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Update IN/EXISTS item.

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Bruce Momjian
2002-10-10 03:15:19 +00:00
parent 5aa7849e1b
commit 52f6918c28
2 changed files with 16 additions and 10 deletions
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doc/FAQ
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@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Sep 30 23:28:35 EDT 2002 Last updated: Wed Oct 9 23:14:53 EDT 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -998,18 +998,21 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
4.22) Why are my subqueries using IN so slow? 4.22) Why are my subqueries using IN so slow?
Currently, we join subqueries to outer queries by sequentially Currently, we join subqueries to outer queries by sequentially
scanning the result of the subquery for each row of the outer query. A scanning the result of the subquery for each row of the outer query.
workaround is to replace IN with EXISTS: 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 * SELECT *
FROM tab FROM tab
WHERE col1 IN (SELECT col2 FROM TAB2) WHERE col IN (SELECT subcol FROM subtab)
to: to:
SELECT * SELECT *
FROM tab FROM tab
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2) WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
We hope to fix this limitation in a future release. For this to be fast, subcol should be an indexed column. We hope to
fix this limitation in a future release.
4.23) How do I perform an outer join? 4.23) How do I perform an outer join?

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doc/src/FAQ/FAQ.html
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alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Mon Sep 30 23:28:35 EDT 2002</P> <P>Last updated: Wed Oct 9 23:14:53 EDT 2002</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@ -1282,22 +1282,25 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>Currently, we join subqueries to outer queries by sequentially <P>Currently, we join subqueries to outer queries by sequentially
scanning the result of the subquery for each row of the outer scanning the result of the subquery for each row of the outer
query. A workaround is to replace <CODE>IN</CODE> with query. If the subquery returns only a few rows and the outer query
returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
speed up other queries, replace <CODE>IN</CODE> with
<CODE>EXISTS</CODE>:</P> <CODE>EXISTS</CODE>:</P>
<PRE> <PRE>
<CODE>SELECT * <CODE>SELECT *
FROM tab FROM tab
WHERE col1 IN (SELECT col2 FROM TAB2) WHERE col IN (SELECT subcol FROM subtab)
</CODE> </CODE>
</PRE> </PRE>
to: to:
<PRE> <PRE>
<CODE>SELECT * <CODE>SELECT *
FROM tab FROM tab
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2) WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
</CODE> </CODE>
</PRE> </PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
We hope to fix this limitation in a future release. We hope to fix this limitation in a future release.
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4> <H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>