Re: [PATCH 10/16] Introduce the concept that wal has a 'origin' node

On 20 June 2012 11:26, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
>> Simon Riggs <simon@2ndQuadrant.com> wrote:
>>> The proposal is to use WAL to generate the logical change stream.
>>> That has been shown in testing to be around x4 faster than having
>>> a separate change stream, which must also be WAL logged (as Jan
>>> noted).
>
>> Sure, that's why I want it.
>
> I think this argument is basically circular.  The reason it's 4x faster
> is that the WAL stream doesn't actually contain all the information
> needed to generate LCRs (thus all the angst about maintaining catalogs
> in sync, what to do about unfriendly datatypes, etc).  By the time the
> dust has settled and you have a workable system, you will have bloated
> WAL and given back a large chunk of that multiple, thereby invalidating
> the design premise.  Or at least that's my prediction.

The tests were conducted with the additional field added, so your
prediction is not verified.

The additional fields do not bloat WAL records - they take up exactly
the same space as before.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services