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

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
index 56943f2a87ae3eb1721cf074e779410adf76e63f..4973396b80d26986a29711c14eb770d5b31c808e 100644 (file)
--- 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))
        {