PostgreSQL 7.1.3 Documentation
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# 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 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 whenever 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 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:

- 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.

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