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.)
