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Update for 7.4 --- prefer IN to EXISTS.

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Bruce Momjian
2003-10-29 20:20:12 +00:00
parent a35deb5400
commit a00d6b23eb
2 changed files with 18 additions and 14 deletions
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doc/FAQ
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@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Fri Oct 10 17:27:02 EDT 2003
Last updated: Wed Oct 29 15:19:43 EST 2003
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -1031,11 +1031,11 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
4.22) Why are my subqueries using IN so slow?
Currently, we join subqueries to outer queries by sequentially
scanning the result of the subquery for each row of the outer query.
If the subquery returns only a few rows and the outer query returns
many rows, IN is fastest. To speed up other queries, replace IN with
EXISTS:
In versions prior to 7.4, subqueries were joined to outer queries by
sequentially scanning the result of the subquery for each row of the
outer query. If the subquery returns only a few rows and the outer
query returns many rows, IN is fastest. To speed up other queries,
replace IN with EXISTS:
SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
@ -1045,8 +1045,10 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
For this to be fast, subcol should be an indexed column. This
preformance problem will be fixed in 7.4.
For this to be fast, subcol should be an indexed column.
In version 7.4 and later, IN actually uses the same sophisticated join
techniques as normal queries, and is prefered to using EXISTS.
4.23) How do I perform an outer join?