Function ExecUnique

src/backend/executor/nodeUnique.c:47–106

---------------------------------------------------------------- * ExecUnique * ---------------------------------------------------------------- */

 * ----------------------------------------------------------------
 */
static TupleTableSlot *			/* return: a tuple or NULL */
ExecUnique(PlanState *pstate)
{
	UniqueState *node = castNode(UniqueState, pstate);
	ExprContext *econtext = node->ps.ps_ExprContext;
	TupleTableSlot *resultTupleSlot;
	TupleTableSlot *slot;
	PlanState  *outerPlan;

	CHECK_FOR_INTERRUPTS();

	/*
	 * get information from the node
	 */
	outerPlan = outerPlanState(node);
	resultTupleSlot = node->ps.ps_ResultTupleSlot;

	/*
	 * now loop, returning only non-duplicate tuples. We assume that the
	 * tuples arrive in sorted order so we can detect duplicates easily. The
	 * first tuple of each group is returned.
	 */
	for (;;)
	{
		/*
		 * fetch a tuple from the outer subplan
		 */
		slot = ExecProcNode(outerPlan);
		if (TupIsNull(slot))
		{
			/* end of subplan, so we're done */
			ExecClearTuple(resultTupleSlot);
			return NULL;
		}

		/*
		 * Always return the first tuple from the subplan.
		 */
		if (TupIsNull(resultTupleSlot))
			break;

		/*
		 * Else test if the new tuple and the previously returned tuple match.
		 * If so then we loop back and fetch another new tuple from the
		 * subplan.
		 */
		econtext->ecxt_innertuple = slot;
		econtext->ecxt_outertuple = resultTupleSlot;
		if (!ExecQualAndReset(node->eqfunction, econtext))
			break;
	}

	/*
	 * We have a new tuple different from the previous saved tuple (if any).
	 * Save it and return it.  We must copy it because the source subplan
	 * won't guarantee that this source tuple is still accessible after
	 * fetching the next source tuple.
	 */