Обсуждение: How long will the query take
I ran a query out of pgAdmin, and (as I expected) it took a long time. In fact, I did not let it finish. I stopped it after a little over an hour. I'm using 8.4.2 on a Mac with a 2.4GHz processor and 2GB of RAM. My question is: is there a way to tell how close the query is to being finished. It would be a great pity if the query would have finished in the 10 seconds after I quit it, but I had no way of telling. As a postscript, I would add that the query was undoubtedly too ambitious. I have a reduced set version which I will run shortly. But I am still curious to know if there is a way to tell how much time is left. Thanks, John
In response to John Gage : > I ran a query out of pgAdmin, and (as I expected) it took a long > time. In fact, I did not let it finish. I stopped it after a little > over an hour. > > I'm using 8.4.2 on a Mac with a 2.4GHz processor and 2GB of RAM. > > My question is: is there a way to tell how close the query is to being > finished. It would be a great pity if the query would have finished > in the 10 seconds after I quit it, but I had no way of telling. > > As a postscript, I would add that the query was undoubtedly too > ambitious. I have a reduced set version which I will run shortly. > But I am still curious to know if there is a way to tell how much time > is left. 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. And yes, have you tuned your postgresql.conf? Regards, Andreas -- Andreas Kretschmer Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header) GnuPG: 0x31720C99, 1006 CCB4 A326 1D42 6431 2EB0 389D 1DC2 3172 0C99
In response to "A. Kretschmer" <andreas.kretschmer@schollglas.com>: > In response to John Gage : > > I ran a query out of pgAdmin, and (as I expected) it took a long > > time. In fact, I did not let it finish. I stopped it after a little > > over an hour. > > > > I'm using 8.4.2 on a Mac with a 2.4GHz processor and 2GB of RAM. > > > > My question is: is there a way to tell how close the query is to being > > finished. It would be a great pity if the query would have finished > > in the 10 seconds after I quit it, but I had no way of telling. > > > > As a postscript, I would add that the query was undoubtedly too > > ambitious. I have a reduced set version which I will run shortly. > > But I am still curious to know if there is a way to tell how much time > > is left. > > 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. -- Bill Moran http://www.potentialtech.com http://people.collaborativefusion.com/~wmoran/
Bill Moran <wmoran@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.
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)
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 can't say how long the query will runs, even an estimate, okay?
Andreas
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In response to Andreas Kretschmer <akretschmer@spamfence.net>: > Bill Moran <wmoran@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/
I will report back on this and attempt to give the particulars. It will take 24 hours due to other time commitments. Thank you very much for explaining :) this to me. When I used only the first 10,000 rows of the 100+ thousand rows in the original table (of two tables) I was working with, I got the result I wanted in 10 minutes, which was really probably 80% of what I wanted. Nevertheless, I do not want to fly blind in the future. John On Mar 29, 2010, at 7:10 PM, Bill Moran wrote: > In response to Andreas Kretschmer <akretschmer@spamfence.net>: > >> Bill Moran <wmoran@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