This article is for SQL Server developers who have experienced the special kind of frustration that only comes from spending hours trying to convince the query optimizer to generate a parallel execution plan.
This situation often occurs when making an apparently innocuous change to the text of a moderately complex query — a change which somehow manages to turn a parallel plan that executes in ten seconds, into a five-minute serially-executing monster.
I came across a SQL Server optimizer bug recently that made me wonder how on earth I never noticed it before.
As the title of this post suggests, the bug occurs in common JOIN and GROUP BY queries. While it does not cause incorrect results to be returned, it will often cause a poor query plan to be selected by the optimizer.
If you are just interested in the bug itself, you will find a description in the section headed “the bug revealed”. It relates to cardinality estimation for serial partial aggregates.
As the regular reader will be expecting though, I am going to work up to it with a bit of background. The lasting value of this post (once the bug is fixed) is in the background details anyway.
Back in 2008, Marc Friedman of the SQL Server Query Processor Team wrote a blog entry entitled “Distinct Aggregation Considered Harmful”.
Marc shows a way to work around the poor performance that often results simply from adding the keyword DISTINCT to an otherwise perfectly reasonable aggregate function in a query.
This post is an update to that work, presenting a query optimizer enhancement in SQL Server 2012 that reduces the need to perform the suggested rewrite manually.