On Mon, Nov 14, 2016 at 3:51 PM, Thomas Munro <thomas.mu...@enterprisedb.com > wrote:
> On Sat, Nov 12, 2016 at 1:56 AM, Rushabh Lathia > <rushabh.lat...@gmail.com> wrote: > > On Fri, Nov 4, 2016 at 8:30 AM, Thomas Munro < > thomas.mu...@enterprisedb.com> > > wrote: > >> + * Portions Copyright (c) 1996-2015, PostgreSQL Global Development > Group > >> + * Portions Copyright (c) 1994, Regents of the University of California > >> > >> Shouldn't this say just "(c) 2016, PostgreSQL Global Development > >> Group"? > > > > Fixed. > > The year also needs updating to 2016 in nodeGatherMerge.h. > Oops sorry, fixed now. > > >> + /* Per-tuple heap maintenance cost */ > >> + run_cost += path->path.rows * comparison_cost * 2.0 * logN; > >> > >> Why multiply by two? The comment above this code says "about log2(N) > >> comparisons to delete the top heap entry and another log2(N) > >> comparisons to insert its successor". In fact gather_merge_getnext > >> calls binaryheap_replace_first, which replaces the top element without > >> any comparisons at all and then performs a sift-down in log2(N) > >> comparisons to find its new position. There is no per-tuple "delete" > >> involved. We "replace" the top element with the value it already had, > >> just to trigger the sift-down, because we know that our comparator > >> function might have a new opinion of the sort order of this element. > >> Very clever! The comment and the 2.0 factor in cost_gather_merge seem > >> to be wrong though -- or am I misreading the code? > >> > > See cost_merge_append. > > That just got tweaked in commit 34ca0905. > Fixed. > > > Looking at the plan I realize that this is happening because wrong > costing > > for Gather Merge. Here in the plan we can see the row estimated by > > Gather Merge is wrong. This is because earlier patch GM was considering > > rows = subpath->rows, which is not true as subpath is partial path. So > > we need to multiple it with number of worker. Attached patch also fixed > > this issues. I also run the TPC-H benchmark with the patch and results > > are same as earlier. > > In create_grouping_paths: > + double total_groups = gmpath->rows * > gmpath->parallel_workers; > > This hides a variable of the same name in the enclosing scope. Maybe > confusing? > > In some other places like create_ordered_paths: > + double total_groups = path->rows * path->parallel_workers; > > Though it probably made sense to use this variable name in > create_grouping_paths, wouldn't total_rows be better here? > > Initially I just copied from the other places. I agree with you that create_ordered_paths - total_rows make more sense. > It feels weird to be working back to a total row count estimate from > the partial one by simply multiplying by path->parallel_workers. > Gather Merge will underestimate the total rows when parallel_workers < > 4, if using partial row estimates ultimately from cost_seqscan which > assume some leader contribution. I don't have a better idea though. > Reversing cost_seqscan's logic certainly doesn't seem right. I don't > know how to make them agree on the leader's contribution AND give > principled answers, since there seems to be some kind of cyclic > dependency in the costing logic (cost_seqscan really needs to be given > a leader contribution estimate from its superpath which knows whether > it will allow the leader to pull tuples greedily/fairly or not, but > that superpath hasn't been created yet; cost_gather_merge needs the > row count from its subpath). Or maybe I'm just confused. > > Yes, I agree with you. But we can't really do changes into cost_seqscan. Another option I can think of is just calculate the rows for gather merge, by using the reverse formula which been used into cost_seqscan. So we can completely remote the rows argument from the create_gather_merge_path() and then inside create_gather_merge_path() - calculate the total_rows using same formula which been used into cost_seqscan. This is working fine - but not quite sure about the approach. So I attached that part of changes as separate patch. Any suggestions? -- Rushabh Lathia www.EnterpriseDB.com
gather_merge_v4_minor_changes.patch
Description: application/download
gm_v4_plus_rows_estimate.patch
Description: application/download
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