Horton

Re: [HACKERS] Partition-wise aggregation/grouping

Jeevan Chalke <jeevan.chalke@enterprisedb.com>

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From: Jeevan Chalke <jeevan.chalke@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>, Dilip Kumar <dilipbalaut@gmail.com>, David Rowley <david.rowley@2ndquadrant.com>, Rajkumar Raghuwanshi <rajkumar.raghuwanshi@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2018-02-26T11:38:30Z
Lists: pgsql-hackers

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Hi Robert,

On Fri, Feb 23, 2018 at 2:53 AM, Robert Haas <robertmhaas@gmail.com> wrote:

> On Thu, Feb 8, 2018 at 8:05 AM, Jeevan Chalke
> <jeevan.chalke@enterprisedb.com> wrote:
> > In this attached version, I have rebased my changes over new design of
> > partially_grouped_rel. The preparatory changes of adding
> > partially_grouped_rel are in 0001.
>
> I spent today hacking in 0001; results attached.  The big change from
> your version is that this now uses generate_gather_paths() to add
> Gather/Gather Merge nodes (except in the case where we sort by group
> pathkeys and then Gather Merge) rather than keeping all of the bespoke
> code.  That turned up to be a bit less elegant than I would have liked
> -- I had to an override_rows argument to generate_gather_paths to make
> it work.  But overall I think this is still a big improvement, since
> it lets us share code instead of duplicating it.  Also, it potentially
> lets us add partially-aggregated but non-parallel paths into
> partially_grouped_rel->pathlist and that should Just Work; they will
> get the Finalize Aggregate step but not the Gather.  With your
> arrangement that wouldn't work.
>
> Please review/test.
>

I have reviewed and tested the patch and here are my couple of points:

     /*
-     * If the input rel belongs to a single FDW, so does the grouped rel.
+     * If the input rel belongs to a single FDW, so does the grouped rel.
Same
+     * for the partially_grouped_rel.
      */
     grouped_rel->serverid = input_rel->serverid;
     grouped_rel->userid = input_rel->userid;
     grouped_rel->useridiscurrent = input_rel->useridiscurrent;
     grouped_rel->fdwroutine = input_rel->fdwroutine;
+    partially_grouped_rel->serverid = input_rel->serverid;
+    partially_grouped_rel->userid = input_rel->userid;
+    partially_grouped_rel->useridiscurrent = input_rel->useridiscurrent;
+    partially_grouped_rel->fdwroutine = input_rel->fdwroutine;

In my earlier mail where I have posted a patch for this partially grouped
rel changes, I forgot to put my question on this.
I was unclear about above changes and thus passed grouped_rel whenever we
wanted to work on partially_grouped_rel to fetch relevant details.

One idea I thought about is to memcpy the struct once we have set all
required fields for grouped_rel so that we don't have to do similar stuff
for partially_grouped_rel.

---

+             * Insert a Sort node, if required.  But there's no point in
+             * sorting anything but the cheapest path.
              */
-            if (root->group_pathkeys)
+            if (!pathkeys_contained_in(root->group_pathkeys,
path->pathkeys))
+            {
+                if (path != linitial(partially_grouped_rel->pathlist))
+                    continue;

Paths in pathlist are added by add_path(). Though we have paths is pathlist
is sorted with the cheapest total path, we generally use
RelOptInfo->cheapest_total_path instead of using first entry, unlike
partial paths. But here you use the first entry like partial paths case.
Will it better to use cheapest total path from partially_grouped_rel? This
will require calling set_cheapest on partially_grouped_rel before we call
this function.

Attached top-up patch doing this along with few indentation fixes.

Rest of the changes look good to me.

Once this gets in, I will re-base my other patches accordingly.

And, thanks for committing 0006.

Thanks


>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>


-- 
Jeevan Chalke
Technical Architect, Product Development
EnterpriseDB Corporation
The Enterprise PostgreSQL Company

Commits

  1. postgres_fdw: Push down partition-wise aggregation.

  2. Remove 'target' from GroupPathExtraData.

  3. Implement partition-wise grouping/aggregation.

  4. Don't pass the grouping target around unnecessarily.

  5. Determine grouping strategies in create_grouping_paths.

  6. Defer creation of partially-grouped relation until it's needed.

  7. Split create_grouping_paths into degenerate and non-degenerate cases.

  8. Pass additional arguments to a couple of grouping-related functions.

  9. Fix logic error in add_paths_to_partial_grouping_rel.

  10. Minor cleanup of code related to partially_grouped_rel.

  11. Add a new upper planner relation for partially-aggregated results.

  12. Charge cpu_tuple_cost * 0.5 for Append and MergeAppend nodes.

  13. Rename enable_partition_wise_join to enable_partitionwise_join

  14. Factor some code out of create_grouping_paths.

  15. Pad XLogReaderState's main_data buffer more aggressively.

  16. Prevent int128 from requiring more than MAXALIGN alignment.

  17. Fix DROP SUBSCRIPTION hang

  18. Inject $(ICU_LIBS) regardless of platform.

  19. Some preliminary refactoring towards partitionwise join.