mirrors/postgres
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Update IN/EXISTS item.

Browse Source
This commit is contained in:
Bruce Momjian 2002-10-10 03:15:19 +00:00
parent 5aa7849e1b
commit 52f6918c28
2 changed files with 16 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
doc/FAQ
View File

@ -1,7 +1,7 @@
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)
@ -998,18 +998,21 @@ 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. A
workaround is to replace IN with EXISTS:
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 col1 IN (SELECT col2 FROM TAB2)
WHERE col IN (SELECT subcol FROM subtab)
to:
SELECT *
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?

11
doc/src/FAQ/FAQ.html
View File

@ -14,7 +14,7 @@
alink="#0000ff">
<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=
"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
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>
<PRE>
<CODE>SELECT *
FROM tab
WHERE col1 IN (SELECT col2 FROM TAB2)
WHERE col IN (SELECT subcol FROM subtab)
</CODE>
</PRE>
to:
<PRE>
<CODE>SELECT *
FROM tab
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
</CODE>
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
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>