An expression index is a database index that is built on a generic expression, rather than on a list of columns. This allows indexes to be defined for common query conditions that depend on data in a table, but are not actually stored in that table.
A common use for an expression index is to support case-insensitive searching or constraints. For example, if a web site wants to make user names case-insensitive, but still preserve the case as originally entered by the user, a unique index can be created on the upper- or lower-case representation of the user name:
CREATE UNIQUE INDEX site_user__user_name_lower ON site_user( lower( user_name ) );
That will create a unique index on "lower(user_name)". Any queries that search on "lower(user_name)" could then make use of that index:
SELECT user_id FROM site_user WHERE lower(user_name) = lower('Decibel');
Famous quotes containing the words expression and/or index:
“Cows sometimes wear an expression resembling wonderment arrested on its way to becoming a question. In the eye of superior intelligence, on the other hand, lies the nil admirari spread out like the monotony of a cloudless sky.”
—Friedrich Nietzsche (18441900)
“Exile as a mode of genius no longer exists; in place of Joyce we have the fragments of work appearing in Index on Censorship.”
—Nadine Gordimer (b. 1923)