Re: Improve WALRead() to suck data directly from WAL buffers when possible

Hi,

On 2023-11-03 20:23:30 +0530, Bharath Rupireddy wrote:
> On Fri, Nov 3, 2023 at 12:35 PM Jeff Davis <pgsql@j-davis.com> wrote:
> >
> > On Thu, 2023-11-02 at 22:38 +0530, Bharath Rupireddy wrote:
> > > > I suppose the question is: should reading from the WAL buffers an
> > > > intentional thing that the caller does explicitly by specific
> > > > callers?
> > > > Or is it an optimization that should be hidden from the caller?
> > > >
> > > > I tend toward the former, at least for now.
> > >
> > > Yes, it's an optimization that must be hidden from the caller.
> >
> > As I said, I tend toward the opposite: that specific callers should
> > read from the buffers explicitly in the cases where it makes sense.
> 
> How about adding a bool flag (read_from_wal_buffers) to
> XLogReaderState so that the callers can set it if they want this
> facility via XLogReaderAllocate()?

That seems wrong architecturally - why should xlogreader itself know about any
of this? What would it mean in frontend code if read_from_wal_buffers were
set? IMO this is something that should happen purely within the read function.

Greetings,

Andres Freund