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https://issues.apache.org/jira/browse/DERBY-481?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12580458#action_12580458
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Rick Hillegas commented on DERBY-481:
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This feature was introduced to the SQL Standard in 2003. There it is called
T175. Here is some information on what other databases do:
DB2 - Appears to implement the full language for T175.
Oracle - Does not appear to implement T175. However, some similar functionality
is available. "Although Oracle does not support generated columns, a
function-based index can be used to index on the result of an expression"
according to
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/ap_standard_sql004.htm
Postgres - Does not appear to implement T175. However, according to section 5.2
of the Postgres 8.3 reference manual, it appears that you can declare the
DEFAULT value of a column to be computed by an expression which can contain
functions.
MySQL - According to section 12.1.10 of the MySQL 6.0 reference manual, MySQL
only allows constants and CURRENT_TIMESTAMP as default values for columns.
> implement SQL generated columns
> -------------------------------
>
> Key: DERBY-481
> URL: https://issues.apache.org/jira/browse/DERBY-481
> Project: Derby
> Issue Type: New Feature
> Components: SQL
> Affects Versions: 10.0.2.1
> Reporter: Rick Hillegas
> Attachments: GeneratedColumns.html
>
>
> Satheesh has pointed out that generated columns, a SQL 2003 feature, would
> satisfy the performance requirements of Expression Indexes (bug 455).
> Generated columns may not be as elegant as Expression Indexes, but they are
> easier to implement. We would allow the following new kind of column
> definition in CREATE TABLE and ALTER TABLE statements:
> columnName GENERATED ALWAYS AS ( expression )
> If expression were an indexableExpression (as defined in bug 455), then we
> could create indexes on it. There is no work for the optimizer to do here.
> The Language merely has to compute the generated column at INSERT/UPDATE time.
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