Does TupleQueueReaderNext() really need to copy its result?

Hi,

A comment in tqueue.c says that the bytes return by shm_mq_receive()
"had better be sufficiently aligned", before assigning the pointer to
htup.t_data. Then it copies htup and returns the copy (and it did so
in the earlier version that had all the remapping stuff, too, but
sometimes it deformed it directly so it really did need to be suitably
aligned in that case IIUC).

Given that shm_mq.c proudly documents that it avoids copying the data
on the receiving side (unless it has to reconstruct a message that was
split up), and given that it promises that the pointed-to data remains
valid until your next call, it seems that it should be safe to return
a pointer to the same HeapTupleData object every time (perhaps a
member of the TupleQueueReader struct) and just adjust its t_data and
t_len members every time, so that the gather node emits tuples
directly from the shared memory queue (and then of course tell the
slot not to pfree()). Alternatively, if the value returned by
shm_mq_receive() is not really suitably aligned, then the comment is a
bit misleading.

--
Thomas Munro
https://enterprisedb.com