Shaul Dar <shaul...@gmail.com> writes:
> I assume this will work but will take a long time:
> DELETE * FROM T1 where T1.PK NOT IN
> (SELECT T1.PK FROM T1, T2 where T1.PK = T2.FK)
Well, yeah, but it's unnecessarily inefficient --- why not just
DELETE FROM T1 where T1.PK NOT IN
(SELECT T2.FK FROM T2)
However, that still won't be tremendously fast unless the subselect fits
in work_mem. As of 8.4 this variant should be reasonable:
DELETE FROM T1 where NOT EXISTS
(SELECT 1 FROM T2 where T1.PK = T2.FK)
Pre-8.4 you should resort to the "left join where is null" trick,
but there's no need to be so obscure as of 8.4.
regards, tom lane
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