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Re: Make tuple deformation faster

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jeff Davis <pgsql@j-davis.com>
Cc: James Hunter <james.hunter.pg@gmail.com>, David Rowley <dgrowleyml@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2025-03-05T21:17:25Z
Lists: pgsql-hackers 
Hi,

On 2025-03-05 12:15:57 -0800, Jeff Davis wrote:
> On Wed, 2025-03-05 at 11:33 -0800, James Hunter wrote:
> > 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.
> 
> Here the bits aren't changing, so we're only talking about mask-and-
> test, right? My intuition is that wouldn't cause much of a problem.

FWIW, I am fairly certain that I looked at this at an earlier state of the
patch, and at least for me the issue wasn't that it was inherently slower to
use the bitmask, but that it was hard to convince the compiler not generate
worse code.

IIRC the compiler generated more complicated address gathering instructions
which are slower on some older microarchitectures, but this is a vague memory.

Greetings,

Andres

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