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.
EnterpriseDB 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 more useful for indexing 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.
Indexes are not used for IS NULL clauses by default.
The best way to use indexes in such cases is to create a partial index
using an IS NULL comparison.
The expression used in the WHERE clause may refer
only to columns of the underlying table, but it can use all columns,
not just the ones 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.
Notes
See Chapter 10 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
EnterpriseDB.) 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.
Use DROP INDEX
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);
To create an index on the column code in the table
films and have the index reside in the tablespace
indexspace:
CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;