[Keystone Slip # 19] NOT IN clause performs badly

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Slip number -----: 19
Problem ---------: NOT IN clause performs badly
Opened by -------: Christian(dot)Rudow(at)thinx(dot)ch on 07/26/99 05:28
Assigned To -----: momjian
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Summary:
table a (id integer
any number/type of other attributes)
unique index on id.
holding 10'000 rows

table b (id integer)
holding a subset of 8'000 rows, that
exist in table a.

trying to select all id values from table a
that do nor exist in table b :

select id from a
where id not in (select id from b);

this takes hours to process, while a simple join

select id from a,b
where a.id = b.id

returns within seconds

explain says : sequential scan on a and b.
still 120 minutes for processing can't be.

system used :
PG 6.4 on SusE 6.0, PII/300mhz.

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