CREATE INDEX() SQL Commands CREATE INDEX()NAME
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. Postgres provides three access
methods for indexes:
BTREE an implementation of Lehman-Yao high-concurrency btrees.
RTREE implements standard rtrees using Guttman's quadratic
split algorithm.
HASH an implementation of Litwin's linear hashing.
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.
ERROR: Cannot create index: 'index_name' already exists.
This error occurs if it is impossible to create the index.
DESCRIPTION
CREATE INDEX constructs an index index_name on the specified 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 functional indices 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.
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 comparison using one of: <, <=,
=, >=, >
The Postgres query optimizer will consider using an rtree index when‐
ever an indexed attribute is involved in a comparison using one of: <<,
&<, &>, >>, @, ~=, &&
The Postgres query optimizer will consider using a hash index whenever
an indexed attribute is involved in a comparison using the = operator.
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).
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 oper‐
ator classes is that for some data types, there could be more than one
meaningful ordering. For example, we might want to sort a complex-num‐
ber 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:
· 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.
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);
COMPATIBILITY
SQL92
CREATE INDEX is a Postgres language extension.
There is no CREATE INDEX command in SQL92.
SQL - Language Statements 29 March 2001 CREATE INDEX()
