From: Tom Lane <tgl@sss.pgh.pa.us>
To: ow <oneway_111@yahoo.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: 7.4 - FK constraint performance
Date: Thu, 12 Feb 2004 19:42:58 -0500
Message-ID: <25331.1076632978@sss.pgh.pa.us> (raw)
In-Reply-To: <20040213002548.19523.qmail@web60808.mail.yahoo.com>
References: <20040213002548.19523.qmail@web60808.mail.yahoo.com>
ow <oneway_111@yahoo.com> writes:
> schemaname tablename attname null_frac avg_width n_distinct most_common_vals
> most_common_freqs histogram_bounds correlation
> my large small_id 0 4 10 {7,3,5,1,4,2,8,10,6,9}
> {0.108667,0.105,0.104333,0.101333,0.100667,0.0983333,0.0983333,0.0983333,0.094,0.091}
> 0.0597573
According to this entry, your small_id column only contains the ten
values 1..10, roughly evenly distributed. So why are you probing for
239??
The planner is certainly going to estimate a probe for an unspecified
value as retrieving 10% of the table, and under that assumption it's
quite right to use a seqscan.
If this estimate is not right, perhaps you could give us a more accurate
view of the column statistics?
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, oneway_111@yahoo.com
Subject: Re: 7.4 - FK constraint performance
In-Reply-To: <25331.1076632978@sss.pgh.pa.us>
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