Fix possible Assert failure in cost_memoize_rescan

david-rowley · david-rowley · commit aa901a37cf8a · 2024-06-19T10:20:24.000+12:00
In cost_memoize_rescan(), when calculating the hit_ratio using the calls and ndistinct estimations, if the value that was set in MemoizePath.calls had not been processed through clamp_row_est(), then it was possible that it was set to some non-integer value which could result in ndistinct being 1 higher than calls due to estimate_num_groups() performing clamp_row_est() on its input_rows. This could result in hit_ratio values slightly below 0.0, which would cause an Assert failure. The value of MemoizePath.calls comes from the final parameter in the create_memoize_path() function, of which we only have one true caller of. That caller passes outer_path->rows. All the core code I looked at always seems to call clamp_row_est() on the Path.rows, so there might have been no issues with any core Paths causing troubles here. The bug report was about a CustomPath with a non-clamped row estimated. The misbehavior as a result of this seems to be mostly limited to the Assert() failing. Aside from that, it seems the Memoize costs would just come out slightly higher than they should have, which is likely fairly harmless. Reported-by: Kohei KaiGai <kaigai@heterodb.com> Discussion: https://postgr.es/m/CAOP8fzZnTU+N64UYJYogb1hN-5hFP+PwTb3m_cnGAD7EsQwrKw@mail.gmail.com Reviewed-by: Richard Guo Backpatch-through: 14, where Memoize was introduced
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
@@ -1618,7 +1618,7 @@ create_memoize_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->param_exprs = param_exprs;
 	pathnode->singlerow = singlerow;
 	pathnode->binary_mode = binary_mode;
-	pathnode->calls = calls;
+	pathnode->calls = clamp_row_est(calls);
 
 	/*
 	 * For now we set est_entries to 0.  cost_memoize_rescan() does all the
