performance But: It is expensive (memory, CPU, time) It is throttled at the server level (see Optimization section in KB ) It can timeout We need plan caching...

Plan caching Attempt to save the optimization effort by caching and reusing execution plans Execution plan consists of: Query plan (one for many users, read-only) Execution context (per-user but cached and reused as well) Plan cache is also called „Procedure Cache”

Query Execution Phases Query Cache lookup Compile query Schema Statistics Check plan Plan valid? Execute query Found Not found Yes No

How cache lookup works Finding an existing plan is based on: ID of the object (for SPs, triggers, functions etc.) Hash of the query text (for ad-hoc queries) Also, the cache-key attributes must match: SET options DBID Language and date settings user_id for non-qualified object names And other...

Inside the Procedure Cache Stores compiled execution plans There is not one ProcCache but 4: CACHESTORE_OBJCP – object plans CACHESTORE_SQLCP – SQL plans CACHESTORE_PHDR – „bound trees” CACHESTORE_XPROC – XProcs ProcCache „steals” pages from the Buffer Pool Entries aged-out based on cost of compilation and usage frequency

Inside the Procedure Cache - DMVs sys.dm_exec_cached_plans sys.dm_exec_sql_text(plan_handle) sys.dm_exec_query_plan(plan_handle) sys.dm_exec_plan_attributes(plan_handle) sys.dm_exec_query_stats You can combine and aggregate those in a number of interesting ways...

Procedure Cache Size Procedure cache size limits SQL 2000: max. 4GB SQL 2005 RTM and SP1: 75% 0-8GB + 50% 8-64GB + 25% >64GB SQL 2005 SP2 and SQL 2008: 75% 0-4GB + 10% 4-64GB + 5% >64GB Example: with 16GB on RTM limit is 10GB, on SP2: 4.2 GB Or number of items: Up to per cache store (SQLCP, OBJCP) on 32-bit Up to per cache store on 64-bit No direct control over the size of cache Workload Governor helps, if you can use it (EE only)

How is Procedure Cache Cleared? Automatically: Entries aged-out due to memory pressure Due to certain database operations: Database restore, changing DB state, etc. See KB for SQL Server 2005 list (or Logs to ERRORLOG: "SQL Server has encountered %d occurrence(s) of cachestore flush for the '%s' cachestore (part of plan cache) due to some database maintenance or reconfigure operations". Fixed in SQL 2008 (in most cases clears plans for one DB only)

How is Procedure Cache Cleared? Manually: DBCC FREEPROCCACHE Clears everything – everything needs to be recompiled Usually an overkill – serious performance effects In SQL 2008 you can be more selective: DBCC FREEPROCCACHE ( { plan_handle | sql_handle | pool_name } ) plan_handle – single plan sql_handle – all plans for particular SQL text pool_name – Resorce Governor pool

How is Procedure Cache Cleared? Manually (cont’d): DBCC FLUSHPROCINDB(dbid) Clears all plans for a single database Better but still an overkill Undocumented and unsupported DBCC FREESYSTEMCACHE('SQL Plans') Clears a particular cache store In this case, the ad-hoc plans cache store Keeps other plans intact Undocumented and unsupported

Managing plan reuse Generally, plan reuse is a good thing Less plans in cache (less memory consumed) Less time spend optimizing and compiling plans Our goal is to improve plan reuse as much as possible (but beware of drawbacks!) Depending on application, can be very easy or very hard to achieve

Controlling plan caching Application-side parameterization Stored Procedures Forced parameterization Database-level option More aggressive in parameterizing ad-hoc SQL Optimize for ad hoc workloads New server option in SQL Server 2008 Only a stub is cached on first execution Full plan cached after second execution

When is plan reuse NOT a good thing? The main downside of plan caching: Cached plan is based on the parameters used during optimization Known as „parameter sniffing” Not always the best plan for subsequent executions with a different parameter values Sometimes difficult to spot – look for queries with greatly varying execution stats Estimated number of rows << Actual number of rows

Working around the „bad plan” issue Recompiling for given execution: OPTION (RECOMPILE) EXECUTE... WITH RECOMPILE Always recompiling (for SPs) Masking parameter values through local variables Specifying value for the optimizer: OPTIMIZE FOR = val) OPTIMIZE FOR UNKNOWN (new in 2008)

Summary Query optimization and execution Plan caching and reuse What is the Procedure Cache? Managing the Procedure Cache Controlling plan reuse Additional reading:

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