On Fri, Aug 25, 2017 at 10:08 PM, Robert Haas <robertmh...@gmail.com> wrote:
> On Mon, Aug 21, 2017 at 5:08 AM, Amit Kapila <amit.kapil...@gmail.com> wrote:
>> (b) I have changed the costing of gather path for path target in
>> generate_gather_paths which I am not sure is the best way. Another
>> possibility could have been that I change the code in
>> apply_projection_to_path as done in the previous patch and just call
>> it from generate_gather_paths. I have not done that because of your
>> comment above thread ("is probably unsafe, because it might confuse
>> code that reaches the modified-in-place path through some other
>> pointer (e.g. code which expects the RelOptInfo's paths to still be
>> sorted by cost)."). It is not clear to me what exactly is bothering
>> you if we directly change costing in apply_projection_to_path.
>
> The point I was trying to make is that if you retroactively change the
> cost of a path after you've already done add_path(), it's too late to
> change your mind about whether to keep the path. At least according
> to my current understanding, that's the root of this problem in the
> first place. On top of that, add_path() and other routines that deal
> with RelOptInfo path lists expect surviving paths to be ordered by
> descending cost; if you frob the cost, they might not be properly
> ordered any more.
>Okay, now I understand your point, but I think we already change the cost of paths in apply_projection_to_path which is done after add_path for top level scan/join paths. I think this matters a lot in case of Gather because the cost of computing target list can be divided among workers. I have changed the patch such that parallel paths for top level scan/join node will be generated after pathtarget is ready. I had kept the costing of path targets local to apply_projection_to_path() as that makes the patch simple. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
parallel_paths_include_tlist_cost_v2.patch
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