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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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44
doc/FAQ
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@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Jan 29 23:15:42 EST 2005
Last updated: Sat Jan 29 23:20:03 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -76,13 +76,12 @@
4.16) Why does my large-object operations get "invalid large obj
descriptor"?
4.17) How do I create a column that will default to the current time?
4.18) Why are my subqueries using IN so slow?
4.19) How do I perform an outer join?
4.20) How do I perform queries using multiple databases?
4.21) How do I return multiple rows or columns from a function?
4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
4.18) How do I perform an outer join?
4.19) How do I perform queries using multiple databases?
4.20) How do I return multiple rows or columns from a function?
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
4.23) What encryption options are available?
4.22) What encryption options are available?
Extending PostgreSQL
@ -922,28 +921,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
4.18) Why are my subqueries using IN so slow?
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, IN is fastest. To speed up other queries,
replace IN with EXISTS:
SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
to:
SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
For this to be fast, subcol should be an indexed column.
In version 7.4 and later, IN actually uses the same sophisticated join
techniques as normal queries, and is prefered to using EXISTS.
4.19) How do I perform an outer join?
4.18) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples:
@ -973,7 +951,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
4.20) How do I perform queries using multiple databases?
4.19) How do I perform queries using multiple databases?
There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
@ -983,12 +961,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
course, a client can make simultaneous connections to different
databases and merge the results on the client side.
4.21) How do I return multiple rows or columns from a function?
4.20) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
PL/PgSQL caches function contents, and an unfortunate side effect is
@ -999,7 +977,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
table access in PL/PgSQL. This will cause the query to be reparsed
every time.
4.23) What encryption options are available?
4.22) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL
queries.

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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