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How well that performs compared to a JOIN can vary massively depending on the data sizes of table_a & tale_b, how table_b is indexed, and what else is going on in the query.

If table_b has an index on id,column2,column3 (or on id INLUDEing column2,column3) I would expect the equivalent JOIN to usually be faster. If you have a clustered index on Id (which is the case more often than not in MS SQL Server and MySQL/InnoDB) then that would count for this unless the table is much wider than those three columns (so the index with its selective data would get many rows per page more than the base data).

Worst (and fairly common) case with sub-selects like that is the query planner deciding to run each subquery one per row from table_a. This is not an issue if you are only returning a few rows, or just one, from table_a, but in more complex examples (perhaps if this fragment is a CTE or view that is joined in a non-sargable manner so filtering predicates can't push down) you might find a lot more rows are processed this way even if few are eventually returned due to other filters.

There are times when the method is definitely faster but be very careful with it (test with realistic data sizes and patterns) because often when it isn't, it really isn't.




> perhaps if this fragment is a CTE or view

Yeah I guess I should have specified that this technique usually works best when done in the outer query, not buried deep inside.

It can be particularly effective if you fetch partial results, ie due to pagination or similar.

That said, these things aren't set in stone. I shared my experience, but my first tip goes first :)




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