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

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index f39e6a9f80d81ac262b5b3e7ddf49cb33f2d9b58..ef7e8281cc9f362a72ce09ae05a9f4102120ba8d 100644 (file)
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -3040,8 +3040,9 @@ 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
@@ -3053,8 +3054,9 @@ 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 */