Comprehensive Performance with Automated Execution Plan

Analysis (ExecStats) Joe Chang

jchang6 @ yahoo www.qdpma.com

About Joe

• SQL Server consultant since 1999• Query Optimizer execution plan cost formulas (2002)• True cost structure of SQL plan operations (2003?)• Database with distribution statistics only,

– no data 2004• Decoding statblob-stats_stream

– writing your own statistics• Disk IO cost structure• Tools for system monitoring, execution plan analysisSee ExecStats http://www.qdpma.com/ExecStats/SQLExecStats.htmlDownload: http://www.qdpma.com/ExecStatsZip.htmlBlog: http://sqlblog.com/blogs/joe_chang/default.aspx

Objectives

• Complexities & depth SQL performance– Cause and Effect

• Focus on the execution plan– Inefficient plans – missing indexes– very large estimate/actual row discrepancies

• Comprehensive Index Strategy– few good indexes, but no more than necessary

Non-Objective

• List of rules to be followed blindly• without consideration for the underlying reason

• and whether rule actually applies in the current circumstance

DBA skill: cause and effect analysis & assessment

Preliminary: Correct Results

• Normalization– Data stored once, avoid anomalies

• Unique Keys– Avoid duplicate rows

• Foreign Keys– Avoid orphaned rows

Incorrect architecture requires use of SELECT DISTINCT etc. to correct architecture deficienciesWhich may cause performance problems as wellCorrect action is to address the architecture mistakes before the performance issue.

Performance Factors

Natural keys with unique indexes, not SQL

The Execution Plan links all the elements of performanceIndex tuning alone has limited valueOver indexing can cause problems as well

Index and Statistics maintenance policy

1 Logic may need more than one execution plan?

Compile cost versus execution cost?

Tables and SQL combined implement business logic

Plan cache bloat?

SQL Tablesnatural keys

Indexes

Execution Plan

Statistics& Compile parameters

Compile Row estimate propagation

errors

Storage Engine

Hardware

DOPMemory

Parallel plans Recompiletemp table /

table variable

Query Optimizer

Performance Strategy

• Tables – support business logic– Normalization, uniqueness etc.

• SQL – clear SARG, Query optimizer interpretable– 1 Logic maps to X Execution plans

• Indexes – good cluster key choice– Good nonclustered indexes, no more than necessary

• Statistics – sample strategy & update frequency• Compile parameter strategy• Temp table / Table variable strategy: Recompile & Row est. prop. error• Parallel execution plans: DOP and CTOP strategy

Indexing Principles

• Good cluster key choice– Grouping + unique, not too wide

• Good nonclustered indexes– For key queries, not necessarily every query – Covered indexes were practical (update overhead)– Create and drop custom indexes for maintenance ops

• No more indexes than necessary– Update overhead– Compile overhead– May tolerate occasional scans to avoid update maintenance

Note emphasis on good, not perfect

Comprehensive Strategy

• Identify (weight) important SQL statements– stored procedure: parameter values & code path• Recompile impact for temp tables

• Execution plan cross references SQL & indexes– Actual plan is better than estimate plan– Compile parameters & skewed statistics• Temp tables - Recompile impact

Automate Execution Plan analysis to fully cross-reference SQL to index usage

Using DMVs – Execution Plan

dm_exec_query_stats

dm_exec_sql_text

dm_exec_query_plandm_exec_text_query_plan

dm_db_index_usage_statsdm_db_index_operational_statsdm_db_index_physical_stats

DBCC SHOW_STATISTICSSTATS_DATE(object_id, stats_id)dm_db_stats_properties

Execution PlanIndexes, joinsCompile parameters

System viewsIndexes, key columns,Include list, filter, XML,Columns store etc.

sys.dm_db_stats_properties, is available in SQL Server 2012 starting with Service Pack 1 and in SQL Server 2008 R2 starting with Service Pack 2.last_updated, rows, rows_sampled, steps, unfiltered_rows, modification_counter

dm_exec_query_profiles2014 Real time query progress?

Using DMVs – Execution Plan

dm_exec_query_stats dm_exec_sql_text

dm_exec_query_plandm_exec_text_query_plan

dm_db_index_usage_statsdm_db_index_operational_statsdm_db_index_physical_stats

DBCC SHOW_STATISTICSSTATS_DATE(object_id, stats_id)dm_db_stats_properties

Execution PlanIndexes, joinsCompile parametersSystem views

Indexes, key columns,Include list, filter, XML,Columns store etc.

sys.dm_db_stats_properties, is available in SQL Server 2012 starting with Service Pack 1 and in SQL Server 2008 R2 starting with Service Pack 2.last_updated, rows, rows_sampled, steps, unfiltered_rows, modification_counter

dm_exec_query_profiles2014 Real time query progress?

SQL & Execution Plan Sources

• Estimated Execution Plan– dm_exec_query_stats• Contents of plan cache + execution statistics

– List of stored procedures• SELECT name FROM sys.procedures

• Any SQL list– Plans not in cache, to be generated– Can also execute SQL for actual plans

sys.dm_exec_query_stats

• sql_handle – token for batch or stored procedure

• statement_start_offset – sql_handle + offset = SQL statement

• plan_handle – SQL (batch) can have multiple plans on recompile

• query_hash – identify queries with similar logic, – differing only by literal values

sys.procedures

• Get list of stored procedures in database– functions are called from procedure?

• Generate estimated execution plan for each– Default parameters

• Full map of index usage to stored procedure• No trigger details in estimated plan

SQL List

• Configuration file has SQL to retrieve SQL list– Can be • explicit SQL • or stored procedures with parameters

– Same procedure, multiple parameter set• To expose different code path (actual plan)• EXEC proc WITH @P1 RECOMPILE (estimated plan)

About ExecStats• General information

• Execution plan sources1. dm_exec_query_stats2. list of all stored procedures (estimated)3. List of SQL in table (estimated or actual plan)4. Trace file

• Correlates execution plans to index usage• Procedures, functions and triggers• Rollup file IO stats by DB, filegroup, disk/vol, data/log• Distribution Statistics• Output to Excel, sqlplan file, (sql in txt file)

ExecStats Output Files

• Txt – runtime info• Log – abbreviated SQL error logs• Excel –• Missing Indexes DMV • SQL plan directory

This can be sent to someone who can identify and fix your problem

Important Items

• Query cost – plan efficiency? Recompiles?– Compile parameters – skewed statistics

• CPU versus Duration (worker – elapsed time)– Disk IO, network transmission, parallel plan?

• Execution count – network roundtrip?• Plan cost – Parallelism– High volume of quick queries is bad, so is excessive DOP

• Index – current rows, rows at time stats generated, sample rows & date

Execution Plans

• Estimated• Actual: estimated cost, actual rows, DOP– Executed stored procedure once for each possible

code path – with appropriate parameters

Execution Plans

• Pay attention to:– Compile parameters– Large table scans: how many rows output?– Predicate • search condition without suitable index

– Rebinds and Rewinds – key lookup– Parallelism

Index Usage – missing IX, excess IX?

• Index usage – seek, scan, lookup & update– Unused indexes (infrequent code?) can be dropped

– Infrequent usage: check plan references– Similar indexes (leading keys)• Same keys, different order• Check plan reference – consolidate if possible

• Scans to large tables or even nonclustered IX– Is it real (SELECT TOP 1 may not be a real scan)

• Lookups – can these be reduced?

SQL Server Skills & Roles

DevelopersSQL code

ArchitectTable structure,

unique keys

Data Architect normalization

DBAIndex + Statistics

MaintenanceHardware & Storage

Performance

• .

SQL Server Performance History

• Before DMVs (SQL Server 2000)– Profiler/Trace to get top SQL– Execution plans – not really exportable– Which indexes are actually used?

• Today– Trace/Extended Events sometimes not necessary• If the dm_exec_query_stats content is good

– Execution plans are exportable– Index Usage Stats

How much can be automated?

• Data collection all, of course– Top resource consumers, etc.

• Assessment sometimes– Is there a problem– Can it be fixed or improved

• Fix/Change sometimes– Indexes– SQL – sometimes– Table structure, architecture no

If problems could be solved by pushing a button, what would be the skill requirements to be a DBA?

Great accomplishments – 99% perspiration 1% inspiration

Performance Approaches

• Check against list of “Best Practices”• Manual DMV scripts approach– Find Top 5 or 10 SQL– Fix it if/when there is a problem

• All Indexes and procedures/SQL– Examine the complete set of stored procedures– Or the full list of SQL statements– Good indexes for all SQL, no more indexes than

Why bother when there are no problems?

• No problems for over 1 year– Never bothered to collect performance baseline

• Problem Today – Find it with DMV, fix it – the problem was xxx– but why did it occur today & not before?

• Probably statistics or compile parameters, but prove it?

• Why ExecStats– SQL scripts? – too much manual work– Third party tools? – only find problem

Rigorous Optimization

• Table structure, SQL, Client-side• Cluster Key• Good (nonclustered) Indexes– All indexes are actually used

• No more indexes than necessary– Consolidate similar indexes

• same keys, same order, or reverse order?

– What SQL is impacted?• Statistics update• Index maintenance

Must consider the full set of SQL/procedures in removing indexes?

SQL versus programming languages

• SQL – great for data access– Not good for everything else– When SQL becomes horribly complicated– What would the code looks like in VB/Java/Cxx

Client-side program C#

Performance Information

• Server, Storage• OS & SQL Server Settings• SQL Server– SQL, query execution statistics, execution plan– Compile parameters– Indexes and index usage statistics– Statistics sampling – when? percentage? skew?