On Mon, Apr 20, 2009 at 9:55 AM, Rafael Domiciano <rafael.domiciano@xxxxxxxxx> wrote: > Hello People, > > I have initiated a work to review the sqls of our internal software. > Lot of them he problem are about sql logic, or join with table unecessary, > and so on. > But software has lot of sql with date, doing thinks like: > [..] > date >= '2009-04-01' AND > date <= '2009-04-15' > [..] > > Redoing the SQL with fix date (date = '2009-04-01') o cost in explain always > still about 200 or less. But with a period the cost is high, about 6000 or > more. > > Select is using Index and the date is using index too. > > There is some way to use date period with less cost? If you have an actual performance problem (as opposed to a big number in EXPLAIN), then it's possible that the planner isn't estimating the number of rows that will be in that range very accurately. In that case, you might need to increase the statistics target for that column, or your default_statistics_target. In 8.3, the default default_statistics_target = 10. In 8.4, it will be 100, so you might try that for a starting point. But date columns can sometimes have highly skewed data, so you might find that you need an even higher value for that particular column. I wouldn't recommend raising the database-wide setting above 100 though (though I know some people have used 200 or 400 without too much pain, especially on Really Big Databases where longer planning time isn't a big deal because the execution times are measured in minutes - it doesn't sound like that's your situation though). The first thing, to do, is see how fast the query actually runs. Try setting \timing in psql and running the query to see how long it actually takes. If it's fast enough, you're done. If not, run EXPLAIN ANALYZE and compare the estimated row counts to t he actual row counts. If they're pretty close, you're out of luck - as others have already said, TANSTAAFL. If they're way off, the try the above. ...Robert -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
