CREATE INDEX(lSQL - Language Statements (2002-11-2CREATE INDEX(l)
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 parent 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.
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CREATE INDEX(lSQL - Language Statements (2002-11-2CREATE INDEX(l)
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
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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(l)] 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 = 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 identifies the operators to be
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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;
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.
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