In response to Andreas Kretschmer <akretsch...@spamfence.net>: > Bill Moran <wmo...@potentialtech.com> wrote: > > > > No, not really. But you can (and should) run EXPLAIN <your query> to > > > obtain the execution plan for that query, und you can show us this plan > > > (and the table-definition for all included tables). Maybe someone is able > > > to tell you what you can do to speed up your query. > > > > To piggyback on this ... EXPLAIN _is_ the way to know how long your > > query will take, but keep in mind it's only an _estimate_. > > > > Given that, in my experience EXPLAIN is pretty accurate 90% of the > > time, as long as you analyze frequently enough. > > As far as i know, EXPLAIN _can't_ say how long a query will take, it > returns only a COST, not a TIME.
Correct. > Or can you tell me how long this query will be take? > > test=# explain select * from foo; > QUERY PLAN > ------------------------------------------------------- > Seq Scan on foo (cost=0.00..34.00 rows=2400 width=4) > (1 Zeile) EXPLAIN ANALYZE a few other queries of various complexity, and I'll be able to translate that estimate to a time. No, it's not 100% accurate, but (as I stated earlier) in my experience, it gives you a pretty good idea. > Okay, it's a really little table and a really simple plan ... but > imagine, i have a table with 100 millions rows and a) a slow disk and b) > a fast SSD. You're absolutely correct, and that's something that I should not have omitted from my previous response. Translating the cost into a time estimate is highly hardware-dependent, and not 100% accurate, so run some tests to get an idea of what your cost -> time ratio is, and take those cost estimates with a grain of salt. -- Bill Moran http://www.potentialtech.com http://people.collaborativefusion.com/~wmoran/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
