<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Unique 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 9.6.10 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Indexes" HREF="indexes.html"><LINK REL="PREVIOUS" TITLE="Combining Multiple Indexes" HREF="indexes-bitmap-scans.html"><LINK REL="NEXT" TITLE="Indexes on Expressions" HREF="indexes-expressional.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="2018-10-16T23:40:43"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="4" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.6.10 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Combining Multiple Indexes" HREF="indexes-bitmap-scans.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="Indexes on Expressions" HREF="indexes-expressional.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="INDEXES-UNIQUE" >11.6. Unique Indexes</A ></H1 ><P > Indexes can also be used to enforce uniqueness of a column's value, or the uniqueness of the combined values of more than one column. </P><PRE CLASS="SYNOPSIS" >CREATE UNIQUE INDEX <TT CLASS="REPLACEABLE" ><I >name</I ></TT > ON <TT CLASS="REPLACEABLE" ><I >table</I ></TT > (<TT CLASS="REPLACEABLE" ><I >column</I ></TT > [<SPAN CLASS="OPTIONAL" >, ...</SPAN >]);</PRE ><P> Currently, only B-tree indexes can be declared unique. </P ><P > When an index is declared unique, multiple table rows with equal indexed values are not allowed. Null values are not considered equal. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows. </P ><P > <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint. </P ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >Note: </B > There's no need to manually create indexes on unique columns; doing so would just duplicate the automatically-created index. </P ></BLOCKQUOTE ></DIV ></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-bitmap-scans.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-expressional.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Combining Multiple 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" >Indexes on Expressions</TD ></TR ></TABLE ></DIV ></BODY ></HTML >