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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Indexes and ORDER BY</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 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Indexes" HREF="indexes.html"><LINK REL="PREVIOUS" TITLE="Multicolumn Indexes" HREF="indexes-multicolumn.html"><LINK REL="NEXT" TITLE="Combining Multiple Indexes" HREF="indexes-bitmap-scans.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="2017-11-06T22:43:11"></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" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Multicolumn Indexes" HREF="indexes-multicolumn.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="indexes.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 11. Indexes</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Combining Multiple Indexes" HREF="indexes-bitmap-scans.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="INDEXES-ORDERING" >11.4. Indexes and <TT CLASS="LITERAL" >ORDER BY</TT ></A ></H1 ><P > In addition to simply finding the rows to be returned by a query, an index may be able to deliver them in a specific sorted order. This allows a query's <TT CLASS="LITERAL" >ORDER BY</TT > specification to be honored without a separate sorting step. Of the index types currently supported by <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN >, only B-tree can produce sorted output — the other index types return matching rows in an unspecified, implementation-dependent order. </P ><P > The planner will consider satisfying an <TT CLASS="LITERAL" >ORDER BY</TT > specification either by scanning an available index that matches the specification, or by scanning the table in physical order and doing an explicit sort. For a query that requires scanning a large fraction of the table, an explicit sort is likely to be faster than using an index because it requires less disk I/O due to following a sequential access pattern. Indexes are more useful when only a few rows need be fetched. An important special case is <TT CLASS="LITERAL" >ORDER BY</TT > in combination with <TT CLASS="LITERAL" >LIMIT</TT > <TT CLASS="REPLACEABLE" ><I >n</I ></TT >: an explicit sort will have to process all the data to identify the first <TT CLASS="REPLACEABLE" ><I >n</I ></TT > rows, but if there is an index matching the <TT CLASS="LITERAL" >ORDER BY</TT >, the first <TT CLASS="REPLACEABLE" ><I >n</I ></TT > rows can be retrieved directly, without scanning the remainder at all. </P ><P > By default, B-tree indexes store their entries in ascending order with nulls last. This means that a forward scan of an index on column <TT CLASS="LITERAL" >x</TT > produces output satisfying <TT CLASS="LITERAL" >ORDER BY x</TT > (or more verbosely, <TT CLASS="LITERAL" >ORDER BY x ASC NULLS LAST</TT >). The index can also be scanned backward, producing output satisfying <TT CLASS="LITERAL" >ORDER BY x DESC</TT > (or more verbosely, <TT CLASS="LITERAL" >ORDER BY x DESC NULLS FIRST</TT >, since <TT CLASS="LITERAL" >NULLS FIRST</TT > is the default for <TT CLASS="LITERAL" >ORDER BY DESC</TT >). </P ><P > You can adjust the ordering of a B-tree index by including the options <TT CLASS="LITERAL" >ASC</TT >, <TT CLASS="LITERAL" >DESC</TT >, <TT CLASS="LITERAL" >NULLS FIRST</TT >, and/or <TT CLASS="LITERAL" >NULLS LAST</TT > when creating the index; for example: </P><PRE CLASS="PROGRAMLISTING" >CREATE INDEX test2_info_nulls_low ON test2 (info NULLS FIRST); CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST);</PRE ><P> An index stored in ascending order with nulls first can satisfy either <TT CLASS="LITERAL" >ORDER BY x ASC NULLS FIRST</TT > or <TT CLASS="LITERAL" >ORDER BY x DESC NULLS LAST</TT > depending on which direction it is scanned in. </P ><P > You might wonder why bother providing all four options, when two options together with the possibility of backward scan would cover all the variants of <TT CLASS="LITERAL" >ORDER BY</TT >. In single-column indexes the options are indeed redundant, but in multicolumn indexes they can be useful. Consider a two-column index on <TT CLASS="LITERAL" >(x, y)</TT >: this can satisfy <TT CLASS="LITERAL" >ORDER BY x, y</TT > if we scan forward, or <TT CLASS="LITERAL" >ORDER BY x DESC, y DESC</TT > if we scan backward. But it might be that the application frequently needs to use <TT CLASS="LITERAL" >ORDER BY x ASC, y DESC</TT >. There is no way to get that ordering from a plain index, but it is possible if the index is defined as <TT CLASS="LITERAL" >(x ASC, y DESC)</TT > or <TT CLASS="LITERAL" >(x DESC, y ASC)</TT >. </P ><P > Obviously, indexes with non-default sort orderings are a fairly specialized feature, but sometimes they can produce tremendous speedups for certain queries. Whether it's worth maintaining such an index depends on how often you use queries that require a special sort ordering. </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-multicolumn.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-bitmap-scans.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Multicolumn Indexes</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="indexes.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Combining Multiple Indexes</TD ></TR ></TABLE ></DIV ></BODY ></HTML >