database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-23 14:01:44 +03:00
Code Activity

Update for 7.4 --- prefer IN to EXISTS.

Browse Source
This commit is contained in:
Bruce Momjian
2003-10-29 20:20:12 +00:00
parent a35deb5400
commit a00d6b23eb
2 changed files with 18 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

14
doc/src/FAQ/FAQ.html
View File

@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Fri Oct 10 17:27:02 EDT 2003</P>
<P>Last updated: Wed Oct 29 15:19:43 EST 2003</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@ -1303,10 +1303,10 @@ BYTEA bytea variable-length byte array (null-byte safe)
<H4><A name="4.22">4.22</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
<P>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, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
<P>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, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
speed up other queries, replace <CODE>IN</CODE> with
<CODE>EXISTS</CODE>:</P>
<PRE> SELECT *
@ -1320,7 +1320,9 @@ BYTEA bytea variable-length byte array (null-byte safe)
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
This preformance problem will be fixed in 7.4.
<P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
sophisticated join techniques as normal queries, and is prefered
to using <CODE>EXISTS</CODE>.
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>