[HACKERS] Proposal: GetOldestXminExtend for ignoring arbitrary vacuum flags

Hi all, 

I propose the patch that adds a function GetOldestXminExtended that is like GetOldestXmin but can ignore arbitrary
vacuumflags. And then, rewrite GetOldestXmin to use it. Note that this is done so as not to change the behavior of
GetOldestXmin.

This change will be useful for features that only reads rows that are visible by all transactions and could not wait
specificprocesses (VACUUM, ANALYZE, etc...) for performance. Our company (Fujitsu) is developing such an extension. In
ourbenchmark, we found that waiting an ANALYZE process created by autovacuum daemon often has a significant impact to
theperformance although the waited process do only reading as to the table. So I hope to ignore it using
GetOldestXminExtendas below. The second argument represents flags to ignore.
 

  GetOldestXminExtended(rel, PROC_IN_VACUUM | PROC_IN_LOGICAL_DECODING | PROC_IN_ANALYZE);

Note: We can ignore VACUUM processes or VACUUM with ANALYZE processes using the second option of GetOldesXmin,
"ignoreVacuum".However, we cannot ignore only ANALYZE processes because the "ignoreVacuum" = true is same to the
following.

  GetOldestXminExtended(rel, PROC_IN_VACUUM | PROC_IN_LOGICAL_DECODING)

This change should have no impact to the existing GetOldestXmin without slight overhead to call the function.

I'm not sure that this feature is useful in general.
Please let me know your opinion if you are interested.

Regards,
Eiji Seki
Fujitsu.

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