Re: WIP: Upper planner pathification

create table t1(c1 int, c2 char(1000));

insert into t1 values(generate_series(1,300000),'aaaa');

analyze t1;

set max_parallel_degree=2;

postgres=# explain select c1, count(c1) from t1 where c1 < 1000 group by c1;

ERROR:  ORDER/GROUP BY expression not found in targetlist

Without setting max_parallel_degree, it works fine and generate the appropriate results.  Here the issue seems to be that the code in grouping_planner doesn't apply the required PathTarget to Path below Gather Path due to which when we generate target list for scan plan, it misses the required information which in this case is sortgrouprefs and the same target list is then propagated for upper nodes which eventually leads to the above mentioned failure.  Due to same reason, if the target list contains some expression, it will give wrong results when parallel query is used.  I could see below ways to solve this issue.

Approach-1

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First way to solve this issue is to jam the PathTarget for partial paths similar to what we are doing for Paths and I have verified that resolves the issue, the patch for same is attached with this mail.  However, it won't work as-is, because this will make target lists pushed to workers as we have applied them below Gather Path which we don't want if the target list has any parallel unsafe functions.  To make this approach work, we need to ensure that we jam the PathTarget for partial paths (or generate a projection) only if they contain parallel-safe expressions.  Now while creating Gather Plan (create_gather_plan()), we need to ensure in some way that if path below doesn't generate the required tlist, then it should generate it's own tlist rather than using it from subplan.

Approach-2