Re: Do all rows from a set have the same TupleDesc?

=?UTF-8?B?UmHDumwgTWFyw61uIFJvZHLDrWd1ZXo=?= <rmrodriguez(at)carto(dot)com> writes:
> I have an user defined aggregate (Postgis' St_AsMVT) that receives rows as
> input and I was wondering if all these tuples have the same TupleDesc.

Yes, they should. It's possible that the tuples themselves might have
visible inconsistencies (primarily, smaller t_natts in some than the
tuple descriptor says), but as long as you confine yourself to using
the standard tuple-disassembly functions that won't matter.

> The idea behind this question is that, if this were true, I could cache
> some information (number of attributes, oids, some internal stuff related
> to each attribute) until the end of the aggregation to avoid having to
> retrieve or calculate them for each one of the rows.

Yeah, caching such info is common. It's traditional to code defensively
about the validity of the cache (see e.g. record_out), but I'm not sure
to what extent that's really necessary for aggregates. In the case of
record_out, paranoia is probably justified because FmgrInfo structs for
I/O functions may get cached and re-used across statements, but I doubt
that can happen for an aggregate.

regards, tom lane