On 24 March 2017 at 00:38, Amit Khandekar <amitdkhan...@gmail.com> wrote: > On 23 March 2017 at 16:26, Amit Khandekar <amitdkhan...@gmail.com> wrote: >> On 23 March 2017 at 05:55, Robert Haas <robertmh...@gmail.com> wrote: >>> >>> So in your example we do this: >>> >>> C[0] += 20; >>> C[1] += 16; >>> C[2] += 10; >>> /* C[2] is smaller than C[0] or C[1] at this point, so we add the next >>> path to C[2] */ >>> C[2] += 8; >>> /* after the previous line, C[1] is now the smallest, so add to that >>> entry next */ >>> C[1] += 3; >>> /* now we've got C[0] = 20, C[1] = 19, C[2] = 18, so add to C[2] */ >>> C[2] += 1; >>> /* final result: C[0] = 20, C[1] = 19, C[2] = 19 */ >>> >>> Now we just take the highest entry that appears in any array, which in >>> this case is C[0], as the total cost. >> >> Wow. The way your final result exactly tallies with my algorithm >> result is very interesting. This looks like some maths or computer >> science theory that I am not aware. >> >> I am currently coding the algorithm using your method. >
> While I was coding this, I was considering if Path->rows also should > be calculated similar to total cost for non-partial subpath and total > cost for partial subpaths. I think for rows, we can just take > total_rows divided by workers for non-partial paths, and this > approximation should suffice. It looks odd that it be treated with the > same algorithm we chose for total cost for non-partial paths. Attached is the patch v12, where Path->rows calculation of non-partial paths is kept separate from the way total cost is done for non-partial costs. rows for non-partial paths is calculated as total_rows divided by workers as approximation. And then rows for partial paths are just added one by one. > > Meanwhile, attached is a WIP patch v10. The only change in this patch > w.r.t. the last patch (v9) is that this one has a new function defined > append_nonpartial_cost(). Just sending this to show how the algorithm > looks like; haven't yet called it. Now append_nonpartial_cost() is used, and it is tested. -- Thanks, -Amit Khandekar EnterpriseDB Corporation The Postgres Database Company
ParallelAppend_v12.patch
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