|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
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9.3 Multicolumn Indexes
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 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
minor together, e.g.,
CREATE INDEX test2_mm_idx ON test2 (major, minor);
Currently, only the B-tree and GiST index types 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’.)
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
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
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 GiST index can be used with query conditions that involve any subset of the index's columns. Conditions on additional columns restrict the entries returned by the index, but the condition on the first column is the most important one for determining how much of the index needs to be scanned. A GiST index will be relatively ineffective if its first column has only a few distinct values, even if there are many distinct values in additional columns.
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 9.4 Combining Multiple Indexes for some discussion of the merits of different index setups.
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