CREATE INDEX - Constructs a secondary index
SYNOPSIS
CREATE [ UNIQUE ] INDEX index_name ON table
[ USING acc_name ] ( column [ ops_name ] [, ...] )
CREATE [ UNIQUE ] INDEX index_name ON table
[ USING acc_name ] ( func_name( column [, ... ]) [ ops_name ] )
INPUTS
UNIQUE 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.
index_name
The name of the index to be created.
table The name of the table to be indexed.
acc_name
The name of the access method to be used for the
index. The default access method is BTREE. Post
gres provides three access methods for indexes:
BTREE an implementation of Lehman-Yao high-concur
rency btrees.
RTREE implements standard rtrees using Guttman's
quadratic split algorithm.
HASH an implementation of Litwin's linear hash
ing.
column The name of a column of the table.
ops_name
An associated operator class. See below for
details.
func_name
A function, which returns a value that can be
indexed.
OUTPUTS
CREATE The message returned if the index is successfully
created.
This error occurs if it is impossible to create the
index.
DESCRIPTION
CREATE INDEX constructs an index index_name on the speci
fied table.
Tip: Indexes are primarily used to enhance database
performance. But inappropriate use will result in
slower performance.
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.
In the second syntax shown above, an index is defined on
the result of a user-specified function func_name applied
to one or more columns of a single table. These func
tional indices can be used to obtain fast access to data
based on operators that would normally require some trans
formation to apply them to the base data.
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).
Use DROP INDEX [drop_index(l)] to remove an index.
NOTES
The Postgres query optimizer will consider using a btree
index whenever an indexed attribute is involved in a com
parison using one of: <, <=, =, >=, >
The Postgres query optimizer will consider using an rtree
index whenever an indexed attribute is involved in a com
parison using one of: <<, &<, &>, >>, @, ~=, &&
The Postgres query optimizer will consider using a hash
index whenever an indexed attribute is involved in a com
parison using the = operator.
column indexes. Up to 16 keys may be specified by default
(this limit can be altered when building Postgres).
An operator class 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 pur
poses:
· 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 mul
tiplication, 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.
The following query shows all defined operator classes:
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
USAGE
To create a btree index on the field title in the table
films:
CREATE UNIQUE INDEX title_idx
ON films (title);
SQL92
CREATE INDEX is a Postgres language extension.
There is no CREATE INDEX command in SQL92.
Man(1) output converted with
man2html