Synopsis

CREATE [UNIQUE] INDEX name ON table
       [USING btree|bitmap|gist]
       ( {column | (expression)} [opclass] [, ...] )
       [ WITH ( FILLFACTOR = value ) ]
       [TABLESPACE tablespace]
       [WHERE predicate]

Description

CREATE INDEX constructs an index on the specified table. Indexes are primarily used to enhance database performance (though inappropriate use can 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.

Greenplum Database provides the index methods B-tree, bitmap, and GiST. 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 is most often selected, you can improve performance by creating an index on just that portion.

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. 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. Their results must depend only on their arguments and never on any outside influence (such as the contents of another table or a parameter value). 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

Checks for duplicate values in the table when the index is created and each time data is added. Duplicate entries will generate an error. Unique indexes only apply to B-tree indexes. In Greenplum Database, unique indexes are allowed only if the columns of the index key are the same as (or a superset of) the Greenplum distribution key. On partitioned tables, a unique index is only supported within an individual partition - not across all partitions.

name

The name of the index to be created. The index is always created in the same schema as its parent table.

table

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

btree | bitmap | gist

The name of the index method to be used. Choices are btree, bitmap, and gist. The default method is btree.

column

The name of a column of the table on which to create the index. Only the B-tree, bitmap, and GiST index methods support multicolumn indexes.

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. 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, a complex-number data type could be sorted by either 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.

FILLFACTOR

The fillfactor for an index is a percentage that determines how full the index method will try to pack index pages. For B-trees, leaf pages are filled to this percentage during initial index build, and also when extending the index at the right (largest key values). If pages subsequently become completely full, they will be split, leading to gradual degradation in the index's efficiency.

B-trees use a default fillfactor of 90, but any value from 10 to 100 can be selected. If the table is static then fillfactor 100 is best to minimize the index's physical size, but for heavily updated tables a smaller fillfactor is better to minimize the need for page splits. The other index methods use fillfactor in different but roughly analogous ways; the default fillfactor varies between methods.

tablespace

The tablespace in which to create the index. If not specified, the default tablespace is used.

predicate

The constraint expression for a partial index.

Notes

When an index is created on a partitioned table, the index is propagated to all the child tables created by Greenplum Database. Creating an index on a table that is created by Greenplum Database for use by a partitioned table is not supported.

UNIQUE indexes are allowed only if the index columns are the same as (or a superset of) the Greenplum distribution key columns.

UNIQUE indexes are not allowed on append-only tables.

A UNIQUE index can be created on a partitioned table. However, uniqueness is enforced only within a partition; uniqueness is not enforced between partitions. For example, for a partitioned table with partitions that are based on year and a subpartitions that are based on quarter, uniqueness is enforced only on each individual quarter partition. Uniqueness is not enforced between quarter partitions

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.

bitmap indexes perform best for columns that have between 100 and 100,000 distinct values. For a column with more than 100,000 distinct values, the performance and space efficiency of a bitmap index decline. The size of a bitmap index is proportional to the number of rows in the table times the number of distinct values in the indexed column.

Columns with fewer than 100 distinct values usually do not benefit much from any type of index. For example, a gender column with only two distinct values for male and female would not be a good candidate for an index.

Prior releases of Greenplum Database also had an R-tree index method. This method has been removed because it had no significant advantages over the GiST method. If USING rtree is specified, CREATE INDEX will interpret it as USING gist.

For more information on the GiST index type, refer to the PostgreSQL documentation.

The use of hash and GIN indexes has been disabled in Greenplum Database.

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 a bitmap index on the column gender in the table employee:

CREATE INDEX gender_bmp_idx ON employee USING bitmap 
(gender);

To create an index on the expression lower(title), allowing efficient case-insensitive searches:

CREATE INDEX lower_title_idx ON films ((lower(title)));

To create an index with non-default fill factor:

CREATE UNIQUE INDEX title_idx ON films (title) WITH 
(fillfactor = 70);

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 Greenplum Database language extension. There are no provisions for indexes in the SQL standard.

Greenplum Database does not support the concurrent creation of indexes (CONCURRENTLY keyword not supported).

See Also

ALTER INDEX, DROP INDEX, CREATE TABLE, CREATE OPERATOR CLASS