As part of a performance investigation for a customer I've noticed an O(N^2) performance issue on COMMITs of transactions that contain many SAVEPOINTs. I've consistently measured COMMIT times of around 9 seconds, with 49% CPU, mostly in LockReassignCurrentOwner().
BEGIN; INSERT... SAVEPOINT ... INSERT... SAVEPOINT ... ... (repeat 10,000 times) COMMIT; The way SAVEPOINTs work is that each is nested within the previous one, so that at COMMIT time we must recursively commit all the subtransactions before we issue final commit. That's a shame because ResourceOwnerReleaseInternal() contains an optimisation to speed up final commit, by calling ProcReleaseLocks(). What we actually do is recursively call LockReassignCurrentOwner() which sequentially scans LockMethodLocalHash at each level of transaction. The comments refer to this as "retail" rather than the wholesale method, which never gets to execute anything worthwhile in this case. This issue does NOT occur in PLpgSQL functions that contain many EXCEPTION clauses in a loop, since in that case the subtransactions are started and committed from the top level so that the subxact nesting never goes too deep. Fix looks like we need special handling for the depth-first case, rather than just a recursion loop in CommitTransactionCommand(). Issues looks like it goes all the way back, no fix for 9.0. I notice also that the nesting model of SAVEPOINTs also means that read-only subtransactions will still generate an xid when followed by a DML statement. That's unnecessary, but required given current design. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Training and Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
