postgres.git / summary / log / commit / refs

commit    ca5f88502096a39f831da74eb02ec9eb3310616d
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Tue Nov 29 23:00:49 2016 +0000

    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


 src/backend/utils/adt/selfuncs.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 56943f2a87a..4973396b80d 100644
--- 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))
 	{


[parent: 96fb4c90e3b2]