Full Stack Development - by Ryan Kirkman

SQL Server CE - Changing INNER JOIN to LEFT JOIN is a 5x speedup

I was recently working on a fairly complex query in SQL CE 3.5. It was around 8 seconds to complete on a Windows CE device. I had a very similar query in terms of complexity, joins and result set that would complete in around 1 second on the same device.

This led me to compare the queries via their execution plans. There are about 10 joins involved in each query.

The start of the first (slow) query results in an estimated query cost of 0.307772 and looks like:

SELECT col1, col2, ...
FROM [Table]
... join other tables ...

The start of the second (fast) query results in an estimated query cost of 0.061847 and looks like:

SELECT col1, col2, ...
FROM [Table]
... join other tables ...

Notice the difference? The second query uses a LEFT JOIN instead of an INNER JOIN. The semantics of the query dictate an INNER JOIN, but it results in a significantly slower query plan. The only other source I've found mentioning such a problem says that it could be a bug in the query optimizer:

Other behavior that I noticed was that if I changed the inner join to a left join, the optimizer came up with a different much more efficient plan. This appears to be a flaw in the optimizer but I would like to speak to someone at Microsoft before making that claim.

Comparison of the impact of INNER JOIN vs LEFT JOIN for my query:

Query Completion Time ~ 8 seconds ~ 1 - 2 seconds
Estimated Query Cost 0.308658 0.0652409
Relative Cost 83% 17%

Interestingly, this drastic difference only appears to happen in SQLCE. In full SQL Server 2008 R2 with the same database structure, foreign keys and indexes, the resulting query plans and estimated costs are identical.

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