Re: Report oldest xmin source when autovacuum cannot remove tuples

HI,

Sorry for the late reply. I've updated the patch to follow Sami's
recommended approach.

Overview:
- Instead of modifying ComputeXidHorizons(), this patch introduces two
new functions: GetXidHorizonBlockers() and GetXidHorizonBlocker().
- GetXidHorizonBlockers() retrieves all potential blockers. This API
design leaves open the possibility of exposing this information
through a dynamic statistics view in the future [0].
- GetXidHorizonBlocker() selects the highest-priority blocker from the
candidates returned by GetXidHorizonBlockers().
- Priority is defined in the XidHorizonBlockerType enum. By
distinguishing whether the blocker matches the horizon via xid or
xmin, the appropriate blocker is selected.

Changes addressed from review comments:
- Fixed unstable regression test (Fujii-san's and Andres's comments).
- When multiple blockers share the same horizon, the blocker with the
highest priority is now selected for output (Fujii-san's comment).
- Removed unnecessary code (Fujii-san's comment).
- Distinguished between active transactions and idle-in-transaction
sessions, and added tests for both (Sami's and Wenhui's comments).
- Added a trailing comma to the last value of the enum (Sami's comment).
- Added a new function GetXidHorizonBlockers(), modeled after
BackendXidGetPid(), instead of modifying ComputeXidHorizons() (Sami's
comment).
- Added a test for a SERIALIZABLE transaction (Sami's comment).

Not addressed:
- Did not switch from int to pid_t for the pid type, because int is
used consistently throughout the PostgreSQL codebase for this purpose
(Sami's comment).

Other changes:
- Changed the TAP test to use VACUUM (VERBOSE) instead of autovacuum.

[0] https://www.postgresql.org/message-id/CAAaqYe9Dy9sicKg3xzCQUMK3VLdEP39g9nMGZheqtFYfNiO5Bg%40mail.gmail.com

--
Best regards,
Shinya Kato
NTT OSS Center