Re: Proposal for Implenting read-only queries during wal replay (SoC 2007)

Tom Lane wrote:
> "Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
>> I plan to submit a proposal for implementing support for
>> read-only queries during wal replay as a "Google Summer of Code 2007"
>> project.
>
> You are discussing this on the wrong list.
So what list would be more appropriate?

>> B) Split StartupXLOG into two steps. The first (Recovery) will process
>> only enough wal to bring the system into a consistent state,
>
> How will you know what that is?
With the same logic that postgres uses now to bring an file-system backup
into a consistent state when doing PITR.

>> C) Combine A) and B), in the simplest possible way.
>> Introduce a global R/W lock, which is taken by the Replay part
>> of B) in write mode before replaying a chunk, then released,
>> and immediatly reaquired before replaying the next chunk.
>
> That seems certain to result in intolerable performance, for both the
> queries and the replay process.

That depends entirely on the usecase. And besides, this limitation could
and probably would be adressed in the future. I think a step-by-step
approach is more likely to be successfull then attempting to solve
all problems at once.

greetings, Florian Pflug