PostgreSQL 8.2.20 Documentation
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# 11.2. Index Types

PostgreSQL provides several index types: B-tree, Hash, GiST and GIN. Each index type uses a different algorithm that is best suited to different types of queries. By default, the `CREATE INDEX` command will create a B-tree index, which fits the most common situations.

B-trees can handle equality and range queries on data that can be sorted into some ordering. In particular, the PostgreSQL query planner will consider using a B-tree index whenever an indexed column is involved in a comparison using one of these operators:

`<`
`<=`
`=`
`>=`
`>`

Constructs equivalent to combinations of these operators, such as `BETWEEN` and `IN`, can also be implemented with a B-tree index search. (But note that `IS NULL` is not equivalent to `=` and is not indexable.)

The optimizer can also use a B-tree index for queries involving the pattern matching operators `LIKE` and `~` if the pattern is a constant and is anchored to the beginning of the string — for example, `col LIKE 'foo%'` or `col ~ '^foo'`, but not `col LIKE '%bar'`. However, if your server does not use the C locale you will need to create the index with a special operator class to support indexing of pattern-matching queries. See Section 11.8 below. It is also possible to use B-tree indexes for `ILIKE` and `~*`, but only if the pattern starts with non-alphabetic characters, i.e. characters that are not affected by upper/lower case conversion.

Hash indexes can only handle simple equality comparisons. The query planner will consider using a hash index whenever an indexed column is involved in a comparison using the `=` operator. The following command is used to create a hash index:

    CREATE INDEX name ON table USING hash (column);

> Note: Testing has shown PostgreSQL's hash indexes to perform no better than B-tree indexes, and the index size and build time for hash indexes is much worse. Furthermore, hash index operations are not presently WAL-logged, so hash indexes may need to be rebuilt with `REINDEX` after a database crash. For these reasons, hash index use is presently discouraged.

GiST indexes are not a single kind of index, but rather an infrastructure within which many different indexing strategies can be implemented. Accordingly, the particular operators with which a GiST index can be used vary depending on the indexing strategy (the operator class). As an example, the standard distribution of PostgreSQL includes GiST operator classes for several two-dimensional geometric data types, which support indexed queries using these operators:

`<<`
`&<`
`&>`
`>>`
`<<|`
`&<|`
`|&>`
`|>>`
`@>`
`<@`
`~=`
`&&`

(See Section 9.10 for the meaning of these operators.) Many other GiST operator classes are available in the `contrib` collection or as separate projects. For more information see Chapter 50.

GIN indexes are inverted indexes which can handle values that contain more than one key, arrays for example. Like GiST, GIN can support many different user-defined indexing strategies and the particular operators with which a GIN index can be used vary depending on the indexing strategy. As an example, the standard distribution of PostgreSQL includes GIN operator classes for one-dimensional arrays, which support indexed queries using these operators:

`<@`
`@>`
`=`
`&&`

(See Section 9.14 for the meaning of these operators.) Other GIN operator classes are available in the `contrib` `tsearch2` and `intarray` modules. For more information see Chapter 51.

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