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CREATE INDEX

Name

CREATE INDEX -- define a new index

Synopsis

CREATE [ UNIQUE ] INDEX name ON table [ USING method ]
    ( { column | ( expression ) } [ opclass ] [, ...] )
    [ TABLESPACE tablespace ]
    [ 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 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. See Section 11.7 for more discussion.

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.

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.

tablespace

The tablespace in which to create the index. If not specified, default_tablespace is used, or the database's default tablespace if default_tablespace is an empty string.

predicate

The constraint expression for a partial index.

Notes

See Chapter 11 for information about when indexes can be used, when they are not used, and in which particular situations they 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 Section 11.6 and in Section 31.14.

Use DROP INDEX to remove an index.

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 predicate.

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;

Compatibility

CREATE INDEX is a PostgreSQL language extension. There are no provisions for indexes in the SQL standard.

Comments


May 31, 2005, 8:11 a.m.

I didn't know about the possibility to create partial indexes until today, and I have only one thing to say about them: they rock!

Above it's stated "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 predicate."

An example of such an index would be:

create index find_null_idx on my_table(attr) where attr is NULL;

This index is perfectly legal, and will help speed up any queries searching for the NULL attributes. However, this index on attr is not useful since attr can take only one value. When creating such an index, it's better to piggyback some other attribute on the index:

create index find_null_idx on my_table(attr2) where attr is NULL;

The second index will require the same resources and will be equally efficient as the first, but as an extra plus, it will also help any query that is restricted or ordered by attr2.

If you have queries that are checking both on "attr is NULL" and "attr is not NULL", it could seem to be an idea to do:

create index attr_is_null on my_table((attr is NULL));

unfortunately, in pg 8.0 and lower it is needed to do a rather explicit check on the boolean value to be able to use the index:

select * from my_table where (attr is NULL)=true;

and in any case, I suppose that in most cases, it makes no sense using an index when we want to retrieve more than 10% of the table.

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