| From: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
|---|---|
| To: | Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com> |
| Cc: | Peter Geoghegan <pg(at)bowt(dot)ie>, Tomas Vondra <tomas(at)vondra(dot)me>, Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Georgios <gkokolatos(at)protonmail(dot)com>, Konstantin Knizhnik <knizhnik(at)garret(dot)ru>, Dilip Kumar <dilipbalaut(at)gmail(dot)com> |
| Subject: | Re: index prefetching |
| Date: | 2025-08-12 19:29:25 |
| Message-ID: | CA+hUKG+9yP4JC_PXdrew7-xQPVy2VGucFjnDveJRsQibPVOyVg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Aug 12, 2025 at 11:22 PM Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com> wrote:
> Unfortunately this doesn't work. We need to handle backwards I/O
> combining in the StartReadBuffersImpl() function too as buffer indexes
> won't have correct blocknums. Also, I think buffer forwarding of split
> backwards I/O should be handled in a couple of places.
Perhaps there could be a flag pending_read_backwards that can only
become set with pending_read_nblocks goes from 1 to 2, and then a new
flag stream->ios[x].backwards (in struct InProgressIO) that is set in
read_stream_start_pending_read(). Then immediately after
WaitReadBuffers(), we reverse the buffers it returned in place if that
flag was set. Oh, I see, you were imagining a flag
READ_BUFFERS_REVERSE that tells WaitReadBuffers() to do that
internally. Hmm. Either way I don't think you need to consider the
forwarded buffers because they will be reversed during a later call
that includes them in *nblocks (output value), no?
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