Comprehensive Indexing via Automated Execution Plan
Analysis (ExecStats) Joe Chang
jchang6 @ yahoo www.qdpma.com
Slide deck here
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
What is not in this session
• 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- Not enthusiastic, prolific, indiscriminate collector of rules
Why this topic on Indexing?
• Ideal – Good indexes, but no more than necessary
• Reality– Too many nonclustered indexes
• How did this happen?– Poor choice of cluster key (my opinion)– Indexes added one at a time for specific query
• Instead of modifying similar existing index
– DBA not inclined to remove indexes • Afraid to touch the cluster key
Indexes can be created or dropped for specific operations
Going from Current to Good Indexes
• Determine the better choice of cluster key– Can eliminate many nonclustered index by itself• Not hard to determine, but down time required
• Drop indexes not used (over long period)• Consolidate indexes – with leading keys in the same order– Determine if indexes with same keys in different
order can be consolidated• Need to find SQL that uses each index?
Simple
Simple
Does this involve work/risk?I don’t like work/risk!Execution plan links SQL to index usage
Notes
• 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
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 Big Picture
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 Tableskeys & const
Indexes
Execution Plan
StatisticsSampling & Re-compute
Compile parameters &
variables
Storage Engine
Hardware
DOP MemoryParallel plans Recompile
temp table / table variable
Query Optimizer
Index & Stats Maintenance
API Server Cursors: open, prepare, execute, close?
SET NO COUNT Information
messages
Row estimate propagation errors
Indexing Principles
• Good cluster key choice– Grouping + unique, not too wide
• Good nonclustered indexes– For key queries, not necessarily every query – Covered indexes where practical– Create and drop custom indexes for maintenance ops/special circum.
• No more indexes than necessary– Update overhead– Compile overhead– May tolerate occasional scans to avoid update maintenance
Note emphasis on good, not perfect
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 SP2. last_updated, rows, rows_sampled, steps, unfiltered_rows, modification_counter
dm_exec_query_profiles2014 Real time query progress?
Execution Plan maps SQL to Indexes
SQL IndexesExecution Plan
dm_db_index_usage_stats
dm_exec_query_statssql_handle & plan_handle
dm_exec_sql_text dm_exec_query_plandm_exec_text_query_plan
Performance Oriented Approach
• Getting Top SQL from dm_exec_query_stats– Manually examining top execution plans
• Index Reduction – dm_db_index_usage_stats– Drop unused indexes (based on long period)– Consolidating indexes with similar keys
– Infrequently used indexes?• Must hunt down SQL, possibly low item in query stats• Can it use another index?
Systematic Approach
• Get full list of:– stored procedures : schema + name– Scalar Functions (FN) schema + name– Inline & Table Valued Functions (IF,TF)• Need parameter list
– Triggers (should be obsolete, my opinion)• Generate execution plan – match to indexes– Alternative, maintain a list of SQL
Real World Example
July 20143.6B rows, 3.4TB, 1.5TB data 1.8TB indexesKey tables have 21, 8, 14, 4 and 14 nonclustered indexes
Index Reduction
Key tables have 6, 4, 3, 2 and 4 nonclustered indexes(some nonclustered indexes compressed)
Dec 2014 4.6B rows (30%), 2.3TB, 1.7TB data 0.5TB indexes
Compression – base tables
May 2015 5.7B rows, 0.99TB, 0.62TB data 0.36TB indexes
Dec 2014 4.6B rows, 2.3TB, 1.7TB data 0.5TB indexes
Table – 21 nonclustered Indexes
Note: All indexes are full rows, not filtered
Table with 21 NC IndexesJul 2014 5 frequently used NC indexes of 21
Dec – 6 Nonclustered Indexes
Infrequently used indexes could probably be removed by re-working the query
Jul 2014 5 frequently used NC indexes of 21
Dec 2014 4 frequently used, 2 Filtered IX, note lead column
May 2015 – 2 NC used
May 2015 3 frequently used NC, 1 disabled pending removal
Jul 2014 5 frequently used NC indexes of 21
Dec 2014 4 frequently used, 2 Filtered IX, note lead column
Note on Filtered Index Strategy
Query is SELECT xx FROM NARSplit WHERE IsActive = 1 AND CessionId IN (list)
Then the index strategy used is CREATE INDEX IX ON table (CessionId, xx, IsActive) WHERE IsActive = 1
Balancing Select vs Update
In the above table, the nonclustered index with several included columns nearly eliminates key lookups
One column, IsActive, in the include list was frequently updatedRemoving that column reduces need for update maintenance
Since a key lookup is needed anyways, may as well remove all include columns
Compression Notes
• Very high compression was achieved– Because all keys were 16-byte GUID– Even on dimensions, when natural key would have
been 1, 2 or 4 bytes!• Core data + indexes – 648GB data 230GB indexes w/o compression– 174GB data 185GB indexes w/compression
• Reduction in I/O even with SSD storage far outweigh compression overhead!
System memory: 256GB (220) Storage: Violin (NAND Flash)
Violin
Compression& statistics
Indexreduction
HDD StorageGDC
Index Theory - Locality
• Example Database– 10 B rows, 80 bytes per row, 800GB, 100M pages• (100M x 8KB/page = 800GB, 100 rows per page)
• Suppose 1% of rows are active, i.e. 100M rows– There could be 1 active row in each page (100M)• Possible if each table were clustered on a row guid
– Possible for active rows to be in only 1M pages• All rows in each of these page happen to be active
• Build the cluster key to tend toward 2nd option
Index Key SARG + Group/Order
SELECT TransactionType, SUM(Amount)FROM Table WHERE ReportDate = ‘value’AND other SARGSGROUP BY TransactionType
Index should lead with the key SARG, then Group or Order, less selective SARGs can be in Include List
Selective SARG
Grouping
Index Key SARG + Order By
SELECT TransactionType, SUM(Amount)FROM Table WHERE ReportDate = ‘value’AND other SARGSGROUP BY TransactionType
Index should lead with the key SARG, then Group or Order, less selective SARGs can be in Include List
Selective SARG
Grouping
Index Example w & w/o PartitioningWithout partitioningRequired indexes leading with:1) ReportDate - for grouping2) RowId - for single row access
With partitioningPartition asCREATE UNIQUE CLUSTERED INDEX UCX ON Table (RowId, ReportDate) ON psdate (ReportDate)
Partition key is not lead key of cluster indexSearch on RowId must check each partition
Oracle Skip-Scan would be nice
ExecStats
Database view
File IO view
Table view columns
Indexes - continued
Number of execution plans that reference the index in Seeks, Scans, Lookups, Insert/Updates and Deletes
Literal identifying the execution plans that reference the index in Seeks, Scans, Lookups, Insert/Upd & Deletes
Query Execution Stats - 1
Query Execution Stats - more
Dataspace – Partition Scheme view
Partition View
Procedure and Functions
• Columns• Dbid, schema, object, object_id, type,• Create date, modify date, • Number of references (NumRef) • (literal) plan reference (from QExec Stats)• Caller reference (Functions only)
Volumes
Slides not used
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
Identity key / alternative: large & small customers
• 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
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 estimate - actual
• Actual: estimated cost, actual rows, DOP– Compile parameters– Actual rows/executions versus estimated
Execute stored procedure once for each possible code path – with appropriate parameters
Execution Plans Analysis
• Predicate– index key columns does not matching full SARG– SQL has function on SARG, data type mismatch– Compile parameters & statistics– Actual and Estimated rows/execution mismatch– Large table scans: how many rows output?– Rebinds and Rewinds – key lookup– Parallelism
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?