Re: DELETE FROM t WHERE EXISTS

--- Dan Langille <dan(at)langille(dot)org> wrote:
> Hi folks,
>
> I wanted to delete "old" rows from a table. These
> are the rows I
> want to keep:
>
> SELECT *
> FROM clp
> ORDER BY commit_date
> LIMIT 100
>
> So I tried this:
>
> DELETE FROM clp
> WHERE NOT EXISTS (
> SELECT *
> FROM clp
> ORDER BY commit_date
> LIMIT 100);
>
> Uhh uhh, nothing deleted. I don't understand why.
>

Your WHERE clause will never evaluate to true in this
case, because something will always be returned by the
subselect.

> OK, I can do this instead:
>
> DELETE from clp
> where commit_log_id NOT in (
> SELECT commit_log_id
> FROM clp
> ORDER BY commit_date
> LIMIT 100);
>
> Can you think of a better way?
> --

AFAIK joins cannot be used with DELETEs (but see
below), so you are stuck with a subselect. If you
don't like the performance of the IN, you could do an
EXISTS using a correlated subselect from a subselect,
but that is ugly. Or you could do it in two steps:

SELECT INTO TEMP sometable *
FROM clp
ORDER BY commit_date
LIMIT 100;

DELETE FROM clp
WHERE commit_date = sometable.commit_date;

That last must be converted into a join clause
somehow, but right now I am too lazy to turn on
logging to find out what :-)

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