On Wed, 2008-02-06 at 11:00 -0500, Tom Lane wrote: > Theo Kramer <theo@xxxxxxxxxxx> writes: > > On Wed, 2008-02-06 at 11:53 +0000, Simon Riggs wrote: > >> Since the SQL is not your fault and difficult to control, it is an > >> argument in favour of an optional planner mode that would perform > >> additional checks for redundant clauses of various kinds. The default > >> for that would be "off" since most people don't suffer from this > >> problem. BO isn't the only SQL generating-client out there, so I think > >> this is a fairly wide problem. > We used to have code that removed duplicate WHERE clauses (check the > revision history for prepqual.c). It was taken out because it consumed > excessive amounts of planning time without accomplishing a darn thing > for most queries. There is no chance that it will be put back in as the > only behavior, or even the default behavior, but I can see the reasoning > for offering an option as Simon suggests. I was wondering if we might do that automatically? It seems easy to imagine a switch, but I wonder if we'd be able to set it correctly in enough situations to make it worthwhile. Say if cost of best plan >= N then recheck query for strangeness. If anything found, re-plan query. That way we only pay the cost of checking for longer queries and we only actually re-plan for queries that will benefit. -- Simon Riggs 2ndQuadrant http://www.2ndQuadrant.com ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
