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Re: Make tuple deformation faster

James Hunter <james.hunter.pg@gmail.com>

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From: James Hunter <james.hunter.pg@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2025-03-05T19:33:02Z
Lists: pgsql-hackers 
On Wed, Mar 5, 2025 at 10:40 AM Jeff Davis <pgsql@j-davis.com> wrote:
>
> On Thu, 2025-03-06 at 01:07 +1300, David Rowley wrote:
> > I've attached the results. The 3990x with clang looks good, but the
> > rest are mostly slower.
>
> I am still curious why.
>
> If it's due to compiler misoptimization, is that kind of thing often
> misoptimized, or is there something we're doing in particular?
>
> Even if we don't have answers, it might be worth adding a brief comment
> that we empirically determined that booleans are faster than bitfields
> or flags. In the future, maybe compilers mostly get this right, and we
> want to change to bitfields.

I haven't run this sort of experiment in years, and CPUs continue to
improve -- but with a Boolean, the CPU can read from or write to that
Boolean directly, one instruction. A Boolean tends to be a byte, and
memory is byte-addressable.

For a bitfield, however, the CPU has to read from or write to the byte
that contains the bit, but then it also has to mask out the *other*
bits in that bitfield. This is a data dependency, so it stalls the CPU
pipeline.

So Booleans tend to be faster than bitfields, because they avoid a
pipeline stall.

James Hunter

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix possible Assert failure in verify_compact_attribute()

  2. Speedup tuple deformation with additional function inlining

  3. Fix race condition in TupleDescCompactAttr assert code

  4. Optimize alignment calculations in tuple form/deform

  5. Remove pg_attribute.attcacheoff column

  6. Introduce CompactAttribute array in TupleDesc, take 2

  7. Introduce CompactAttribute array in TupleDesc

  8. Use TupleDescAttr macro consistently