> I notice that ProcessUtility() calls SetQuerySnapshot() for FETCH
> and COPY TO statements, and nothing else.
>
> Seems to me this is very broken. Isn't a query snapshot needed for
> any utility command that might do database accesses?
Not needed. We don't support multi-versioning for schema operations.
More of that, sometimes it would be better to read *dirty* data from
system tables - so, no snapshot required.
What is really, hm, not good is that first SetQuerySnapshot defines
serializable snapshot for *all* transactions, even for ones with read
committed
isolevel: in the times of 6.5 I thought about ability to switch between
isolevels
inside single xaction - this is not required by standard and *bad* for
system:
just remember that vacuum doesn't clean up deleted tuples if there is some
transaction *potentially* interested in them. For read committed xactions
must be no serializable snapshot defined and MyProc->xmin must be
updated when *each* top-level query begins.
Vadim