Optimizing SQL, part 2
Regarding my previous question, I noticed a slight mistake on my behalf: In each case, only one record should be retrieved by the select statement to be deleted by the outer query. The statement should look like this:

delete from table_name
 where exists(select 1 from table a, table b
              where a.column_name = b.column_name and
			   a.column_name = bind_variable)

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Even with the addition of the bind variable my explanation of how the optimizer will run and cost the two queries is still the same. When using exists, it executes the interior select for every row in . The interior query really only returns true or false (true if 1 or more rows is returned and false if 0 rows are returned) BUT it still must be executed for every row in the table being deleted from. That's where your performance hit comes from... EXISTS vs IN. In this case, IN is the clear winner.

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This was first published in March 2003

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