Function push_down_restrict

src/backend/optimizer/path/allpaths.c:3945–4005

* push_down_restrict * push down restrictinfo to subquery if any. * * If there are any restriction clauses that have been attached to the * subquery relation, consider pushing them down to become WHERE or HAVING * quals of the subquery itself. This transformation is useful because it * may allow us to generate a better plan for the subquery than evaluating * all the subquery output rows

 * push down a pushable qual, because it'd result in a worse plan?
 */
static Query *
push_down_restrict(PlannerInfo *root, RelOptInfo *rel,
				   RangeTblEntry *rte, Index rti, Query *subquery)
{
	pushdown_safety_info safetyInfo;

	/* Nothing to do here if it doesn't have qual at all */
	if (rel->baserestrictinfo == NIL)
		return subquery;

	/*
	 * Zero out result area for subquery_is_pushdown_safe, so that it can set
	 * flags as needed while recursing.  In particular, we need a workspace
	 * for keeping track of unsafe-to-reference columns.  unsafeColumns[i]
	 * will be set true if we find that output column i of the subquery is
	 * unsafe to use in a pushed-down qual.
	 */
	memset(&safetyInfo, 0, sizeof(safetyInfo));
	safetyInfo.unsafeColumns = (bool *)
		palloc0((list_length(subquery->targetList) + 1) * sizeof(bool));

	/*
	 * If the subquery has the "security_barrier" flag, it means the subquery
	 * originated from a view that must enforce row-level security.  Then we
	 * must not push down quals that contain leaky functions.  (Ideally this
	 * would be checked inside subquery_is_pushdown_safe, but since we don't
	 * currently pass the RTE to that function, we must do it here.)
	 */
	safetyInfo.unsafeLeaky = rte->security_barrier;

	if (subquery_is_pushdown_safe(subquery, subquery, &safetyInfo))
	{
		/* OK to consider pushing down individual quals */
		List	   *upperrestrictlist = NIL;
		ListCell   *l;

		foreach(l, rel->baserestrictinfo)
		{
			RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);
			Node	   *clause = (Node *) rinfo->clause;

			if (!rinfo->pseudoconstant &&
				qual_is_pushdown_safe(subquery, rti, rinfo, &safetyInfo))
			{
				/* Push it down */
				subquery_push_qual(subquery, rte, rti, clause);
			}
			else
			{
				/* Keep it in the upper query */
				upperrestrictlist = lappend(upperrestrictlist, rinfo);
			}
		}
		rel->baserestrictinfo = upperrestrictlist;
		/* We don't bother recomputing baserestrict_min_security */
	}

	pfree(safetyInfo.unsafeColumns);