Cardinality How many rows? Distribution How many distinct values? density How many rows for each distinct value? Used by optimizer A histogram 200 steps max DBCC SHOWSTATISTICS

Index statistics User defined statistics Optimizer created statistics AKA column statistics Filtered FULLSCAN vs SAMPLE Default sampling rate

DBCC SHOW_STATISTICS sys.stats view DMF sys.dm_db_stats_properties (object_id, stats_id) modification_counter sys.sysrscols (DAC) sys.system_internals_partition_columns Which stats were actually used by optimizer?

UPDATE STATISTICS A single one or all stats on a table WITH FULLSCAN | SAMPLE | STATS_STREAM, ROWCOUNT, PAGECOUNT ALL | COLUMN | INDEX NORECOMPUTE Index rebuild (but not reorganize!) Doesn’t update column statistics FULLSCAN is what you get sp_updatestats Updates all stats in the database where rowmodctr > 0 SAMPLE by default

ALTER DATABASE SET AUTO_CREATE_STATISTICS ON SET AUTO_UPDATE_STATISTICS ON SET AUTO_UPDATE_STATISTICS_ASYNC ON Stale statistics during optimization

DBCC SHOW_STATISTICS WITH STATS_STREAM SSMS: Database -> Tasks -> Generate Scripts

Fullscan vs sample Maintenance plans Ascending keys Stale statistics How many row updates trigger update? Do we really need AUTO_UPDATE and AUTO_CREATE?