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Remove IN/slow FAQ item because it only applies to <=7.3.X.

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Bruce Momjian
2005-01-30 04:20:52 +00:00
parent bfc536217d
commit d79f2d4036
2 changed files with 22 additions and 70 deletions
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doc/src/FAQ/FAQ.html
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@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sat Jan 29 23:15:42 EST 2005</P>
<P>Last updated: Sat Jan 29 23:20:03 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@ -114,16 +114,14 @@
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.17">4.17</A>) How do I create a column that will
default to the current time?<BR>
<A href="#4.18">4.18</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
<A href="#4.19">4.19</A>) How do I perform an outer join?<BR>
<A href="#4.20">4.20</A>) How do I perform queries using multiple
<A href="#4.18">4.18</A>) How do I perform an outer join?<BR>
<A href="#4.19">4.19</A>) How do I perform queries using multiple
databases?<BR>
<A href="#4.21">4.21</A>) How do I return multiple rows or columns
<A href="#4.20">4.20</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#4.22">4.22</A>) Why can't I reliably create/drop
<A href="#4.21">4.21</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
<A href="#4.23">4.23</A>) What encryption options are available?<BR>
<A href="#4.22">4.22</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
@ -1155,31 +1153,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</CODE>
</PRE>
<H4><A name="4.18">4.18</A>) Why are my subqueries using
<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
<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 *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
</PRE>
to:
<PRE> SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
<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="4.19">4.19</A>) How do I perform an outer join?</H4>
<H4><A name="4.18">4.18</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
@ -1219,7 +1193,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
ORDER BY col1
</PRE>
<H4><A name="4.20">4.20</A>) How do I perform queries using
<H4><A name="4.19">4.19</A>) How do I perform queries using
multiple databases?</H4>
<P>There is no way to query a database other than the current one.
@ -1231,7 +1205,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
connections to different databases and merge the results on the
client side.</P>
<H4><A name="4.21">4.21</A>) How do I return multiple rows or
<H4><A name="4.20">4.20</A>) How do I return multiple rows or
columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a
@ -1239,7 +1213,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<H4><A name="4.22">4.22</A>) Why can't I reliably create/drop
<H4><A name="4.21">4.21</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that
@ -1249,7 +1223,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
<H4><A name="4.23">4.23</A>) What encryption options are available?
<H4><A name="4.22">4.22</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for