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 .