A reloption for partitioned tables - parallel_workers

[Seamus Abshere]

hi,

It turns out parallel_workers may be a useful reloption for certain uses of 
partitioned tables, at least if they're made up of fancy column store 
partitions (see 
https://www.postgresql.org/message-id/7d6fdc20-857c-4cbe-ae2e-c0ff9520ed55%40www.fastmail.com).

Would somebody tell me what I'm doing wrong? I would love to submit a patch but 
I'm stuck:

diff --git a/src/backend/optimizer/path/allpaths.c 
b/src/backend/optimizer/path/allpaths.c
index cd3fdd259c..f1ade035ac 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -3751,6 +3751,7 @@ compute_parallel_worker(RelOptInfo *rel, double 
heap_pages, double index_pages,
         * If the user has set the parallel_workers reloption, use that; 
otherwise
         * select a default number of workers.
         */
+       // I want to affect this
        if (rel->rel_parallel_workers != -1)
                parallel_workers = rel->rel_parallel_workers;
        else

so I do this

diff --git a/src/backend/access/common/reloptions.c 
b/src/backend/access/common/reloptions.c
index c687d3ee9e..597b209bfb 100644
--- a/src/backend/access/common/reloptions.c
+++ b/src/backend/access/common/reloptions.c
@@ -1961,13 +1961,15 @@ build_local_reloptions(local_relopts *relopts, Datum 
options, bool validate)
 bytea *
 partitioned_table_reloptions(Datum reloptions, bool validate)
 {
-       /*
-        * There are no options for partitioned tables yet, but this is able to 
do
-        * some validation.
-        */
+       static const relopt_parse_elt tab[] = {
+               {"parallel_workers", RELOPT_TYPE_INT,
+               offsetof(StdRdOptions, parallel_workers)},
+       };
+
        return (bytea *) build_reloptions(reloptions, validate,
                                                                          
RELOPT_KIND_PARTITIONED,
-                                                                         0, 
NULL, 0);
+                                                                         
sizeof(StdRdOptions),
+                                                                         tab, 
lengthof(tab));
 }

That "works":

postgres=# alter table test_3pd_cstore_partitioned set (parallel_workers = 33);
ALTER TABLE
postgres=# select relname, relkind, reloptions from pg_class where relname = 
'test_3pd_cstore_partitioned';
           relname           | relkind |      reloptions       
-----------------------------+---------+-----------------------
 test_3pd_cstore_partitioned | p       | {parallel_workers=33}
(1 row)

But it seems to be ignored:

diff --git a/src/backend/optimizer/path/allpaths.c 
b/src/backend/optimizer/path/allpaths.c
index cd3fdd259c..c68835ce38 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -3751,6 +3751,8 @@ compute_parallel_worker(RelOptInfo *rel, double 
heap_pages, double index_pages,
         * If the user has set the parallel_workers reloption, use that; 
otherwise
         * select a default number of workers.
         */
+       // I want to affect this, but this assertion always passes
+       Assert(rel->rel_parallel_workers == -1)
        if (rel->rel_parallel_workers != -1)
                parallel_workers = rel->rel_parallel_workers;
        else

Thanks and please forgive my code pasting etiquette as this is my first post to 
pgsql-hackers and I'm not quite sure what the right format is.

Thank you,
Seamus