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commit    00fecc24c140b965e0bdd00942f07f0d43983d7d
Author:   David Rowley <drowley@postgresql.org>
Date:     Mon Oct 19 11:03:16 2020 +0000
First released in: 9.5.24, 9.6.20, 10.15, 11.10, 12.5, 13.1

    Relax some asserts in merge join costing code
    
    In the planner, it was possible, given an extreme enough case containing a
    large number of joins for the number of estimated rows to become infinite.
    This could cause problems in initial_cost_mergejoin() where we perform
    some calculations based on those row estimates.
    
    A problem case, presented by Onder Kalaci showed an Assert failure from
    an Assert checking outerstartsel <= outerendsel.  In his test case this
    was effectively NaN <= Inf, which is false.  The NaN outerstartsel came
    from multiplying the infinite outer_path_rows by 0.0.
    
    In master, this problem was fixed by a90c950fc, however, that fix was too
    invasive for the backbranches.  Here we just relax the Asserts to allow
    them to pass.  The worst that appears to happen from this is that we show
    NaN cost values and infinite row estimates in EXPLAIN.  add_path() would
    have had a hard time doing anything useful with such costs, but that does
    not really matter as if the row estimates were even close to accurate,
    such plan would not complete this side of the heat death of the universe.
    
    Reported-by: Onder Kalaci
    Backpatch: 9.5 to 13
    Discussion: https://postgr.es/m/DM6PR21MB1211FF360183BCA901B27F04D80B0@DM6PR21MB1211.namprd21.prod.outlook.com


 src/backend/optimizer/path/costsize.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index ff304e8624e..9558f1b6ca3 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -2276,8 +2276,9 @@ initial_cost_mergejoin(PlannerInfo *root, JoinCostWorkspace *workspace,
 	outer_rows = clamp_row_est(outer_path_rows * outerendsel);
 	inner_rows = clamp_row_est(inner_path_rows * innerendsel);
 
-	Assert(outer_skip_rows <= outer_rows);
-	Assert(inner_skip_rows <= inner_rows);
+	/* skip rows can become NaN when path rows has become infinite */
+	Assert(outer_skip_rows <= outer_rows || isnan(outer_skip_rows));
+	Assert(inner_skip_rows <= inner_rows || isnan(inner_skip_rows));
 
 	/*
 	 * Readjust scan selectivities to account for above rounding.  This is
@@ -2289,8 +2290,9 @@ initial_cost_mergejoin(PlannerInfo *root, JoinCostWorkspace *workspace,
 	outerendsel = outer_rows / outer_path_rows;
 	innerendsel = inner_rows / inner_path_rows;
 
-	Assert(outerstartsel <= outerendsel);
-	Assert(innerstartsel <= innerendsel);
+	/* start sel can become NaN when path rows has become infinite */
+	Assert(outerstartsel <= outerendsel || isnan(outerstartsel));
+	Assert(innerstartsel <= innerendsel || isnan(innerstartsel));
 
 	/* cost of source data */
 


[parent: 994a02f7f77c]