Function SubqueryToJoinWalker

src/backend/cdb/cdbsubselect.c:336–456

* This method recursively walks down the quals of an expression subquery to see if it can be pulled up to a join * and constructs the pieces necessary to perform the pullup. * E.g. SELECT * FROM outer o WHERE o.a < (SELECT avg(i.x) FROM inner i WHERE o.b = i.y) * This extracts interesting pieces of the subquery so as to create SELECT i.y, avg(i.x) from inner i GROUP by i.y */

 * This extracts interesting pieces of the subquery so as to create SELECT i.y, avg(i.x) from inner i GROUP by i.y
 */
static void
SubqueryToJoinWalker(Node *node, ConvertSubqueryToJoinContext *context)
{
	Assert(context);
	Assert(context->safeToConvert);

	if (node == NULL)
	{
		return;
	}

	if (IsA(node, BoolExpr))
	{

		/**
		 * Be extremely conservative. If there are any outer vars under an or or a not expression, then give up.
		 */
		if (is_notclause(node)
			|| is_orclause(node))
		{
			if (contain_vars_of_level_or_above(node, 1))
			{
				context->safeToConvert = false;
				return;
			}
			context->innerQual = make_and_qual(context->innerQual, node);
			return;
		}

		Assert(is_andclause(node));

		BoolExpr   *bexp = (BoolExpr *) node;
		ListCell   *lc = NULL;

		foreach(lc, bexp->args)
		{
			Node	   *arg = (Node *) lfirst(lc);

			/**
			 * If there is an outer var anywhere in the boolean expression, walk recursively.
			 */
			if (contain_vars_of_level_or_above(arg, 1))
			{
				SubqueryToJoinWalker(arg, context);

				if (!context->safeToConvert)
				{
					return;
				}
			}
			else
			{
				/**
				 * This qual should be part of the subquery's inner qual.
				 */
				context->innerQual = make_and_qual(context->innerQual, arg);
			}
		}