I've been investigating making performance improvements on a series of procedures, and recently a colleague mentioned that he had achieved significant performance improvements when utilising an INNER JOIN in place of EXISTS.
As part of the investigation as to why this might be I thought I would ask the question here.
So:
And really, any other experience people can bring to bear on this question.
I would appreciate if any answers could address this question specifically without any suggestion of other possible performance improvements. We've had quite a degree of success already, and I was just interested in this one item.
Any help would be much appreciated.
Generally speaking, INNER JOIN
and EXISTS
are different things.
The former returns duplicates and columns from both tables, the latter returns one record and, being a predicate, returns records from only one table.
If you do an inner join on a UNIQUE
column, they exhibit same performance.
If you do an inner join on a recordset with DISTINCT
applied (to get rid of the duplicates), EXISTS
is usually faster.
IN
and EXISTS
clauses (with an equijoin correlation) usually employ one of the several SEMI JOIN
algorithms which are usually more efficient than a DISTINCT
on one of the tables.
See this article in my blog: