Improve eqjoinsel_semi's behavior for small inner relations with no stats.

tglsfdc · tglsfdc · commit ca5f88502096 · 2016-11-29T18:00:56.000-05:00
If we don't have any MCV statistics for the inner relation, and we don't trust its numdistinct estimate either, eqjoinsel_semi falls back to a very conservative estimate (that 50% of the outer rows have matches). This is particularly problematic if the inner relation is completely empty, since then even an explicit ANALYZE won't produce any pg_statistic entries, so there's no way to budge the planner off the bad estimate. We'd produce a better estimate in such cases if we used the nd2/nd1 selectivity heuristic, so an easy fix is to treat the nd2 estimate as non-default if we derive it from clamping to the inner rel's rowcount estimate. This won't fix every related case (mainly because the rowcount estimate might be larger than DEFAULT_NUM_DISTINCT), but it seems like a sane extension of the existing logic, so let's apply the change in HEAD and see if anyone complains. Per bug #14438 from Nikolay Nikitin. Report: https://postgr.es/m/20161128182113.6527.58926@wrigleys.postgresql.org Discussion: https://postgr.es/m/31089.1480384713@sss.pgh.pa.us
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
@@ -2511,10 +2511,24 @@ eqjoinsel_semi(Oid operator,
 	 * We can apply this clamping both with respect to the base relation from
 	 * which the join variable comes (if there is just one), and to the
 	 * immediate inner input relation of the current join.
+	 *
+	 * If we clamp, we can treat nd2 as being a non-default estimate; it's not
+	 * great, maybe, but it didn't come out of nowhere either.  This is most
+	 * helpful when the inner relation is empty and consequently has no stats.
 	 */
 	if (vardata2->rel)
-		nd2 = Min(nd2, vardata2->rel->rows);
-	nd2 = Min(nd2, inner_rel->rows);
+	{
+		if (nd2 >= vardata2->rel->rows)
+		{
+			nd2 = vardata2->rel->rows;
+			isdefault2 = false;
+		}
+	}
+	if (nd2 >= inner_rel->rows)
+	{
+		nd2 = inner_rel->rows;
+		isdefault2 = false;
+	}
 
 	if (HeapTupleIsValid(vardata1->statsTuple))
 	{
