Posts by Brent Ozar

In this week’s Query Exercise challenge, I explained SQL Server’s 201 buckets problem. SQL Server’s statistics only handle up to ~201 outliers, which means that outliers ~202-300 get wildly inaccurate estimates. In our example, I had an index on Location and perfectly accurate statistics, but even still, this query gets bad estimates because Lithuania is…

When you run a query, SQL Server needs to estimate the number of matching rows it’ll find – so that it can decide which indexes to use, whether to go parallel, how much memory to grant, and more. For example, take any Stack Overflow database, and let’s say I have an index on Location, and…

These two queries both get the same answer from the Stack Overflow database: Transact-SQL SELECT TOP 1 LastAccessDate FROM dbo.Users ORDER BY LastAccessDate DESC; SELECT MAX(LastAccessDate) FROM dbo.Users; 123456 SELECT TOP 1 LastAccessDate    FROM dbo.Users    ORDER BY LastAccessDate DESC; SELECT MAX(LastAccessDate)    FROM dbo.Users; But do they go about their work the same way? As it turns out, no –…