On Sunday 10 October 2010 05:02:03 ÐÑÑÐÐÑÐ ÐÐÐÑÐÑÐÐ wrote: > 2010/10/10 Neil Whelchel <neil.whelchel@xxxxxxxxx> > > > On Saturday 09 October 2010 18:47:34 Scott Marlowe wrote: > > > On Sat, Oct 9, 2010 at 5:26 PM, Neil Whelchel <neil.whelchel@xxxxxxxxx> > > > > wrote: > > > > I know that there haven been many discussions on the slowness of > > > > count(*) > > > > > > even when an index is involved because the visibility of the rows has > > > > to > > > > > > be checked. In the past I have seen many suggestions about using > > > > triggers and tables to keep track of counts and while this works fine > > > > in > > > > > > a situation where you know what the report is going to be ahead of > > > > time, > > > > > > this is simply not an option when an unknown WHERE clause is to be > > > > used (dynamically generated). I ran into a fine example of this when > > > > I was searching this mailing list, "Searching in 856,646 pages took > > > > 13.48202 seconds. Site search powered by PostgreSQL 8.3." Obviously > > > > at some > > > > point > > > > > > count(*) came into play here because the site made a list of pages (1 > > > > 2 3 4 5 6 > next). I very commonly make a list of pages from search > > > > results, and the biggest time killer here is the count(*) portion, > > > > even worse yet, I sometimes have to hit the database with two SELECT > > > > statements, one with OFFSET and LIMIT to get the page of results I > > > > need and another to get the amount of total rows so I can estimate > > > > how many pages of results are available. The point I am driving at > > > > here is that since building a list of pages of results is such a > > > > common thing to do, there need to be some specific high speed ways > > > > to do this in one query. Maybe an estimate(*) that works like count > > > > but gives an answer from the index without checking visibility? I am > > > > sure that this would be good enough to make a page list, it is > > > > really no big deal if it errors on > > > > the > > > > > > positive side, maybe the list of pages has an extra page off the end. > > > > I can live with that. What I can't live with is taking 13 seconds to > > > > get > > > > a > > > > > > page of results from 850,000 rows in a table. > > > > > > 99% of the time in the situations you don't need an exact measure, and > > > assuming analyze has run recently, select rel_tuples from pg_class for > > > a given table is more than close enough. I'm sure wrapping that in a > > > simple estimated_rows() function would be easy enough to do. > > > > This is a very good approach and it works very well when you are counting > > the > > entire table, but when you have no control over the WHERE clause, it > > doesn't > > help. IE: someone puts in a word to look for in a web form. > > > > From my perspective, this issue is the biggest problem there is when > > using Postgres to create web pages, and it is so commonly used, I think > > that there > > should be a specific way to deal with it so that you don't have to run > > the same WHERE clause twice. > > IE: SELECT count(*) FROM <table> WHERE <clause>; to get the total amount > > of items to make page navigation links, then: > > SELECT <columns> FROM table WHERE <clause> LIMIT <items_per_page> OFFSET > > <(page_no-1)*items_per_page>; to get the actual page contents. > > > > How about > > select * from (select *, count(*) over () as total_count from <table> where > <clause) a LIMIT <items_per_page> OFFSET > <(page_no-1)*items_per_page> > It will return you total_count column with equal value in each row. You may > have problems if no rows are returned (e.g. page num is too high). I have done this before, but the speedup from the two hits to the database that I mentioned above is tiny, just a few ms. It seems to end up doing about the same thing on the database end. The reason that I don't commonly do this is what you said about not getting a count result if you run off the end. -Neil- -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
