Charlie Savage <cfis@xxxxxxxxxxxxx> writes: > 1. Postgresql estimates the index scan will be 50 times more costly > than the seq scan (112870376 vs 2229858) yet in fact it only takes 3 > times longer to execute (2312426 s vs. 768403 s). My understanding is > that postgresql assumes, via the random_page_cost parameter, that an > index scan will take 4 times longer than a sequential scan. So why is > the analyzer estimating it is 50 times slower? The other factors that are likely to affect this are index correlation and effective cache size. It's fairly remarkable that a full-table index scan only takes 3 times longer than a seqscan; you must have both a high correlation and a reasonably large cache. You showed us your effective_cache_size setting, but what's the pg_stats entry for completechain.tlid contain? Can you quantify what the physical ordering of tlid values is likely to be? regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
