NAME¶

CREATE INDEX - define a new index

SYNOPSIS¶

CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] name ON table [ USING method ]


    ( { column | ( expression ) } [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )


    [ WITH ( storage_parameter = value [, ... ] ) ]


    [ TABLESPACE tablespace ]


    [ WHERE predicate ]

DESCRIPTION¶

CREATE INDEX constructs an index named name on the specified table. Indexes are primarily used to enhance database performance (though inappropriate use can result in slower performance).

The key field(s) for the index are specified as column names, or alternatively as expressions written in parentheses. Multiple fields can be specified if the index method supports multicolumn indexes.

An index field can be an expression computed from the values of one or more columns of the table row. This feature can be used to obtain fast access to data based on some transformation of the basic data. For example, an index computed on upper(col) would allow the clause WHERE upper(col) = 'JIM' to use an index.

PostgreSQL provides the index methods B-tree, hash, GiST, and GIN. Users can also define their own index methods, but that is fairly complicated.

When the WHERE clause is present, a partial index is created. A partial index is an index that contains entries for only a portion of a table, usually a portion that is more useful for indexing than the rest of the table. For example, if you have a table that contains both billed and unbilled orders where the unbilled orders take up a small fraction of the total table and yet that is an often used section, you can improve performance by creating an index on just that portion. Another possible application is to use WHERE with UNIQUE to enforce uniqueness over a subset of a table. See in the documentation for more discussion.

The expression used in the WHERE clause can refer only to columns of the underlying table, but it can use all columns, not just the ones being indexed. Presently, subqueries and aggregate expressions are also forbidden in WHERE. The same restrictions apply to index fields that are expressions.

All functions and operators used in an index definition must be ``immutable'', that is, their results must depend only on their arguments and never on any outside influence (such as the contents of another table or the current time). This restriction ensures that the behavior of the index is well-defined. To use a user-defined function in an index expression or WHERE clause, remember to mark the function immutable when you create it.

PARAMETERS¶

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.

CONCURRENTLY

When this option is used, PostgreSQL will build the index without taking any locks that prevent concurrent inserts, updates, or deletes on the table; whereas a standard index build locks out writes (but not reads) on the table until it's done. There are several caveats to be aware of when using this option — see Building Indexes Concurrently [create_index(7)].

name

The name of the index to be created. No schema name can be included here; the index is always created in the same schema as its parent table.

table

The name (possibly schema-qualified) of the table to be indexed.

method

The name of the index method to be used. Choices are btree, hash, gist, and gin. The default method is btree.

column

The name of a column of the table.

expression

An expression based on one or more columns of the table. The expression usually must be written with surrounding parentheses, as shown in the syntax. However, the parentheses can be omitted if the expression has the form of a function call.

opclass

The name of an operator class. See below for details.

ASC

Specifies ascending sort order (which is the default).

DESC

Specifies descending sort order.

NULLS FIRST

Specifies that nulls sort before non-nulls. This is the default when DESC is specified.

NULLS LAST

Specifies that nulls sort after non-nulls. This is the default when DESC is not specified.

storage_parameter

The name of an index-method-specific storage parameter. See Index Storage Parameters [create_index(7)] for details.

tablespace

The tablespace in which to create the index. If not specified, default_tablespace is consulted, or temp_tablespaces for indexes on temporary tables.

predicate

The constraint expression for a partial index.

postgres=# \d tab


       Table "public.tab"


 Column |  Type   | Modifiers 
--------+---------+-----------


 col    | integer | 
Indexes:


    "idx" btree (col) INVALID

NOTES¶

See in the documentation for information about when indexes can be used, when they are not used, and in which particular situations they can be useful.

Currently, only the B-tree, GiST and GIN index methods support multicolumn indexes. Up to 32 fields can be specified by default. (This limit can be altered when building PostgreSQL.) Only B-tree currently supports unique indexes.

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 B-tree 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 column'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. More information about operator classes is in in the documentation and in in the documentation.

For index methods that support ordered scans (currently, only B-tree), the optional clauses ASC, DESC, NULLS FIRST, and/or NULLS LAST can be specified to modify the sort ordering of the index. Since an ordered index can be scanned either forward or backward, it is not normally useful to create a single-column DESC index — that sort ordering is already available with a regular index. The value of these options is that multicolumn indexes can be created that match the sort ordering requested by a mixed-ordering query, such as SELECT ... ORDER BY x ASC, y DESC. The NULLS options are useful if you need to support ``nulls sort low'' behavior, rather than the default ``nulls sort high'', in queries that depend on indexes to avoid sorting steps.

For most index methods, the speed of creating an index is dependent on the setting of maintenance_work_mem. Larger values will reduce the time needed for index creation, so long as you don't make it larger than the amount of memory really available, which would drive the machine into swapping. For hash indexes, the value of effective_cache_size is also relevant to index creation time: PostgreSQL will use one of two different hash index creation methods depending on whether the estimated index size is more or less than effective_cache_size. For best results, make sure that this parameter is also set to something reflective of available memory, and be careful that the sum of maintenance_work_mem and effective_cache_size is less than the machine's RAM less whatever space is needed by other programs.

Use DROP INDEX [drop_index(7)] to remove an index.

Prior releases of PostgreSQL also had an R-tree index method. This method has been removed because it had no significant advantages over the GiST method. If USING rtree is specified, CREATE INDEX will interpret it as USING gist, to simplify conversion of old databases to GiST.

EXAMPLES¶

To create a B-tree index on the column title in the table films:

CREATE UNIQUE INDEX title_idx ON films (title);

To create an index on the expression lower(title), allowing efficient case-insensitive searches:

CREATE INDEX lower_title_idx ON films ((lower(title)));

To create an index with non-default sort ordering of nulls:

CREATE INDEX title_idx_nulls_low ON films (title NULLS FIRST);

To create an index with non-default fill factor:

CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);

To create a GIN index with fast updates disabled:

CREATE INDEX gin_idx ON documents_table USING gin (locations) WITH (fastupdate = off);

To create an index on the column code in the table films and have the index reside in the tablespace indexspace:

CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;

To create a GiST index on a point attribute so that we can efficiently use box operators on the result of the conversion function:

CREATE INDEX pointloc


    ON points USING gist (box(location,location));
SELECT * FROM points


    WHERE box(location,location) && '(0,0),(1,1)'::box;


  

To create an index without locking out writes to the table:

CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);

COMPATIBILITY¶

CREATE INDEX is a PostgreSQL language extension. There are no provisions for indexes in the SQL standard.

SEE ALSO¶

ALTER INDEX [alter_index(7)], DROP INDEX [drop_index(7)]