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Asked: 2026-06-04T06:11:37+00:00

I have 4 full-outer joins in my query and its really slow, So does

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I have 4 full-outer joins in my query and its really slow, So does the order of FULL OUTER JOIN make a difference in performance / result ?

FULL OUTER JOIN = ⋈

Then,

I have a situation : A ⋈ B ⋈ C ⋈ D

All joins occur on a key common to all k contained in all A,B,C,D

Then:

  • Will changing the order of ⋈ joins make a difference to performance ?
  • Will changing the order of ⋈ change the result ?

I feel that it should not affect the result, but will it affect the performance or not I am not sure !

Update:

Will SQL Server automatically rearrange the joins for better performance assuming the result set will be independent of the order ?

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1 Answer

  1. Editorial Team

    No, rearranging the JOIN orders should not affect the performance. MSSQL (as with other DBMS) has a query optimizer whose job it is to find the most efficient query plan for any given query. Generally, these do a pretty good job – so you’re unlikely to beat the optimizer easily.

    That said, they do get it wrong occasionally. That’s where reading an execution plan comes into play. You can add JOIN hints to tell MSSQL how to join your tables (at which point, ordering does matter). You’d generally order from smallest to largest table (though, with a FULL JOIN, it’s not likely to matter very much) and follow the rules of thumb for join types.

    Since you’re doing FULL JOINS, you’re basically reading the entirety of 4 tables off disk. That’s likely to be very expensive. You may want to re-examine the problem, and see if it can be accomplished in a different way.

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