Re: hyrax vs. RelationBuildPartitionDesc

Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> writes:
> On 2019/04/15 2:38, Tom Lane wrote:
>> To my mind there are only two trustworthy solutions to the problem of
>> wanting time-extended usage of a relcache subsidiary data structure: one
>> is to copy it, and the other is to reference-count it. I think that going
>> over to a reference-count-based approach for many of these structures
>> might well be something we should do in future, maybe even the very near
>> future. In the mean time, though, I'm not really satisfied with inserting
>> half-baked kluges, especially not ones that are different from our other
>> half-baked kluges for similar purposes. I think that's a path to creating
>> hard-to-reproduce bugs.

> +1 to reference-count-based approach.

> I've occasionally wondered why there is both keep_tupdesc kludge and
> refcounting for table TupleDescs. I guess it's because *only* the
> TupleTableSlot mechanism in the executor uses TupleDesc pinning (that is,
> refcounting) and the rest of the sites depend on the shaky guarantee
> provided by keep_tupdesc.

The reason for that is simply that at the time we added TupleDesc
refcounts, we didn't want to do the extra work of making all uses
of relcache entries' tupdescs deal with refcounting; keep_tupdesc
is certainly a kluge, but it works for an awful lot of callers.
We'd have to go back and deal with that more honestly if we go down
this path.

I'm inclined to think we could still allow many call sites to not
do an incr/decr-refcount dance as long as they're just fetching
scalar values out of the relcache's tupdesc, and not keeping any
pointer into it. However, it's hard to see how to enforce such
a rule mechanically, so it might be impractically error-prone
to allow that.

regards, tom lane