Horton

Re: Improving connection scalability: GetSnapshotData()

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Bruce Momjian <bruce@momjian.us>
Cc: Robert Haas <robertmhaas@gmail.com>, "Jonathan S. Katz" <jkatz@postgresql.org>, Alexander Korotkov <a.korotkov@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Peter Geoghegan <pg@bowt.ie>, Alvaro Herrera <alvherre@2ndquadrant.com>, Thomas Munro <thomas.munro@gmail.com>, David Rowley <dgrowleyml@gmail.com>
Date: 2020-04-08T22:25:34Z
Lists: pgsql-hackers 
Hi,

On 2020-04-08 18:06:23 -0400, Bruce Momjian wrote:
> If we don't commit this, where does this leave us with the
> old_snapshot_threshold feature?  We remove it in back branches and have
> no working version in PG 13?  That seems kind of bad.

I don't think this patch changes the situation for
old_snapshot_threshold in a meaningful way.

Sure, this patch makes old_snapshot_threshold scale better, and triggers
fewer unnecessary query cancellations.  But there still are wrong query
results, the tests still don't test anything meaningful, and the
determination of which query is cancelled is still wrong.

- Andres

Commits

  1. Try to unbreak 021_row_visibility.pl on mingw.

  2. Fix and test snapshot behavior on standby.

  3. Fix race condition in snapshot caching when 2PC is used.

  4. snapshot scalability: cache snapshots using a xact completion counter.

  5. Fix use of wrong index in ComputeXidHorizons().

  6. Make vacuum a bit more verbose to debug BF failure.

  7. snapshot scalability: Introduce dense array of in-progress xids.

  8. snapshot scalability: Move PGXACT->vacuumFlags to ProcGlobal->vacuumFlags.

  9. snapshot scalability: Move subxact info to ProcGlobal, remove PGXACT.

  10. snapshot scalability: Move PGXACT->xmin back to PGPROC.

  11. snapshot scalability: Don't compute global horizons while building snapshots.

  12. BRIN: Handle concurrent desummarization properly

  13. Track latest completed xid as a FullTransactionId.

  14. Rename VariableCacheData.nextFullXid to nextXid.

  15. snapshot scalability: Move delayChkpt from PGXACT to PGPROC.

  16. Report progress of CREATE INDEX operations