Index for Section INDEX

Alphabetical listing for C

Bottom of page

CREATE
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-concurrency 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 partial
  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 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.

  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 parentheses, 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 the chapter called ``Indexes'' in the documentation for information
  about when indexes can be used, when they are not used, and in which
  particular situations can be useful.

  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 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 the sections
  called ``Operator Classes'' and ``Interfacing Extensions to Indexes'' in
  the documentation.

  Use DROP INDEX [drop_index(5)] 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 provisions
  for indexes in the SQL standard.

Index for Section INDEX

Alphabetical listing for C

Top of page