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Fix add_partial_path interaction with disabled_nodes

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Commit e222534679 adjusted the logic in
add_path() to keep the path list sorted by disabled_nodes and then by
total_cost, but failed to make the corresponding adjustment to
add_partial_path. As a result, add_partial_path might sort the path list
just by total cost, which could lead to later planner misbehavior.

In principle, this should be back-patched to v18, but we are typically
reluctant to back-patch planner fixes for fear of destabilizing working
installations, and it is unclear to me that this has sufficiently
serious consequences to justify an exception, so for now, no back-patch.

Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: http://postgr.es/m/CAMbWs4-mO3jMK4t_LgcJ+7Eo=NmGgkxettgRaVbJzZvVZ1koMA@mail.gmail.com
This commit is contained in:
Robert Haas 2026-02-19 13:46:10 -05:00
parent fc3896c786
commit 6e466e1e83
1 changed files with 11 additions and 5 deletions
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16
src/backend/optimizer/util/pathnode.c
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@ -758,9 +758,10 @@ add_path_precheck(RelOptInfo *parent_rel, int disabled_nodes,
* parallel such that each worker will generate a subset of the path's
* overall result.
*
* As in add_path, the partial_pathlist is kept sorted with the cheapest
* total path in front. This is depended on by multiple places, which
* just take the front entry as the cheapest path without searching.
* As in add_path, the partial_pathlist is kept sorted first by smallest
* number of disabled nodes and then by lowest total cost. This is depended
* on by multiple places, which just take the front entry as the cheapest
* path without searching.
*
* We don't generate parameterized partial paths for several reasons. Most
* importantly, they're not safe to execute, because there's nothing to
@ -878,8 +879,13 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
}
else
{
/* new belongs after this old path if it has cost >= old's */
if (new_path->total_cost >= old_path->total_cost)
/*
* new belongs after this old path if it has more disabled nodes
* or if it has the same number of nodes but a greater total cost
*/
if (new_path->disabled_nodes > old_path->disabled_nodes ||
(new_path->disabled_nodes == old_path->disabled_nodes &&
new_path->total_cost >= old_path->total_cost))
insert_at = foreach_current_index(p1) + 1;
}