On Fri, Jun 07, 2019 at 06:01:12PM -0700, Andres Freund wrote:
>Hi,
>
>On 2019-06-07 20:52:38 -0400, Chapman Flack wrote:
>> It seems they had ended up designing a whole 'nother "protocol level"
>> involving queries wrapping their results as JSON and an app layer that
>> unwraps again, after trying a simpler first approach that was foiled by the
>> inability to see into arrays and anonymous record types in the 'describe'
>> response.
>
>I suspect quite a few people would have to have left the projectbefore
>this would happen.
>
>
>> I thought, in a new protocol rev, why not let the driver send additional
>> 'describe' messages after the first one, to drill into structure of
>> individual columns mentioned in the first response, before sending the
>> 'execute' message?
>>
>> If it doesn't want the further detail, it doesn't have to ask.
>>
>> > And then we suddenly need tracking for all these, so we don't always
>> > send out that information when we previously already did
>>
>> If it's up to the client driver, it can track what it needs or already has.
>
>> I haven't looked too deeply into the replication protocol ... it happens
>> under a kind of copy-both, right?, so maybe there's a way for the receiver
>> to send some inquiries back, but maybe in a windowed, full-duplex way where
>> it might have to buffer some incoming messages before getting the response
>> to an inquiry message it sent.
>
>That'd be a *lot* of additional complexity, and pretty much prohibitive
>from a performance POV. We'd have to not continue decoding on the server
>side *all* the time to give the client a chance to inquire additional
>information.
>
I kinda agree with this, and I think it's an argument why replication
solutions that need such additional metadata (e.g. because they have no
database to query) should not rely on pgoutput but should invent their own
decoding plugin. Which is why it's a plugin.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services