Thread: Understanding explains
Is there a tutorial or reference to the different terms that appear on the explain output? Items such as "Nested Loop", "Hash".. Also is there a way to easily tell which of two explains is "worse". Example I am running a query with "set enable_seqscan to off;" and i see the explain now shows index scans, but not sure if is any faster now. I tried "explain analyze" and the "total runtime" for the one with seq_scan off was faster, but after repeathing them they both dropped in time, likely due to data getting cached. Even after the time drops for both the one with seqscan off was always faster. Is there any disadvantage of having the enable_seqscan off?
while you weren't looking, Francisco Reyes wrote: > Is there any disadvantage of having the enable_seqscan off? Plenty. The planner will choose whichever plan looks "cheapest", based on the information it has available (table size, statistics, &c). If a sequential scan looks cheaper, and in your case above it clearly is, the planner will choose that query plan. Setting enable_seqscan = false doesn't actually disable sequential scans; it merely makes them seem radically more expensive to the planner, in hopes of biasing its choice towards another query plan. In your case, that margin made an index scan look less expensive than sequential scan, but your query runtimes clearly suggest otherwise. In general, it's best to let the planner make the appropriate choice without any artificial constraints. I've seen pathalogical cases where the planner makes the wrong choice(s), but upon analysis, they're almost always attributable to poor statistics, long un-vacuumed tables, &c. /rls -- :wq
On Mon, 11 Oct 2004, Rosser Schwarz wrote: > In general, it's best to let the planner make the appropriate choice > without any artificial constraints. As someone suggested ran with Explain analyze. With seqscan_off was better. Ran a vacuum analyze this afternoon so the stats were up to date. Although I will leave the setting as it's default for most of everything I do, it seems that for some reason in this case it mases sense to turn it off.