CREATE 
INDEX(7) 						   SQL Commands 						   CREATE INDEX(7)

NAME

       CREATE INDEX - define a new index

SYNOPSIS

       CREATE [ UNIQUE ] INDEX index_name ON table
	   [ USING acc_method ] ( column [ ops_name ] [, ...] )
	   [ WHERE predicate ]
       CREATE [ UNIQUE ] INDEX index_name ON table
	   [ USING acc_method ] ( func_name( column [, ... ]) [ ops_name ] )
	   [ WHERE predicate ]

   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. No schema name can be included here; the index is always created in the same schema as its par-
	      ent table.

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

       acc_method
	      The name of the access method to be used for the index. The default access method is BTREE.  PostgreSQL provides four access methods
	      for indexes:

	      BTREE  an implementation of Lehman-Yao high-concurrency B-trees.

	      RTREE  implements standard R-trees using Guttman's quadratic split algorithm.

	      HASH   an implementation of Litwin's linear hashing.

	      GIST   Generalized Index Search Trees.

       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.

       predicate
	      Defines the constraint expression for a partial index.

   OUTPUTS
       CREATE INDEX
	      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 multicolumn 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 indexes 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. For example, a functional index on upper(col) would allow the clause WHERE upper(col) =
       'JIM' to use an index.

       PostgreSQL provides B-tree, R-tree, hash, and GiST access methods for indexes. The B-tree access method is an implementation of	Lehman-Yao
       high-concurrency  B-trees.  The R-tree access method implements standard R-trees 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).

       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.

       All functions and operators used in an index definition must be immutable, that is, their results must depend only on their input 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, remember to mark the function immutable when you create it.

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

   NOTES
       The PostgreSQL query optimizer will consider using a B-tree index whenever an indexed attribute is involved in a comparison using  one  of:
       <, <=, =, >=, >

       The  PostgreSQL query optimizer will consider using an R-tree index whenever an indexed attribute is involved in a comparison using one of:
       <<, &<, &>, >>, @, ~=, &&

       The PostgreSQL query optimizer will consider using a hash index whenever an indexed attribute is involved in a comparison using the = oper-
       ator.

       Testing has shown PostgreSQL's hash indexes to be similar or slower than B-tree indexes, and the index size and build time for hash indexes
       is much worse. Hash indexes also suffer poor performance under high concurrency. For these reasons, hash index use is discouraged.

       Currently, only the B-tree and gist access methods support multicolumn indexes. Up to 32 keys 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 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:

       o The operator classes box_ops and bigbox_ops both support R-tree indexes on the box data type.	The difference between them is	that  big-
	 box_ops  scales  box  coordinates  down,  to avoid floating-point exceptions from doing multiplication, addition, and subtraction on very
	 large floating-point coordinates. (Note: this was true some time ago, but currently the two operator classes both use floating point  and
	 are effectively identical.)

       The following query shows all defined operator classes:

       SELECT am.amname AS acc_method,
	      opc.opcname AS ops_name
	   FROM pg_am am, pg_opclass opc
	   WHERE opc.opcamid = am.oid
	   ORDER BY acc_method, ops_name;

USAGE

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

       CREATE UNIQUE INDEX title_idx
	   ON films (title);

COMPATIBILITY

   SQL92
       CREATE INDEX is a PostgreSQL language extension.

       There is no CREATE INDEX command in SQL92.

SQL - Language Statements					    2002-11-22							   CREATE INDEX(7)