Why would a delete involving a subselect run so much longer than the
individual delete commands?
My situation: table A has 200,000 rows. I've made up a temporary table
which holds the single-valued primary key for 80,000 rows which I want
to delete.
DELETE FROM a WHERE id IN (select ID from tmp LIMIT 800);
runs for several minutes. But if I do
\o tmpfile
\t
SELECT 'DELETE FROM a WHERE id = ' || id || ';' from tmp limit 800;
\o
\i tmpfile
this completes in about 15 seconds, or 1/50 of the time for the
single-statement delete above.
In trying to optimize this process, I disabled all the relational
integrity triggers (foreign keys) involving the table, and then I
dropped all the indexes EXCEPT that of the primary key. All the
experiments were done within a single transaction using BEGIN. The
database version is 7.1.3, and the table was vacuumed very recently.
