diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/ref/alter_table.sgml | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml index 263e5024b62..e5856d9bfb4 100644 --- a/doc/src/sgml/ref/alter_table.sgml +++ b/doc/src/sgml/ref/alter_table.sgml @@ -1,5 +1,5 @@ <!-- -$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.73 2004/07/11 23:13:51 tgl Exp $ +$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.74 2004/10/22 17:20:04 tgl Exp $ PostgreSQL documentation --> @@ -460,6 +460,22 @@ ALTER TABLE table ALTER COLUMN anycol TYPE anytype; </para> <para> + The <literal>USING</literal> option of <literal>ALTER TYPE</> can actually + specify any expression involving the old values of the row; that is, it + can refer to other columns as well as the one being converted. This allows + very general conversions to be done with the <literal>ALTER TYPE</> + syntax. Because of this flexibility, the <literal>USING</literal> + expression is not applied to the column's default value (if any); the + result might not be a constant expression as required for a default. + This means that when there is no implicit or assignment cast from old to + new type, <literal>ALTER TYPE</> may fail to convert the default even + though a <literal>USING</literal> clause is supplied. In such cases, + drop the default with <literal>DROP DEFAULT</>, perform the <literal>ALTER + TYPE</>, and then use <literal>SET DEFAULT</> to add a suitable new + default. + </para> + + <para> If a table has any descendant tables, it is not permitted to add, rename, or change the type of a column in the parent table without doing the same to the descendants. That is, <command>ALTER TABLE ONLY</command> |