Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Alvaro Herrera <alvherre@alvh.no-ip.org>

View thread View on postgr.es
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Michael Paquier <michael@paquier.xyz>, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-05-15T15:14:15Z
Lists: pgsql-hackers 
On 2024-May-14, Melanie Plageman wrote:

> On Tue, May 14, 2024 at 4:05 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> > I do wonder how do we _know_ that the test is testing what it wants to
> > test:

> We include the explain output (the plan) to ensure it is still
> using a bitmap heap scan. The test exercises the skip fetch
> optimization in bitmap heap scan when not all of the inner tuples are
> emitted.

I meant -- the query returns an empty resultset, so how do we know it's
the empty resultset that we want and not a different empty resultset
that happens to be identical?  (This is probably not a critical point
anyhow.)

> Without the patch, the test fails, so it is protection against someone
> adding back that assert in the future. It is not protection against
> someone deleting the line
> scan->rs_empty_tuples_pending = 0
> That is, it doesn't verify that the empty unused tuples count is
> discarded. Do you think that is necessary?

I don't think that's absolutely necessary.  I suspect there are
thousands of lines that you could delete that would break things, with
no test directly raising red flags.

At this point I would go with the RMT's recommendation of pushing this
now to make sure the bug is fixed for beta1, even if the test is
imperfect.  You can always polish the test afterwards if you feel like
it ...

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"No necesitamos banderas
 No reconocemos fronteras"                  (Jorge González)

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.