Extra Result node on top of CustomScan on PG15

[sb230132]

On PG15 CustomScan by default is not projection capable, thus wraps this node in Result node. This change in PG15 causes tests result files which have EXPLAIN output to fail. This patch fixes the plan outputs.

Fixes #4833

[codecov]

Codecov Report

Merging #4928 (8921c67) into main (6c73b61) will decrease coverage by 0.02%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main    #4928      +/-   ##
==========================================
- Coverage   89.51%   89.49%   -0.02%     
==========================================
  Files         226      226              
  Lines       50818    50826       +8     
==========================================
- Hits        45490    45487       -3     
- Misses       5328     5339      +11     

Impacted Files	Coverage Δ
src/chunk.h	100.00% <ø> (ø)
src/nodes/chunk_append/planner.c	93.08% <ø> (ø)
src/chunk.c	91.07% <16.66%> (-0.09%)	⬇️
src/bgw/job.c	93.23% <50.00%> (-0.75%)	⬇️
src/process_utility.c	90.17% <100.00%> (ø)
src/bgw/scheduler.c	83.63% <0.00%> (-1.80%)	⬇️
tsl/src/nodes/data_node_dispatch.c	93.03% <0.00%> (-0.22%)	⬇️
tsl/src/bgw_policy/job.c	88.31% <0.00%> (-0.05%)	⬇️
src/loader/bgw_message_queue.c	88.63% <0.00%> (+2.27%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b955765...8921c67. Read the comment docs.

[akuzm]

Sven explained the difference with earlier PG versions:

On PG <= 14 postgres will modify targetlists of any custom nodes which requires us to fix up targetlists after planning to not end up with broken plans. On PG 15 when the custom node is not marked as projection capable postgres will not mess with the targetlist of the custom node and instead inject a result node if any projection needs to happen. This means we no longer have to do the post-planning fixup. Those targetlist modifications of postgres were source of lots of subtle bugs.

So we want to actually disable these TL modifications to avoid bug-prone code, at least in the future.