On Sun, Dec 4, 2016 at 7:36 PM, Haribabu Kommi <kommi.harib...@gmail.com> wrote: > On Thu, Nov 24, 2016 at 11:12 PM, Rushabh Lathia <rushabh.lat...@gmail.com> > wrote: >> PFA latest patch with fix as well as few cosmetic changes. > > Moved to next CF with "needs review" status.
I spent quite a bit of time on this patch over the last couple of days. I was hoping to commit it, but I think it's not quite ready for that yet and I hit a few other issues along the way. Meanwhile, here's an updated version with the following changes: * Adjusted cost_gather_merge because we don't need to worry about less than 1 worker. * Don't charge double maintenance cost of the heap per 34ca0905. This was pointed out previous and Rushabh said it was fixed, but it wasn't fixed in v5. * cost_gather_merge claimed to charge a slightly higher IPC cost because we have to block, but didn't. Fix it so it does. * Move several hunks to more appropriate places in the file, near related code or in a more logical position relative to surrounding code. * Fixed copyright dates for the new files. One said 2015, one said 2016. * Removed unnecessary code from create_gather_merge_plan that tried to handle an empty list of pathkeys (shouldn't happen). * Make create_gather_merge_plan more consistent with create_merge_append_plan. Remove make_gather_merge for the same reason. * Changed generate_gather_paths to generate gather merge paths. In the previous coding, only the upper planner nodes ever tried to generate gather merge nodes, but that seems unnecessarily limiting, since it could be useful to generate a gathered path with pathkeys at any point in the tree where we'd generate a gathered path with no pathkeys. * Rewrote generate_ordered_paths() logic to consider only the one potentially-useful path not now covered by the new code in generate_gather_paths(). * Reverted changes in generate_distinct_paths(). I think we should add something here but the existing logic definitely isn't right considering the change to generate_gather_paths(). * Assorted cosmetic cleanup in nodeGatherMerge.c. * Documented the new GUC enable_gathermerge. * Improved comments. Dropped one that seemed unnecessary. * Fixed parts of the patch to be more pgindent-clean. Testing this against the TPC-H queries at 10GB with max_parallel_workers_per_gather = 4, seq_page_cost = 0.1, random_page_cost = 0.1, work_mem = 64MB initially produced somewhat demoralizing results. Only Q17, Q4, and Q8 picked Gather Merge, and of those only Q17 got faster. Investigating this led to me realizing that join costing for parallel joins is all messed up: see https://www.postgresql.org/message-id/CA+TgmoYt2pyk2CTyvYCtFySXN=jsorgh8_mjttlowu5qkjo...@mail.gmail.com With that patch applied, in my testing, Gather Merge got picked for Q3, Q4, Q5, Q6, Q7, Q8, Q10, and Q17, but a lot of those queries get a little slower instead of a little faster. Here are the timings -- these are with EXPLAIN ANALYZE, so take them with a grain of salt -- first number is without Gather Merge, second is with Gather Merge: Q3 16943.938 ms -> 18645.957 ms Q4 3155.350 ms -> 4179.431 ms Q5 13611.484 ms -> 13831.946 ms Q6 9264.942 ms -> 8734.899 ms Q7 9759.026 ms -> 10007.307 ms Q8 2473.899 ms -> 2459.225 ms Q10 13814.950 ms -> 12255.618 ms Q17 49552.298 ms -> 46633.632 ms I haven't really had time to dig into these results yet, so I'm not sure how "real" these numbers are and how much is run-to-run jitter, EXPLAIN ANALYZE distortion, or whatever. I think this overall concept is good, because there should be cases where it's substantially cheaper to preserve the order while gathering tuples from workers than to re-sort afterwards. But this particular set of results is a bit lackluster. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
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