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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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rhillegas edited comment on DERBY-481 at 3/24/08 6:55 AM:
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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. In addition, according to section 11.7, Postgres lets you define
indexes on expressions made up of functions and columns in the base row.
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.
was (Author: rhillegas):
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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