> Tom Lane wrote: > > The only really effective way the planner knows to optimize an > "IN (sub-SELECT)" is to turn it into a semi-join, which is not possible > here because of the unrelated OR clause. You might consider replacing > this with a UNION of two scans of "contexts". (And yes, I know it'd be > nicer if the planner did that for you.) In moving our application from Oracle to Postgres, we've discovered that a large number of our reports fall into this category. If we rewrite them as a UNION of two scans, it would be quite a big undertaking. Is there a way to tell the planner explicitly to use a semi-join (I may not grasp the concepts here)? If not, would your advice be to hunker down and rewrite the queries? -- View this message in context: http://postgresql.1045698.n5.nabble.com/hashed-subplan-5000x-slower-than-two-sequential-operations-tp3297790p3346652.html Sent from the PostgreSQL - performance mailing list archive at Nabble.com. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
