MAINHACK

Viewing / Editing File: indexes-unique.html

File is not writable. Editing disabled.
<!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 9.2.24 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="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="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
>    The preferred way to add a unique constraint to a table is
    <TT
CLASS="LITERAL"
>ALTER TABLE ... ADD CONSTRAINT</TT
>.  The use of
    indexes to enforce unique constraints could be considered an
    implementation detail that should not be accessed directly.
    One should, however, be aware that 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
>
Cancel / Back
[ MAINHACK ]

Viewing / Editing File: indexes-unique.html

Server Info

System Features

User Info
