Message-ID: <56B1669A.3040909@ssinger.info>
Date: Tue, 02 Feb 2016 21:31:54 -0500
From: Steve Singer <steve@ssinger.info>
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To: Craig Ringer <craig@2ndquadrant.com>
CC: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: pglogical - logical replication contrib module
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On 01/26/2016 10:43 AM, Craig Ringer wrote:
> On 23 January 2016 at 11:17, Steve Singer <steve@ssinger.info 
> <mailto:steve@ssinger.info>> wrote:
>

>     ** Schema changes involving rewriting big tables
>
>     Sometimes you have a DDL change on a large table that will involve
>     a table rewrite and the best way of deploying the change is to
>     make the DDL change
>     on a replicate then once it is finished promote the replica to the
>     origin in some controlled fashion.  This avoids having to lock the
>     table on the origin for hours.
>
>     pglogical seems to allow minor schema changes on the replica such
>     as changing a type but it doesn't seem to allow a DO INSTEAD
>     trigger on the replica.  I don't think pglogical currently meets
>     this use case particularly well
>
>
> I'm not sure I fully understand that one.

Say you have a table A with column b

and the next version of your application you want to create a second 
table B that has column B

create table B (b text);
insert into B select b from a;
alter table a drop column b;

but you want to do this on a replica because it is a very big table and 
you want to minimize downtown.

You could have a trigger on the replica that performed updates on B.b 
instead of A except triggers don't seem to get run on the replica.


>  Craig Ringer http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services

Steve


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