Avoid call MaintainOldSnapshotTimeMapping, if old_snapshot_threshold is disabled.

Hi,

On 2021-06-17 21:27:15 -0300, Ranier Vilela wrote:

While another long thread discusses the situation of old_snapshot_threshold,
I believe we can improve procarray.c by avoiding calling
MaintainOldSnapshotTimeMapping (src/backend/utils/time/snapmgr.c).

There's a very explicit comment there, which says (line 1866):
"Never call this function when old snapshot checking is disabled."

Well, assert should never be used to validate a condition that certainly
occurs at runtime.

I don't see how it can happen at runtime currently?

Since old_snapshot_threshold is -1, it is disabled, so
MaintainOldSnapshotTimeMapping doesn't need to be run, right?

It *isn't* run, the caller checks OldSnapshotThresholdActive() first.

Greetings,

Andres Freund

Em qui., 17 de jun. de 2021 às 22:08, Andres Freund <andres@anarazel.de>
escreveu:

Hi,

On 2021-06-17 21:27:15 -0300, Ranier Vilela wrote:

While another long thread discusses the situation of

old_snapshot_threshold,

I believe we can improve procarray.c by avoiding calling
MaintainOldSnapshotTimeMapping (src/backend/utils/time/snapmgr.c).

There's a very explicit comment there, which says (line 1866):
"Never call this function when old snapshot checking is disabled."

Well, assert should never be used to validate a condition that certainly
occurs at runtime.

I don't see how it can happen at runtime currently?

Since old_snapshot_threshold is -1, it is disabled, so
MaintainOldSnapshotTimeMapping doesn't need to be run, right?

It *isn't* run, the caller checks OldSnapshotThresholdActive() first.

True. My mistake.
I didn't check GetSnapshotDataInitOldSnapshot correctly.

regards,
Ranier Vilela