Allow parallelism for deferrable serializable txns

[Andres Freund]

Hi,

The code currently says:

         * We can't use parallelism in serializable mode because the predicate
         * locking code is not parallel-aware.  It's not catastrophic if someone
         * tries to run a parallel plan in serializable mode; it just won't get
         * any workers and will run serially.  But it seems like a good 
heuristic
         * to assume that the same serialization level will be in effect at plan
         * time and execution time, so don't generate a parallel plan if we're 
in
         * serializable mode.
         */
        if ((cursorOptions & CURSOR_OPT_PARALLEL_OK) != 0 &&
                IsUnderPostmaster &&
                parse->commandType == CMD_SELECT &&
                !parse->hasModifyingCTE &&
                max_parallel_workers_per_gather > 0 &&
                !IsParallelWorker() &&
                !IsolationIsSerializable())
        {
                /* all the cheap tests pass, so scan the query tree */
                glob->maxParallelHazard = max_parallel_hazard(parse);
                glob->parallelModeOK = (glob->maxParallelHazard != 
PROPARALLEL_UNSAFE);
        }

afaict for deferrable READ ONLY DEFERRABLE transactions we could
trivially allow parallelism?  Am I missing something?

Greetings,

Andres Freund