"Guillaume Smet" <guillaume.smet@xxxxxxxxx> writes: > Isn't there any way to make PostgreSQL have a better estimation here: > -> Index Scan using models_brands_brand on models_brands > (cost=0.00..216410.97 rows=92372 width=0) (actual time=0.008..0.008 > rows=0 loops=303) > Index Cond: (brand = $0) Note that the above plan extract is pretty misleading, because it doesn't account for the implicit "LIMIT 1" of an EXISTS() clause. What the planner is *actually* imputing to this plan is 216410.97/92372 cost units, or about 2.34. However that applies to the seqscan variant as well. I think the real issue with Kevin's example is that when doing an EXISTS() on a brand_id that doesn't actually exist in the table, the seqscan plan has worst-case behavior (ie, scan the whole table) while the indexscan plan still manages to be cheap. Because his brands table has so many brand_ids that aren't in the table, that case dominates the results. Not sure how we could factor that risk into the cost estimates. The EXISTS code could probably special-case it reasonably well for the simplest seqscan and indexscan subplans, but I don't see what to do with more general subqueries (like joins). regards, tom lane
