Re: [HACKERS] Logical Replication WIP - Mailing list pgsql-hackers

On 13/12/16 21:42, Peter Eisentraut wrote:

> On 12/10/16 2:48 AM, Petr Jelinek wrote:
>> Attached new version with your updates and rebased on top of the current
>> HEAD (the partitioning patch produced quite a few conflicts).
>
> I have attached a few more "fixup" patches, mostly with some editing of
> documentation and comments and some compiler warnings.
>
> In 0006 in the protocol documentation I have left a "XXX ???" where I
> didn't understand what it was trying to say.
>

Ah so you didn't understand the
> +                Identifies the following TupleData submessage as a key.
> +                This field is optional and is only present if
> +                the update changed the REPLICA IDENTITY index. XXX???

So what happens here is that the update message can contain one or two
out of 3 possible tuple submessages. It always contains 'N' message
which is the new data. Then it can optionally contain 'O' message with
old data if the table has REPLICA IDENTITY FULL (ie, not REPLICA
IDENTITY index like pkey, etc). Or it can include 'K' message that only
contains old data for the columns in the REPLICA IDENTITY index. But if
the REPLICA IDENTITY index didn't change (ie, old and new would be same
for those columns) we simply omit the 'K' message and let the downstream
take the key data from the 'N' message to save space.