<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Multicolumn Indexes</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 8.0.11 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Indexes" HREF="indexes.html"><LINK REL="PREVIOUS" TITLE="Index Types" HREF="indexes-types.html"><LINK REL="NEXT" TITLE="Unique Indexes" HREF="indexes-unique.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2007-02-02T03:57:22"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" >PostgreSQL 8.0.11 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="indexes-types.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="indexes.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 11. Indexes</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="indexes.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="indexes-unique.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="INDEXES-MULTICOLUMN" >11.3. Multicolumn Indexes</A ></H1 ><A NAME="AEN14145" ></A ><P > An index can be defined on more than one column. For example, if you have a table of this form: </P><PRE CLASS="PROGRAMLISTING" >CREATE TABLE test2 ( major int, minor int, name varchar );</PRE ><P> (say, you keep your <TT CLASS="FILENAME" >/dev</TT > directory in a database...) and you frequently make queries like </P><PRE CLASS="PROGRAMLISTING" >SELECT name FROM test2 WHERE major = <TT CLASS="REPLACEABLE" ><I >constant</I ></TT > AND minor = <TT CLASS="REPLACEABLE" ><I >constant</I ></TT >;</PRE ><P> then it may be appropriate to define an index on the columns <TT CLASS="STRUCTFIELD" >major</TT > and <TT CLASS="STRUCTFIELD" >minor</TT > together, e.g., </P><PRE CLASS="PROGRAMLISTING" >CREATE INDEX test2_mm_idx ON test2 (major, minor);</PRE ><P> </P ><P > Currently, only the B-tree and GiST implementations support multicolumn indexes. Up to 32 columns may be specified. (This limit can be altered when building <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN >; see the file <TT CLASS="FILENAME" >pg_config_manual.h</TT >.) </P ><P > The query planner can use a multicolumn index for queries that involve the leftmost column in the index definition plus any number of columns listed to the right of it, without a gap. For example, an index on <TT CLASS="LITERAL" >(a, b, c)</TT > can be used in queries involving all of <TT CLASS="LITERAL" >a</TT >, <TT CLASS="LITERAL" >b</TT >, and <TT CLASS="LITERAL" >c</TT >, or in queries involving both <TT CLASS="LITERAL" >a</TT > and <TT CLASS="LITERAL" >b</TT >, or in queries involving only <TT CLASS="LITERAL" >a</TT >, but not in other combinations. (In a query involving <TT CLASS="LITERAL" >a</TT > and <TT CLASS="LITERAL" >c</TT > the planner could choose to use the index for <TT CLASS="LITERAL" >a</TT >, while treating <TT CLASS="LITERAL" >c</TT > like an ordinary unindexed column.) Of course, each column must be used with operators appropriate to the index type; clauses that involve other operators will not be considered. </P ><P > Multicolumn indexes can only be used if the clauses involving the indexed columns are joined with <TT CLASS="LITERAL" >AND</TT >. For instance, </P><PRE CLASS="PROGRAMLISTING" >SELECT name FROM test2 WHERE major = <TT CLASS="REPLACEABLE" ><I >constant</I ></TT > OR minor = <TT CLASS="REPLACEABLE" ><I >constant</I ></TT >;</PRE ><P> cannot make use of the index <TT CLASS="STRUCTNAME" >test2_mm_idx</TT > defined above to look up both columns. (It can be used to look up only the <TT CLASS="STRUCTFIELD" >major</TT > column, however.) </P ><P > Multicolumn indexes should be used sparingly. Most of the time, an index on a single column is sufficient and saves space and time. Indexes with more than three columns are unlikely to be helpful unless the usage of the table is extremely stylized. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="indexes-types.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="indexes-unique.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Index Types</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="indexes.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Unique Indexes</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
