On Tue, Jun 20, 2006 at 02:06:19AM -0700, [EMAIL PROTECTED] wrote: > Such construction is very slow but when I modify SQL to: > OPEN cursor1 FOR SELECT * FROM alias WHERE mask>=alias_out > ORDER BY mask LIMIT 100; > > it works very fast. It is strange for me becuase I've understood so far > that when cursor is open select is executed but Postgres does not > select all rows - only cursor is positioned on first row, when you > execute fetch next row is read. But this example shows something > different.
PostgreSQL tries to optimise for overall query time. Without the limit it tries to find a plan that will return the whole set as quick as possible. With the LIMIT it might take a different approach, which might be worse if you read the whole lot, but better for a limited set. A fast-start plan so to speak. To see detail I'd suggest doing an EXPLAIN ANALYZE over the query with and with limit to see the changes. Have a nice day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate.
signature.asc
Description: Digital signature
