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An expression index, also known as a function based index, is a database index that is built on a generic expression, rather than one or more 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, an index can be created on the lower-case representation of the user name: : CREATE INDEX users__last_name_lower ON users( lower( last_name ) );That will create a unique index on "lower(last_name)". Any queries that search on "lower(last_name)" could then make use of that index: : SELECT user_id FROM users WHERE lower( last_name ) = lower( 'Smith' );==Database support== Major databases which support expression indexes include: DB2 (since version 10.5〔(【引用サイトリンク】accessdate=2015-08-26 )〕), Oracle Database (since release 8i.〔(【引用サイトリンク】accessdate=2015-08-26 )〕) and PostgreSQL (since at least version 7〔(【引用サイトリンク】accessdate=2015-08-26 )〕). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Expression index」の詳細全文を読む スポンサード リンク
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