Function gm_readnext_tuple

src/backend/executor/nodeGatherMerge.c:714–740

* Attempt to read a tuple from given worker. */

 * Attempt to read a tuple from given worker.
 */
static MinimalTuple
gm_readnext_tuple(GatherMergeState *gm_state, int nreader, bool nowait,
				  bool *done)
{
	TupleQueueReader *reader;
	MinimalTuple tup;

	/* Check for async events, particularly messages from workers. */
	CHECK_FOR_INTERRUPTS();

	/*
	 * Attempt to read a tuple.
	 *
	 * Note that TupleQueueReaderNext will just return NULL for a worker which
	 * fails to initialize.  We'll treat that worker as having produced no
	 * tuples; WaitForParallelWorkersToFinish will error out when we get
	 * there.
	 */
	reader = gm_state->reader[nreader - 1];
	tup = TupleQueueReaderNext(reader, nowait, done);

	/*
	 * Since we'll be buffering these across multiple calls, we need to make a
	 * copy.
	 */
	return tup ? heap_copy_minimal_tuple(tup) : NULL;
}

/*
 * We have one slot for each item in the heap array.  We use SlotNumber