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Updating index statistics sql

Good stats can make the difference between a query that runs in less than a second or one that takes an hour to complete. When you create an index, a set of statistics for that index is also created. Stats created in this way are called, not surprisingly, ‘Index stats’ to differentiate them from ‘column stats’.Column stats can be created automatically or manually.” The obvious way to do that it to tell SQL Server to do more than just take a sample of the data: instead to do it with FULLSCAN.So they use a command like this: of extra IO to the work of updating statistics. It has only six statistics on it: I queried information about them with a query like this.

To gather information for them, SQL Server runs this query: SELECT Stat Man([SC0], [SC1], [SC2], [SC3]) FROM (SELECT TOP 100 PERCENT [State Code] AS [SC0], [Gender] AS [SC1], [First Name By Birth Date Id] AS [SC2], [Fake Birth Date Stamp] AS [SC3] FROM [dbo].[First Name By Birth Date_1966_2015] WITH (READUNCOMMITTED) ORDER BY [SC0], [SC1], [SC2], [SC3] ) AS _MS_UPDSTATS_TBL OPTION (MAXDOP 4) This gets a plan that scans the associated nonclustered index: Whew, I’m glad this is a narrow table. This might look like a duplicate stat to the index statistic — but note that the index statistic leads on State Code.This will allow the optimizer to (hopefully) come up with a reasonably good execution plan.These automatically created stats will have a name of this form: _WA_00003_xxxxx Not that it matters much but the components of the name are WA for the state of Washington where the original developers of SQL Server lived and worked, then a number that corresponds to the column id.With Auto Create Statistics enabled, when you query a column that is not a leading column in an index, SQL Server will create stats on that column.For example: Assuming that Middle Name is not the first column of any existing index, SQL Server will create stats for that column.I ran a trace when I updated statistics with FULLSCAN, and here’s what I saw, stat by stat… Those two columns match up with this query in the trace: SELECT Stat Man([SC0], [SC1]) FROM (SELECT TOP 100 PERCENT [First Name By Birth Date Id] AS [SC0], [Fake Birth Date Stamp] AS [SC1] FROM [dbo].[First Name By Birth Date_1966_2015] WITH (READUNCOMMITTED) ORDER BY [SC0], [SC1] ) AS _MS_UPDSTATS_TBL OPTION (MAXDOP 4) Here’s the plan I saw in the trace: There are only two columns output from the clustered index scan: First Name By Birth Date Id and Fake Birth Date Stamp, the two columns needed to update the stat.(Note: this plan does seem a bit odd to me, as it could have found these columns in a non-clustered index: these are the key columns in the clustered index, which will be present in each nonclustered index, whether we ask for it or not. “We just got that info when updating the stat for the clustered index! But we’re going to go scan something else, anyway, because that’s how we roll. We’ve scanned every row in the nonclustered index on First Name Id twice already. SELECT Stat Man([SC0], [SC1], [SC2]) FROM (SELECT TOP 100 PERCENT [First Name Id] AS [SC0], [First Name By Birth Date Id] AS [SC1], [Fake Birth Date Stamp] AS [SC2] FROM [dbo].[First Name By Birth Date_1966_2015] WITH (READUNCOMMITTED) ORDER BY [SC0], [SC1], [SC2] ) AS _MS_UPDSTATS_TBL OPTION (MAXDOP 4) And the plan was…Automatic creation depends on the database setting Auto Create Statistics being enabled.This is the default and in most cases it should remain enabled.The three statistics with the funny names beginning in _WA_Sys are column statistics that SQL Server automatically created when I ran queries with joins or where clauses using those columns.The other three statistics were automatically created when I created indexes.

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  1. Jan 24, 2008. See “Making the Most of Automatic Statistics Updating,” October 2007, InstantDoc ID 96767 for more information about automatic updating of index statistics. In addition to. providing statistics for indexes, SQL Server can create statistics on unindexed table columns—called column statistics. Along with.

  2. Jul 1, 2013. Greg Larsen shows you how to identify when your index statistics were updated last, what method was used to gather statistics, and how to update your statistics.

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