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Commit52f6918

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Update IN/EXISTS item.
1 parent5aa7849 commit52f6918

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‎doc/FAQ‎

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated:Mon Sep 30 23:28:35 EDT 2002
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Last updated:Wed Oct 9 23:14:53 EDT 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@@ -998,18 +998,21 @@ 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 join subqueries to outer queries by sequentially
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scanning the result of the subquery for each row of the outer query. A
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workaround is to replace IN with EXISTS:
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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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SELECT *
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FROM tab
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WHEREcol1 IN (SELECTcol2 FROMTAB2)
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WHEREcol IN (SELECTsubcol FROMsubtab)
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to:
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SELECT *
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FROM tab
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WHERE EXISTS (SELECTcol2 FROMTAB2 WHEREcol1 =col2)
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WHERE EXISTS (SELECTsubcol FROMsubtab WHEREsubcol =col)
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We hope to fix this limitation in a future release.
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For this to be fast, subcol should be an indexed column. We hope to
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fix this limitation in a future release.
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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:Mon Sep 3023:28:35 EDT 2002</P>
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<P>Last updated:Wed Oct 923:14:53 EDT 2002</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>
@@ -1282,22 +1282,25 @@ <H4><A name="4.22">4.22</A>) Why are my subqueries using
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<P>Currently, we join subqueries 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. A workaround is to replace<CODE>IN</CODE> with
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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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speed up other queries, replace<CODE>IN</CODE> with
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<CODE>EXISTS</CODE>:</P>
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<PRE>
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<CODE>SELECT *
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FROM tab
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WHEREcol1 IN (SELECTcol2 FROMTAB2)
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WHEREcol IN (SELECTsubcol FROMsubtab)
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</CODE>
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</PRE>
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to:
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<PRE>
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<CODE>SELECT *
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FROM tab
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WHERE EXISTS (SELECTcol2 FROMTAB2 WHEREcol1 =col2)
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WHERE EXISTS (SELECTsubcol FROMsubtab WHEREsubcol =col)
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</CODE>
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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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We hope to fix this limitation in a future release.
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<H4><Aname="4.23">4.23</A>) How do I perform an outer join?</H4>

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