On Tue, 2005-03-29 at 09:40 -0500, Tom Lane wrote: > Simon Riggs <[EMAIL PROTECTED]> writes: > > ...but, I see no way for OidFunctionCall8 to ever return an answer of > > "always just 1 row, no matter how big the relation"...so tuples_fetched > > is always proportional to the size of the relation. Are unique indexes > > treated just as very-low-selectivity indexes? > > Yeah. It is not the job of amcostestimate to estimate the number of > rows, only the index access cost. (IIRC there is someplace in the > planner that explicitly considers unique indexes as a part of developing > selectivity estimates ... but it's not that part.)
Well, I mention this because costsize.c:cost_index *does* calculate the number of rows returned. If unique indexes are handled elsewhere then this would not cause problems for them...but for LIMIT queries..? cost_index gets the selectivity then multiplies that by number of tuples in the relation to calc tuples_fetched, so it can use that in the Mackert & Lohman formula. There's no consideration of the query limits. That implies to me that LIMIT queries are not considered correctly in the M&L formula and thus we are more likely to calculate a too-high cost for using an index in those circumstances....and thus more likely to SeqScan for medium sized relations? Best Regards, Simon Riggs ---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend
