SQL Server Statistics DEMO SQL Server Statistics SREENI JULAKANTI,MCTS.MCITP SQL SERVER Database Administration

SQL Server Statistics What are SQL Server STATISTICS? What do they look like? When\How do they get created? When\How do they get updated? When SQL Server KICKs Off AUTO UPDATES? SQL Server Statistics Best Practices. SQL Server Statistics Maintenance. AGENDA

SQL Server Statistics New Terms Statistics Header Vector Density Histogram Cardinality

SQL Server Statistics Statistics contain a map of the data distribution for a column or an Index. The query optimizer uses these statistics to estimate the cardinality. cardinality :The total number of rows processed at each level of a query plan. What are SQL Server STATISTICS?

SQL Server Statistics These cardinality estimates enable the query optimizer to create a high-quality query plan. SQL Server estimates cardinalities come primarily from histograms A histogram with distribution of values in the first column. A histogram measures the frequency of occurrence for each distinct value in a data set What are SQL Server STATISTICS?

SQL Server Statistics Statistics Stats – Header Vector Density HISTOGRAM

SQL Server Statistics Histogram

SQL Server Statistics The range includes all possible column values between boundary values, excluding the boundary values themselves. The lowest of the sorted column values is the upper boundary value for the first histogram step

SQL Server Statistics STMT Syntax check (Parsing)Algebrizer (Binding) Optimizer Plan Execution Result Estimated Row Count Hardware Configuration Query Hints Indexes Partitioning Filegroups/Files

SQL Server Statistics The Different way stats retrieved DBCC SHOW_STATISTICS (tablename.statsname) Ex: DBCC SHOW_STATISTICS (STATSIMPACT,IxSTATSIMPACT_Colx ) SP_Autostats table name Ex: SP_Autostats [STATSIMPACT] Sys.indexes Sys.stats sys.dm_db_stats_properties (object_id, stateside) How do you see them?

SQL Server Statistics The Different way stats being created The query optimizer creates statistics for indexes on tables when the index is created. Index statistics are created on the key or Lead columns of the index. The query optimizer creates statistics for single columns in query predicates. Composite indexes creates Multi column statistics. sp_createstats stored procedure Create Statistics Statement Creating Statistics

SQL Server Statistics Detailed look at the utilization of statistics. How SQL Server Thinks about Stats? How SQL Server Comes to a conclusion? DEMO -1 How SQL Server STATISTICS Impacts?

SQL Server Statistics When STASTS Updated SQL Server update stats on default threshold (20%+500) A query compiles for the first time, and a statistic used in the plan is out of date A query has an existing query plan, but a statistic in the plan is out of date Auto_update_statistics Sp_Updatestats stored procedure Update statistics Command Updating Statistics

When Stats Not Updated Index reorg will not Update index linked stats Auto create will not work on Read-only DB’s Even if a statistic becomes outdated as the result of a modification, it will not automatically update after the modification completes. Note:The statistic will automatically update the next time a query plan uses it. SQL Server Statistics Updating Statistics

SQL Server Statistics Updating Statistics Index Rebuild SQL Server update stats on default threshold (20%+500) A query compiles for the first time, and a statistic used in the plan is out of date A query has an existing query plan, but a statistic in the plan is out of date

SQL Server Statistics When AUTO Update Kicks off Demo - 2

SQL Server Statistics How Granular are AUTO Update Statistics Or NOT Updated DEMO -3

SQL Server Statistics Even Distribution is easy ( it is fairly consistent ) Un-even distribution is HARDER This is all over the place and varies over time as well The Histogram does a much better job having steps and average distribution per step but what if there are well over 200 distinct values and millions of rows with heavy skew between steps Simply put the averages just aren’t going to cut it anymore Data Distribution Matters

SQL Server Statistics Impact of Skewed estimates 1.Spills to Disk(Under estimates) 2.IndexScan Vs. Index Seek Decisions 3.Inflated Memory Grants 4.Serial vs. Parallel Operations 5.Least Selective tables Joined first 6. outer/inner choice 7.Inappropriate Join algorithm selected Data Distribution Matters

SQL Server Statistics Skewed data Distribution

SQL Server Statistics Statistics Best Practices DEMO -4

SQL Server Statistics Optimize for (Unknown) Recompile Plan guides filtered statistics Declare local Variables. Skewed data Distribution

SQL Server Statistics Keep AUTO_CREATE_STATISTICS ON by default Keep AUTO_UPDATE_STATISTICS ON by default Try AUTO_UPDATE_STATISTICS_ASYNC for large OLTP tables if auto updating is an issue Eliminate any duplicate statistics If you notice estimate VS actual are off while troubleshooting, try updating the statistics Make Sure Your Data is not SKEWED (Important) Statistics Best Practices

SQL Server Statistics Schedule an Update statistics job with large sample or FULLSCAN if feasible Nightly for larger/OLTP tables Weekly for smaller, less updated tables Statistics Maintenance

So the Auto Update stats will fire for every % change in table rows. Of course, we have an improved algorithm in SQL 2012 which is SQRT(1000 * Table rows) which is much better. When it fires it will use the default sampling rate and here is the algorithm how it calculates the sampling rate. 1) If the table < 8MB then it updates the statistics with a full scan. 2) If the table > 8MB, it follows an algorithm. It reduces the sampling rate as the number of rows in the table are increased to make sure we are not scanning too much data. This is not a fixed value but is under the control of optimizer. It is not a linear algorithm either. Example: if we have 1,000,000 rows it would use a sampling rate of 30% but when the number of rows increase to 8,000,000 it would reduce the sampling rate to 10%. These sampling rates are not under the DBAs control but optimizer decides it. SQL Server Statistics SQL Server 2012 algorithm

SQL Server Statistics Q&A