Can an INNER JOIN offer better performance than EXISTS

Question

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:

• Can an INNER JOIN offer better performance than EXISTS?
• What circumstances would this happen?
• How might I set up a test case as proof?
• Do you have any useful links to further documentation?

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.

Solution 1

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:

• IN vs. JOIN vs. EXISTS

Solution 2

Maybe, maybe not.

• The same plan will be generated most likely
• An INNER JOIN may require a DISTINCT to get the same output
• EXISTS deals with NULL

Solution 3

In sql server 2019 queries with IN, EXIST, JOIN statements have different plans (if correct indexes added). So performence also is different. It is shown in article https://www.mssqltips.com/sqlservertip/6659/sql-exists-vs-in-vs-join-performance-comparison/ that JOIN is some faster.

P.S. I understand that question was about sql server 2005 (in tags), but people mostly looks for answer by article title.

Comments

[EricI]

This is somewhat off topic, but I would suggest avoiding DISTINCT and use GROUP BY for overall better performance when returning distinct lists. DISTINCT doesn't perform as well as GROUP BY in general. It may help make up some of the difference between INNER JOIN and EXISTS as well.

[Quassnoi]

@EricI: could you please provide an example of a query which is less efficient with DISTINCT than GROUP BY, provided that the outputs are identical? Thanks!