Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>
Date: 2024-03-14T13:00:49Z
Lists: pgsql-hackers 
On 14/03/2024 14:34, Robert Haas wrote:
> On Thu, Mar 14, 2024 at 6:37 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> If es_snapshot was different from the active snapshot, things would get
>> weird, even without parallel query. The scans would use es_snapshot for
>> the visibility checks, but any functions you execute in quals would use
>> the active snapshot.
> 
> Hmm, that's an interesting point.
> 
> The case where the query is suspended and resumed - i.e. cursors are
> used - probably needs more analysis. In that case, perhaps there's
> more room for the snapshots to diverge.

The portal code is pretty explicit about it, the ExecutorRun() call in 
PortalRunSelect() looks like this:

             PushActiveSnapshot(queryDesc->snapshot);
             ExecutorRun(queryDesc, direction, (uint64) count,
                         portal->run_once);
             nprocessed = queryDesc->estate->es_processed;
             PopActiveSnapshot();

I looked at all the callers of ExecutorRun(), and they all have the 
active snapshot equal to queryDesc->snapshot, either because they called 
CreateQueryDesc() with the active snapshot before ExecutorRun(), or they 
set the active snapshot like above.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Commits

  1. Fix bitmapheapscan incorrect recheck of NULL tuples

  2. Increase default maintenance_io_concurrency to 16

  3. Separate TBM[Shared|Private]Iterator and TBMIterateResult

  4. Improve read_stream.c advice for dense streams.

  5. Increase default effective_io_concurrency to 16

  6. Delay extraction of TIDBitmap per page offsets

  7. Add lossy indicator to TBMIterateResult

  8. Move BitmapTableScan per-scan setup into a helper

  9. Add and use BitmapHeapScanDescData struct

  10. Fix bitmap table scan crash on iterator release

  11. Bitmap Table Scans use unified TBMIterator

  12. Add common interface for TBMIterators

  13. Make table_scan_bitmap_next_block() async-friendly

  14. Move EXPLAIN counter increment to heapam_scan_bitmap_next_block

  15. Refactor tidstore.c iterator buffering.

  16. BitmapHeapScan: Remove incorrect assert and reset field

  17. Change BitmapAdjustPrefetchIterator to accept BlockNumber

  18. BitmapHeapScan: Use correct recheck flag for skip_fetch

  19. BitmapHeapScan: Push skip_fetch optimization into table AM

  20. BitmapHeapScan: postpone setting can_skip_fetch

  21. BitmapHeapScan: begin scan after bitmap creation

  22. Fix EXPLAIN Bitmap heap scan to count pages with no visible tuples

  23. Remove redundant snapshot copying from parallel leader to workers

  24. Remove some obsolete smgrcloseall() calls.

  25. Remove the "snapshot too old" feature.

  26. Compute XID horizon for page level index vacuum on primary.