Re: Restore replication settings when modifying a field type

Hello.

# The patch no longer applies on the current master. Needs a rebasing.

At Sat, 26 Oct 2019 16:50:48 +0800, Quan Zongliang <quanzongliang(at)gmail(dot)com> wrote in
> In fact, the replication property of the table has not been modified,
> and it is still 'i'(REPLICA_IDENTITY_INDEX). But the previously
> specified index property 'indisreplident' is set to false because of
> the rebuild.

I suppose that the behavior is intended. Change of column types on the
publisher side can break the agreement on replica identity with
subscribers. Thus replica identity setting cannot be restored
unconditionally. For (somewhat artifitial :p) example:

P=# create table t (c1 integer, c2 text unique not null);
P=# alter table t replica identity using index t_c2_key;
P=# create publication p1 for table t;
P=# insert into t values (0, '00'), (1, '01');
S=# create table t (c1 integer, c2 text unique not null);
S=# alter table t replica identity using index t_c2_key;
S=# create subscription s1 connection '...' publication p1;

Your patch allows change of the type of c2 into integer.

P=# alter table t alter column c2 type integer using c2::integer;
P=# update t set c1 = c1 + 1 where c2 = '01';

This change doesn't affect perhaps as expected.

S=# select * from t;
c1 | c2
----+----
0 | 00
1 | 01
(2 rows)

> So I developed a patch. If the user modifies the field type. The
> associated index is REPLICA IDENTITY. Rebuild and restore replication
> settings.

Explicit setting of replica identity premises that they are sure that
the setting works correctly. Implicit rebuilding after a type change
can silently break it.

At least we need to guarantee that the restored replica identity
setting is truly compatible with all existing subscribers. I'm not
sure about potential subscribers..

Anyway I think it is a problem that replica identity setting is
dropped silently. Perhaps a message something like "REPLICA IDENTITY
setting is lost, please redefine after confirmation of compatibility
with subscribers." is needed.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center