PostgreSQL 7.4.3 Documentation
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# 11.3. Multicolumn Indexes

An index can be defined on more than one column. For example, if you have a table of this form:

    CREATE TABLE test2 (
      major int,
      minor int,
      name varchar
    );

(say, you keep your `/dev` directory in a database...) and you frequently make queries like

    SELECT name FROM test2 WHERE major = constant AND minor = constant;

then it may be appropriate to define an index on the columns `major` and `minor` together, e.g.,

    CREATE INDEX test2_mm_idx ON test2 (major, minor);

Currently, only the B-tree and GiST implementations support multicolumn indexes. Up to 32 columns may be specified. (This limit can be altered when building PostgreSQL; see the file `pg_config_manual.h`.)

The query planner can use a multicolumn index for queries that involve the leftmost column in the index definition plus any number of columns listed to the right of it, without a gap. For example, an index on `(a, b, c)` can be used in queries involving all of `a`, `b`, and `c`, or in queries involving both `a` and `b`, or in queries involving only `a`, but not in other combinations. (In a query involving `a` and `c` the planner could choose to use the index for `a`, while treating `c` like an ordinary unindexed column.) Of course, each column must be used with operators appropriate to the index type; clauses that involve other operators will not be considered.

Multicolumn indexes can only be used if the clauses involving the indexed columns are joined with `AND`. For instance,

    SELECT name FROM test2 WHERE major = constant OR minor = constant;

cannot make use of the index `test2_mm_idx` defined above to look up both columns. (It can be used to look up only the `major` column, however.)

Multicolumn indexes should be used sparingly. Most of the time, an index on a single column is sufficient and saves space and time. Indexes with more than three columns are unlikely to be helpful unless the usage of the table is extremely stylized.

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