Re: {CREATE INDEX, REINDEX} CONCURRENTLY improvements

On 2020-Dec-04, Dmitry Dolgov wrote:

> * This one is mostly for me to understand. There are couple of places
> with a commentary that 'PROC_IN_SAFE_IC is not necessary, because the
> transaction only takes a snapshot to do some catalog manipulation'.
> But for some of them I don't immediately see in the relevant code
> anything related to snapshots. E.g. one in DefineIndex is followed by
> WaitForOlderSnapshots (which seems to only do waiting, not taking a
> snapshot), index_set_state_flags and CacheInvalidateRelcacheByRelid.
> Is taking a snapshot hidden somewhere there inside?

Well, they're actually going to acquire an Xid, not a snapshot, so the
comment is slightly incorrect; I'll fix it, thanks for pointing that
out. The point stands: because those transactions are of very short
duration (at least of very short duration after the point where the XID
is obtained), it's not necessary to set the PROC_IN_SAFE_IC flag, since
it won't cause any disruption to other processes.

It is possible that I copied the wrong comment in DefineIndex. (I only
noticed that one after I had mulled over the ones in
ReindexRelationConcurrently, each of which is skipping setting the flag
for slightly different reasons.)