.\\" auto-generated by docbook2man-spec $Revision: 1.25 $
.TH "CREATE INDEX" "" "29 March 2001" "SQL - Language Statements" "SQL Commands"
.SH NAME
CREATE INDEX \- Constructs a secondary index
.SH SYNOPSIS
.sp
.nf
CREATE [ UNIQUE ] INDEX \fIindex_name\fR ON \fItable\fR
    [ USING \fIacc_name\fR ] ( \fIcolumn\fR [ \fIops_name\fR ] [, ...] )
CREATE [ UNIQUE ] INDEX \fIindex_name\fR ON \fItable\fR
    [ USING \fIacc_name\fR ] ( \fIfunc_name\fR( \fIcolumn\fR [, ... ]) [ \fIops_name\fR ] )
  
.sp
.fi
.SS "INPUTS"
.PP
.TP
\fBUNIQUE\fR
Causes the system to check for
duplicate values in the table when the index is created (if data
already exist) and each time data is added. Attempts to
insert or update data which would result in duplicate entries
will generate an error.
.TP
\fB\fIindex_name\fB\fR
The name of the index to be created.
.TP
\fB\fItable\fB\fR
The name of the table to be indexed.
.TP
\fB\fIacc_name\fB\fR
The name of the access method to be used for
the index. The default access method is BTREE.
Postgres provides three access methods for indexes:
.RS
.TP
\fBBTREE\fR
an implementation of Lehman-Yao
high-concurrency btrees.
.TP
\fBRTREE\fR
implements standard rtrees using Guttman's
quadratic split algorithm.
.TP
\fBHASH\fR
an implementation of Litwin's linear hashing.
.RE
.PP
.TP
\fB\fIcolumn\fB\fR
The name of a column of the table.
.TP
\fB\fIops_name\fB\fR
An associated operator class. See below for details.
.TP
\fB\fIfunc_name\fB\fR
A function, which returns a value that can be indexed.
.PP
.SS "OUTPUTS"
.PP
.TP
\fBCREATE\fR
The message returned if the index is successfully created.
.TP
\fBERROR: Cannot create index: 'index_name' already exists.\fR
This error occurs if it is impossible to create the index.
.PP
.SH "DESCRIPTION"
.PP
\fBCREATE INDEX\fR constructs an index 
\fIindex_name\fR
on the specified \fItable\fR.
.sp
.RS
.B "Tip:"
Indexes are primarily used to enhance database performance.
But inappropriate use will result in slower performance.
.RE
.sp
.PP
In the first syntax shown above, the key field(s) for the
index are specified as column names.
Multiple fields can be specified if the index access method supports
multi-column indexes.
.PP
In the second syntax shown above, an index is defined
on the result of a user-specified function
\fIfunc_name\fR applied
to one or more columns of a single table.
These \fIfunctional indices\fR
can be used to obtain fast access to data
based on operators that would normally require some
transformation to apply them to the base data.
.PP
Postgres provides btree, rtree and hash access methods for
indices. The btree access method is an implementation of
Lehman-Yao high-concurrency btrees. The rtree access method
implements standard rtrees using Guttman's quadratic split algorithm.
The hash access method is an implementation of Litwin's linear
hashing. We mention the algorithms used solely to indicate that all
of these access methods are fully dynamic and do not have to be
optimized periodically (as is the case with, for example, static hash
access methods).
.PP
Use DROP INDEX [\fBdrop_index\fR(l)]
to remove an index.
.SS "NOTES"
.PP
The Postgres
query optimizer will consider using a btree index whenever
an indexed attribute is involved in a comparison using one of:
<, <=, =, >=, >

.PP
The Postgres
query optimizer will consider using an rtree index whenever
an indexed attribute is involved in a comparison using one of:
<<, &<, &>, >>, @, ~=, &&

.PP
The Postgres
query optimizer will consider using a hash index whenever
an indexed attribute is involved in a comparison using
the = operator.
.PP
Currently, only the btree access method supports multi-column
indexes. Up to 16 keys may be specified by default (this limit
can be altered when building Postgres).
.PP
An \fIoperator class\fR can be specified for each
column of an index. The operator class identifies the operators to
be used by the index for that column. For example, a btree index on
four-byte integers would use the int4_ops class;
this operator class includes comparison functions for four-byte
integers. In practice the default operator class for the field's
data type is usually sufficient. The main point of having operator classes
is that for some data types, there could be more than one meaningful
ordering. For example, we might want to sort a complex-number data type
either by absolute value or by real part. We could do this by defining
two operator classes for the data type and then selecting the proper
class when making an index. There are also some operator classes with
special purposes:
.TP 0.2i
\(bu
The operator classes box_ops and
bigbox_ops both support rtree indices on the
box data type.
The difference between them is that bigbox_ops
scales box coordinates down, to avoid floating-point exceptions from
doing multiplication, addition, and subtraction on very large
floating-point coordinates. If the field on which your rectangles lie
is about 20,000 units square or larger, you should use
bigbox_ops.
.PP
.PP
The following query shows all defined operator classes:
.sp
.nf
SELECT am.amname AS acc_name,
       opc.opcname AS ops_name,
       opr.oprname AS ops_comp
    FROM pg_am am, pg_amop amop,
         pg_opclass opc, pg_operator opr
    WHERE amop.amopid = am.oid AND
          amop.amopclaid = opc.oid AND
          amop.amopopr = opr.oid
    ORDER BY acc_name, ops_name, ops_comp
    
.sp
.fi
.SH "USAGE"
.PP
To create a btree index on the field title
in the table films:
.sp
.nf
CREATE UNIQUE INDEX title_idx
    ON films (title);
  
.sp
.fi
.SH "COMPATIBILITY"
.SS "SQL92"
.PP
CREATE INDEX is a Postgres language extension.
.PP
There is no \fBCREATE INDEX\fR command in SQL92.
