Horton

Re: Move pg_attribute.attcompression to earlier in struct for reduced size?

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-06-09T03:24:57Z
Lists: pgsql-hackers 
On Tue, Jun 08, 2021 at 10:39:24AM -0400, Alvaro Herrera wrote:
> My unverified guess is that this code causes too many pipeline stalls
> while executing the big per-column loop.  Maybe it would be better to
> scan the attribute array twice: one to collect all data from
> Form_pg_attribute for each column into nicely packed arrays, then in a
> second loop process all the recompressions together ... the idea being
> that the first loop can run without stalling.

You mean for attlen and attcompression, right?  I agree that it would
help.

A extra set of things worth it here would be to move the allocation
and memset(0) of values_free from reform_and_rewrite_tuple(), and do
the round of pfree() calls if at least one value has been detoasted.

> Maybe at this point reverting is the only solution.

That's a safe bet at this point.  It would be good to conclude this
one by beta2 IMO.
--
Michael

Commits

  1. Remove forced toast recompression in VACUUM FULL/CLUSTER

  2. Rethink definition of pg_attribute.attcompression.

  3. Fix memory leak when de-toasting compressed values in VACUUM FULL/CLUSTER

  4. Re-order pg_attribute columns to eliminate some padding space.

  5. Add more TAP tests for pg_dump with attribute compression