PostgreSQL 7.2.1 Documentation
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# 7.5. Functional Indexes

For a functional index, an index is defined on the result of a function applied to one or more columns of a single table. Functional indexes can be used to obtain fast access to data based on the result of function calls.

For example, a common way to do case-insensitive comparisons is to use the `lower` function:

    SELECT * FROM test1 WHERE lower(col1) = 'value';

This query can use an index, if one has been defined on the result of the `lower(column)` operation:

    CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));

The function in the index definition can take more than one argument, but they must be table columns, not constants. Functional indexes are always single-column (namely, the function result) even if the function uses more than one input field; there cannot be multicolumn indexes that contain function calls.

> Tip: The restrictions mentioned in the previous paragraph can easily be worked around by defining a custom function to use in the index definition that computes any desired result internally.

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