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Commita00d6b2

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Update for 7.4 --- prefer IN to EXISTS.
1 parenta35deb5 commita00d6b2

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‎doc/FAQ

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated:Fri Oct10 17:27:02 EDT 2003
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Last updated:Wed Oct29 15:19:43 EST 2003
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@@ -1031,11 +1031,11 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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4.22) Why are my subqueries using IN so slow?
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Currently, we joinsubqueries to outer queries by sequentially
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scanning the result of the subquery for each row of the outer query.
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If the subquery returns only a few rows and the outer query returns
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many rows, IN is fastest. To speed up other queries, replace IN with
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EXISTS:
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In versions prior to 7.4,subquerieswere joinedto outer queries by
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sequentiallyscanning the result of the subquery for each row of the
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outer query.If the subquery returns only a few rows and the outer
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query returnsmany rows, IN is fastest. To speed up other queries,
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replace IN withEXISTS:
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SELECT *
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FROM tab
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WHERE col IN (SELECT subcol FROM subtab);
@@ -1045,8 +1045,10 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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FROM tab
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WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
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For this to be fast, subcol should be an indexed column. This
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preformance problem will be fixed in 7.4.
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For this to be fast, subcol should be an indexed column.
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In version 7.4 and later, IN actually uses the same sophisticated join
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techniques as normal queries, and is prefered to using EXISTS.
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4.23) How do I perform an outer join?
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‎doc/src/FAQ/FAQ.html

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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated:Fri Oct10 17:27:02 EDT 2003</P>
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<P>Last updated:Wed Oct29 15:19:43 EST 2003</P>
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<P>Current maintainer: Bruce Momjian (<Ahref=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -1303,10 +1303,10 @@ <H4><A name="4.21">4.21</A>) How do I create a column that will
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<H4><Aname="4.22">4.22</A>) Why are my subqueries using
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<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
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<P>Currently, we joinsubqueries to outer queries by sequentially
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scanning the result of the subquery for each row of the outer
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query. If the subquery returns only a few rows and the outer query
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returns many rows,<CODE><SMALL>IN</SMALL></CODE> is fastest. To
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<P>In versions prior to 7.4,subquerieswere joinedto outer queries
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by sequentiallyscanning the result of the subquery for each row of
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the outerquery. If the subquery returns only a few rows and the outer
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queryreturns many rows,<CODE><SMALL>IN</SMALL></CODE> is fastest. To
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speed up other queries, replace<CODE>IN</CODE> with
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<CODE>EXISTS</CODE>:</P>
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<PRE> SELECT *
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</PRE>
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For this to be fast,<CODE>subcol</CODE> should be an indexed column.
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This preformance problem will be fixed in 7.4.
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<P>In version 7.4 and later,<CODE>IN</CODE> actually uses the same
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sophisticated join techniques as normal queries, and is prefered
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to using<CODE>EXISTS</CODE>.
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<H4><Aname="4.23">4.23</A>) How do I perform an outer join?</H4>
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