Horton

Re: new heapcheck contrib module

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Mark Dilger <mark.dilger@enterprisedb.com>, Dilip Kumar <dilipbalaut@gmail.com>, Peter Geoghegan <pg@bowt.ie>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-07-31T16:42:51Z
Lists: pgsql-hackers 
On Fri, Jul 31, 2020 at 12:33 PM Andres Freund <andres@anarazel.de> wrote:
> I'm not sure what I was thinking "back then", but right now I'd argue
> that the best lock against vacuum isn't a SUE, but announcing the
> correct ->xmin, so you can be sure that clog entries won't be yanked out
> from under you. Potentially with the right flag sets to avoid old enough
> tuples eing pruned.

Suppose we don't even do anything special in terms of advertising
xmin. What can go wrong? To have a problem, we've got to be running
concurrently with a vacuum that truncates clog. The clog truncation
must happen before our XID lookups, but vacuum has to remove the XIDs
from the heap before it can truncate. So we have to observe the XIDs
before vacuum removes them, but then vacuum has to truncate before we
look them up. But since we observe them and look them up while holding
a ShareLock on the buffer, this seems impossible. What's the flaw in
this argument?

If we do need to do something special in terms of advertising xmin,
how would you do it? Normally it happens by registering a snapshot,
but here all we would have is an XID; specifically, the value of
relfrozenxid that we observed.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Add pg_amcheck, a CLI for contrib/amcheck.

  2. Refactor and generalize the ParallelSlot machinery.

  3. Generalize parallel slot result handling.

  4. Move some code from src/bin/scripts to src/fe_utils to permit reuse.

  5. Factor pattern-construction logic out of processSQLNamePattern.

  6. Doc: clean up verify_heapam() documentation.

  7. Fix more portability issues in new amcheck code.

  8. Fix portability issues in new amcheck test.

  9. Try to avoid a compiler warning about using fxid uninitialized.

  10. Extend amcheck to check heap pages.

  11. Adjust walsender usage of xlogreader, simplify APIs

  12. Improve checking of child pages in contrib/amcheck.

  13. Sanitize line pointers within contrib/amcheck.

  14. Fix possible sorting error when aborting use of abbreviated keys.