From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | ow <oneway_111(at)yahoo(dot)com> |
Cc: | pgsql-sql(at)postgresql(dot)org |
Subject: | Re: 7.4 - FK constraint performance |
Date: | 2004-02-13 00:42:58 |
Message-ID: | 25331.1076632978@sss.pgh.pa.us |
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Lists: | pgsql-hackers pgsql-sql |
ow <oneway_111(at)yahoo(dot)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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