In response to Shaul Dar : > Hi, > > I have two large tables T1 and T2, such that T2 has a FK to T1 (i.e. T2.FK --> > T1.PK, possibly multiple T2 rows may reference the same T1 row). I have deleted > about 2/3 of table T2. I now want to delete all rows in T1 that are not > referenced by T2, i.e. all rows in T1 that cannot join with (any row in) T2 on > the condition T2.FK = T1.PK (the opposite of a join...) > > 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) > > What is an efficient way to do this? > Thanks, Maybe this one: (my id is your pk): delete from t1 where t1.id in (select t1.id from t1 left join t2 using (id) where t2.id is null); Try it, and/or use explain for both versions and see which which is faster. Andreas -- Andreas Kretschmer Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header) -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
