create_index
CREATE INDEX() SQL Commands CREATE INDEX()
NAME
CREATE INDEX - define a new index
SYNOPSIS
CREATE [ UNIQUE ] INDEX name ON table [ USING method ]
( { column | ( expression ) } [ opclass ] [, ...] )
[ WHERE predicate ]
DESCRIPTION
CREATE INDEX constructs an index index_name on the specified table.
Indexes are primarily used to enhance database performance (though
inappropriate use will 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, R-tree, hash, and GiST.
The B-tree index method is an implementation of Lehman-Yao high-con-
currency B-trees. The R-tree index method implements standard R-trees
using Guttman’s quadratic split algorithm. The hash index method is an
implementation of Litwin’s linear hashing. 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 par-
tial index is an index that contains entries for only a portion of a
table, usually a portion that is somehow more interesting 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.
The expression used in the WHERE clause may refer only to columns of
the underlying table (but it can use all columns, not only the one(s)
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 argu-
ments 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.
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 method to be used for the index. Choices are
btree, hash, rtree, and gist. 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 parenthe-
ses, as shown in the syntax. However, the parentheses may be
omitted if the expression has the form of a function call.
opclass
The name of an operator class. See below for details.
predicate
The constraint expression for a partial index.
NOTES
See [XRef to INDEXES] for information about when indexes can be used,
when they are not used, and in which particular situations can be use-
ful.
Currently, only the B-tree and GiST index methods support multicolumn
indexes. Up to 32 fields may 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 func-
tions 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 infor-
mation about operator classes is in [XRef to INDEXES-OPCLASS] and in
[XRef to XINDEX].
Use DROP INDEX [drop_index(7)] to remove an index.
EXAMPLES
To create a B-tree index on the column title in the table films:
CREATE UNIQUE INDEX title_idx ON films (title);
COMPATIBILITY
CREATE INDEX is a PostgreSQL language extension. There are no provi-
sions for indexes in the SQL standard.
SQL - Language Statements 2008-01-03 CREATE INDEX()
