Horton

Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Rushabh Lathia <rushabh.lathia@gmail.com>, Thomas Munro <thomas.munro@enterprisedb.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>, Corey Huinker <corey.huinker@gmail.com>
Date: 2018-01-12T14:14:43Z
Lists: pgsql-hackers 
On Fri, Jan 12, 2018 at 8:19 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> 1.
> + if (!IsBootstrapProcessingMode() && !indexInfo->ii_Concurrent)
>   {
> - snapshot = RegisterSnapshot(GetTransactionSnapshot());
> - OldestXmin = InvalidTransactionId; /* not used */
> + OldestXmin = GetOldestXmin(heapRelation, true);
>
> I think leader and workers should have the same idea of oldestXmin for
> the purpose of deciding the visibility of tuples.  I think this is
> ensured in all form of parallel query as we do share the snapshot,
> however, same doesn't seem to be true for Parallel Index builds.

Hmm.  Does it break anything if they use different snapshots?  In the
case of a query that would be disastrous because then you might get
inconsistent results, but if the snapshot is only being used to
determine what is and is not dead then I'm not sure it makes much
difference ... unless the different snapshots will create confusion of
some other sort.

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

Commits

  1. Support parallel btree index builds.

  2. Report an ERROR if a parallel worker fails to start properly.

  3. Transfer state pertaining to pending REINDEX operations to workers.

  4. Add a barrier primitive for synchronizing backends.

  5. Allow DML commands that create tables to use parallel query.

  6. Refactor GetOldestXmin() to use flags

  7. Fix regression in parallel planning against inheritance tables.

  8. Don't create "holes" in BufFiles, in the new logtape code.

  9. Simplify the code for logical tape read buffers.

  10. Fix excessive memory consumption in the new sort pre-reading code.

  11. Implement binary heap replace-top operation in a smarter way.

  12. Cosmetic code cleanup in commands/extension.c.

  13. Speed up planner's scanning for parallel-query hazards.

  14. Read from the same worker repeatedly until it returns no tuple.

  15. Improve tuplesort.c to support variable merge order. The original coding