[PG19] Stale state after the join removal restart gives wrong results

Hi,

This is High 1 from my list in [1].

Since 2ebf25e7d70, query_planner() jumps back to its restart label
after it removes a join and runs standard_qp_callback() again. Two
things from the first pass are left behind. Both are on master, on
REL_19_STABLE and on REL_18_STABLE (9f25197bf27), 18.6 does not have
them.

1. adjust_group_pathkeys_for_groupagg() sets aggpresorted on the
aggregates it picks and nothing clears it. When self join elimination
removes a rel and the second pass picks a different presorted group,
the stale flag makes the executor skip the sort.

create table sj (id int primary key, p int, q int);
insert into sj select i, i % 3, i from generate_series(1, 30) i;
select count(distinct a.p), count(distinct a.q), sum(distinct a2.q)
from sj a join sj a2 on a.id = a2.id;

master returns 29 for the first column, the answer is 3.
enable_self_join_elimination = off gives 3. 0001 clears the flag at
the top of that function.

2. A removed rel keeps its tablesample. When the sample argument
refers to another rel that the second pass then removes,
ChangeVarNodes hits Assert(context->new_index != INVALID_VAR) in
rewriteManip.c. Non assert builds write INVALID_VAR into a dead rte
and seem fine. Richard looked at the rte fields in [2] and thought
tablesample was safe, this is the case that is not. 0002 clears
rte->tablesample next to securityQuals.

0003 adds both cases to join.sql. The patches apply to master,
REL_19_STABLE and REL_18_STABLE.

[1]
https://postgr.es/m/CAGRkXqSFdjqZP_kwRz7dv6LHX-CSrkDnynNXgntRsCEnQTKxmg@mail.gmail.com
[2]
https://postgr.es/m/CAMbWs4-wxRAFx8RcsRBJ6mawf7mbOp9J-ms5n3Di0U8T=SxCHg@mail.gmail.com

Thanks,
Shihao