PostgreSQL 8.1.3 Documentation
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# 11.2. Index Types

PostgreSQL provides several index types: B-tree, R-tree, Hash, and GiST. 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.

R-tree indexes are suited for queries on two-dimensional spatial data. To create an R-tree index, use a command of the form

    CREATE INDEX name ON table USING rtree (column);

The PostgreSQL query planner will consider using an R-tree index whenever an indexed column is involved in a comparison using one of these operators:

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

(See Section 9.10 for the meaning of these operators.)

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);

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). The standard distribution of PostgreSQL includes GiST operator classes equivalent to the R-tree operator classes, and many other GiST operator classes are available in the `contrib` collection or as separate projects. For more information see Chapter 49.

> 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.

> Similarly, R-tree indexes do not seem to have any performance advantages compared to the equivalent operations of GiST indexes. Like hash indexes, they are not WAL-logged and may need reindexing after a database crash.

> While the problems with hash indexes may be fixed eventually, it is likely that the R-tree index type will be retired in a future release. Users are encouraged to migrate applications that use R-tree indexes to GiST indexes.

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