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In ALTER COLUMN TYPE, strip any implicit coercion operations appearing

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at the top level of the column's old default expression before adding
an implicit coercion to the new column type.  This seems to satisfy the
principle of least surprise, as per discussion of bug #1290.
This commit is contained in:
Tom Lane
2004-10-22 17:20:05 +00:00
parent 4733dcc592
commit 9309d5f2ba
4 changed files with 65 additions and 5 deletions
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18
doc/src/sgml/ref/alter_table.sgml
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@ -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
-->
@ -459,6 +459,22 @@ ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
data.
</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