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Commitd79f2d4

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Remove IN/slow FAQ item because it only applies to <=7.3.X.
1 parentbfc5362 commitd79f2d4

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‎doc/FAQ‎

Lines changed: 11 additions & 33 deletions
Original file line numberDiff line numberDiff line change
@@ -1,7 +1,7 @@
11

22
Frequently Asked Questions (FAQ) for PostgreSQL
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4-
Last updated: Sat Jan 29 23:15:42 EST 2005
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Last updated: Sat Jan 29 23:20:03 EST 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@@ -76,13 +76,12 @@
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4.16) Why does my large-object operations get "invalid large obj
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descriptor"?
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4.17) How do I create a column that will default to the current time?
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4.18) Why are my subqueries using IN so slow?
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4.19) How do I perform an outer join?
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4.20) How do I perform queries using multiple databases?
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4.21) How do I return multiple rows or columns from a function?
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4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
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4.18) How do I perform an outer join?
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4.19) How do I perform queries using multiple databases?
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4.20) How do I return multiple rows or columns from a function?
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4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
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functions?
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4.23) What encryption options are available?
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4.22) What encryption options are available?
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Extending PostgreSQL
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@@ -922,28 +921,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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Use CURRENT_TIMESTAMP:
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CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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4.18) Why are my subqueries using IN so slow?
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In versions prior to 7.4, subqueries were joined to outer queries by
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sequentially scanning the result of the subquery for each row of the
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outer query. If the subquery returns only a few rows and the outer
930-
query returns many rows, IN is fastest. To speed up other queries,
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replace IN with EXISTS:
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SELECT *
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FROM tab
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WHERE col IN (SELECT subcol FROM subtab);
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to:
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SELECT *
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FROM tab
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WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
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For this to be fast, subcol should be an indexed column.
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In version 7.4 and later, IN actually uses the same sophisticated join
944-
techniques as normal queries, and is prefered to using EXISTS.
945-
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4.19) How do I perform an outer join?
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4.18) How do I perform an outer join?
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PostgreSQL supports outer joins using the SQL standard syntax. Here
949927
are two examples:
@@ -973,7 +951,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
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ORDER BY col1
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976-
4.20) How do I perform queries using multiple databases?
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4.19) How do I perform queries using multiple databases?
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There is no way to query a database other than the current one.
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Because PostgreSQL loads database-specific system catalogs, it is
@@ -983,12 +961,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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course, a client can make simultaneous connections to different
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databases and merge the results on the client side.
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4.21) How do I return multiple rows or columns from a function?
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4.20) How do I return multiple rows or columns from a function?
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In 7.3, you can easily return multiple rows or columns from a
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function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
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4.22) Why can't I reliably create/drop temporary tables in PL/PgSQL
969+
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
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functions?
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994972
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 );
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table access in PL/PgSQL. This will cause the query to be reparsed
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every time.
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4.23) What encryption options are available?
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4.22) What encryption options are available?
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* contrib/pgcrypto contains many encryption functions for use in SQL
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queries.

‎doc/src/FAQ/FAQ.html‎

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Original file line numberDiff line numberDiff line change
@@ -10,7 +10,7 @@
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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated: Sat Jan 29 23:15:42 EST 2005</P>
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<P>Last updated: Sat Jan 29 23:20:03 EST 2005</P>
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<P>Current maintainer: Bruce Momjian (<Ahref=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -114,16 +114,14 @@ <H2 align="center">Operational Questions</H2>
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<I>"invalid large obj descriptor"</I>?<BR>
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<Ahref="#4.17">4.17</A>) How do I create a column that will
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default to the current time?<BR>
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<Ahref="#4.18">4.18</A>) Why are my subqueries using
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<CODE><SMALL>IN</SMALL></CODE> so slow?<BR>
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<Ahref="#4.19">4.19</A>) How do I perform an outer join?<BR>
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<Ahref="#4.20">4.20</A>) How do I perform queries using multiple
117+
<Ahref="#4.18">4.18</A>) How do I perform an outer join?<BR>
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<Ahref="#4.19">4.19</A>) How do I perform queries using multiple
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databases?<BR>
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<Ahref="#4.21">4.21</A>) How do I return multiple rows or columns
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<Ahref="#4.20">4.20</A>) How do I return multiple rows or columns
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from a function?<BR>
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<Ahref="#4.22">4.22</A>) Why can't I reliably create/drop
122+
<Ahref="#4.21">4.21</A>) Why can't I reliably create/drop
125123
temporary tables in PL/PgSQL functions?<BR>
126-
<Ahref="#4.23">4.23</A>) What encryption options are available?<BR>
124+
<Ahref="#4.22">4.22</A>) What encryption options are available?<BR>
127125

128126

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<H2align="center">Extending PostgreSQL</H2>
@@ -1155,31 +1153,7 @@ <H4><A name="4.17">4.17</A>) How do I create a column that will
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</CODE>
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</PRE>
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<H4><Aname="4.18">4.18</A>) Why are my subqueries using
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<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
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1161-
<P>In versions prior to 7.4, subqueries were joined to outer queries
1162-
by sequentially scanning the result of the subquery for each row of
1163-
the outer query. If the subquery returns only a few rows and the outer
1164-
query returns many rows,<CODE><SMALL>IN</SMALL></CODE> is fastest. To
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speed up other queries, replace<CODE>IN</CODE> with
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<CODE>EXISTS</CODE>:</P>
1167-
<PRE> SELECT *
1168-
FROM tab
1169-
WHERE col IN (SELECT subcol FROM subtab);
1170-
</PRE>
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to:
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<PRE> SELECT *
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FROM tab
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WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
1175-
</PRE>
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1177-
For this to be fast,<CODE>subcol</CODE> should be an indexed column.
1178-
<P>In version 7.4 and later,<CODE>IN</CODE> actually uses the same
1179-
sophisticated join techniques as normal queries, and is prefered
1180-
to using<CODE>EXISTS</CODE>.
1181-
1182-
<H4><Aname="4.19">4.19</A>) How do I perform an outer join?</H4>
1156+
<H4><Aname="4.18">4.18</A>) How do I perform an outer join?</H4>
11831157

11841158
<P>PostgreSQL supports outer joins using the SQL standard syntax.
11851159
Here are two examples:</P>
@@ -1219,7 +1193,7 @@ <H4><A name="4.19">4.19</A>) How do I perform an outer join?</H4>
12191193
ORDER BY col1
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</PRE>
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1222-
<H4><Aname="4.20">4.20</A>) How do I perform queries using
1196+
<H4><Aname="4.19">4.19</A>) How do I perform queries using
12231197
multiple databases?</H4>
12241198

12251199
<P>There is no way to query a database other than the current one.
@@ -1231,15 +1205,15 @@ <H4><A name="4.20">4.20</A>) How do I perform queries using
12311205
connections to different databases and merge the results on the
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client side.</P>
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1234-
<H4><Aname="4.21">4.21</A>) How do I return multiple rows or
1208+
<H4><Aname="4.20">4.20</A>) How do I return multiple rows or
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columns from a function?</H4>
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12371211
<P>In 7.3, you can easily return multiple rows or columns from a
12381212
function,
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<ahref="http://techdocs.postgresql.org/guides/SetReturningFunctions">
12401214
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
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1242-
<H4><Aname="4.22">4.22</A>) Why can't I reliably create/drop
1216+
<H4><Aname="4.21">4.21</A>) Why can't I reliably create/drop
12431217
temporary tables in PL/PgSQL functions?</H4>
12441218
<P>PL/PgSQL caches function contents, and an unfortunate side effect
12451219
is that if a PL/PgSQL function accesses a temporary table, and that
@@ -1249,7 +1223,7 @@ <H4><A name="4.22">4.22</A>) Why can't I reliably create/drop
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<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
12501224
will cause the query to be reparsed every time.</P>
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1252-
<H4><Aname="4.23">4.23</A>) What encryption options are available?
1226+
<H4><Aname="4.22">4.22</A>) What encryption options are available?
12531227
</H4>
12541228
<UL>
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<LI><I>contrib/pgcrypto</I> contains many encryption functions for

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