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List:       pgsql-bugs
Subject:    Re: Multiple inheritance and ALTER TABLE issue
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2019-07-27 16:53:21
Message-ID: 31561.1564246401 () sss ! pgh ! pa ! us
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Manuel Rigger <rigger.manuel@gmail.com> writes:
> Consider the test case below:

> CREATE TABLE t0(c0 boolean);
> CREATE TABLE t1(c0 boolean);
> CREATE TABLE t2(c0 boolean) INHERITS(t0, t1);
> ALTER TABLE t0 ALTER c0 TYPE TEXT;
> UPDATE t1 SET c0 = TRUE; -- ERROR:  attribute "c0" of relation "t2"
> does not match parent's type

> The ALTER TABLE leaves t1 behind in an unusable state, which is
> somewhat unexpected. I would expect that either the ALTER TABLE fails
> unless also t1 is explicitly updated, or that the ALTER TABLE updates
> the column in t1. Updating the other tables does not cause a problem:

> UPDATE t2 SET c0 = TRUE; -- no error
> UPDATE t0 SET c0 = 'asdf'; -- no error

> Is this behavior intended?

Hm.  I would say that the ALTER COLUMN TYPE operation should have thrown
an error instead of trying to change the type of a multiply-inherited
column.  As you say, no good can come of that.

Given such a restriction, there would be no way to change c0's type while
the multi-inheritance situation exists.  You'd have to disinherit t2 from
one parent or the other, change the type in both parents, and then
re-inherit from the removed parent.  That is possible, and given the lack
of prior complaints, it seems like it'd be a sufficient answer to anyone
who needs to do it.

			regards, tom lane


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