An index can be defined on more than one column of a table. 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 issue queries like:

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

then it might 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 GIN index types support multicolumn indexes. Up to 32 columns can be specified. (This limit can be altered when building LightDB; see the file lt_config_manual.h.)

A multicolumn B-tree index can be used with query conditions that involve any subset of the index's columns, but the index is most efficient when there are constraints on the leading (leftmost) columns. The exact rule is that equality constraints on leading columns, plus any inequality constraints on the first column that does not have an equality constraint, will be used to limit the portion of the index that is scanned. Constraints on columns to the right of these columns are checked in the index, so they save visits to the table proper, but they do not reduce the portion of the index that has to be scanned. For example, given an index on (a, b, c) and a query condition WHERE a = 5 AND b >= 42 AND c < 77, the index would have to be scanned from the first entry with a = 5 and b = 42 up through the last entry with a = 5. Index entries with c >= 77 would be skipped, but they'd still have to be scanned through. This index could in principle be used for queries that have constraints on b and/or c with no constraint on a — but the entire index would have to be scanned, so in most cases the planner would prefer a sequential table scan over using the index.

A multicolumn GIN index can be used with query conditions that involve any subset of the index's columns. Unlike B-tree, index search effectiveness is the same regardless of which index column(s) the query conditions use.

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 should be used sparingly. In most situations, 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. See also Section 12.5 and Section 12.9 for some discussion of the merits of different index configurations.