From: Tom Lane <tgl@sss.pgh.pa.us>
To: Edin Kadribasic <edink@emini.dk>
Cc: pgsql-performance@postgresql.org
Subject: Re: Optimizer wrongly picks Nested Loop Left Join
Date: Wed, 11 May 2005 10:18:34 -0400
Message-ID: <3742.1115821114@sss.pgh.pa.us> (raw)
In-Reply-To: <001f01c55604$494d2e60$1200000a@intra.emini.dk>
References: <001f01c55604$494d2e60$1200000a@intra.emini.dk>
"Edin Kadribasic" <edink@emini.dk> writes:
> I have a query that is giving the optimizer (and me) great headache.
The main problem seems to be that the rowcount estimates for
axp_temp_order_match and axp_dayschedule are way off:
> -> Index Scan using axp_temp_order_match_idx1 on
> axp_temp_order_match a (cost=0.00..209.65 rows=426 width=4) (actual
> time=0.277..0.512 rows=6 loops=1)
> Index Cond: (sid = 16072)
> -> Index Scan using axp_dayschedule_day_idx on
> axp_dayschedule ds (cost=0.00..3.02 rows=1 width=8) (actual
> time=0.036..3.973 rows=610 loops=1)
> Index Cond: (("day" >= '2005-05-12'::date)
> AND ("day" <= '2005-05-12'::date))
> Filter: (used = B'1'::"bit")
> -> Index Scan using axp_temp_order_match_idx1 on
> axp_temp_order_match a (cost=0.00..2.45 rows=1 width=4) (actual
> time=0.027..2.980 rows=471 loops=1)
> Index Cond: (sid = 16092)
> -> Index Scan using axp_dayschedule_day_idx on
> axp_dayschedule ds (cost=0.00..3.02 rows=1 width=8) (actual
> time=0.015..3.557 rows=606 loops=471)
> Index Cond: (("day" >= '2005-05-13'::date) AND
> ("day" <= '2005-05-13'::date))
> Filter: (used = B'1'::"bit")
Do you ANALYZE these tables on a regular basis? If so, it may be
necessary to increase the statistics target to the point where you
get better estimates.
> Please note that sometimes when I get "bad plan" in the logfile, I just
> re-run the query and the optimizer chooses the more efficient one.
That's fairly hard to believe, unless you've got autovacuum running
in the background.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, edink@emini.dk
Subject: Re: Optimizer wrongly picks Nested Loop Left Join
In-Reply-To: <3742.1115821114@sss.pgh.pa.us>
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