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

       CREATE INDEX - define a new index

       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 ]

       UNIQUE Causes the system to check for duplicate values in the table when the index is cre-
	      ated (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.

	      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.

	      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.

	      An associated operator class. See below for details.

	      A function, which returns a value that can be indexed.

	      Defines the constraint expression for a partial 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.

       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

       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 algo-
       rithms 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  con-
       tains  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, remem-
       ber to mark the function immutable when you create it.

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

       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 = operator.

       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 identi-
       fies 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  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 bigbox_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;

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

       CREATE UNIQUE INDEX title_idx
	   ON films (title);

       CREATE INDEX is a PostgreSQL language extension.

       There is no CREATE INDEX command in SQL92.

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