Re: Partition-wise aggregation/grouping

From: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
To: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Partition-wise aggregation/grouping
Date: 2017-08-23 11:13:04
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Attached is the patch to implement partition-wise aggregation/grouping.

As explained earlier, we produce a full aggregation for each partition when
partition keys are leading group by clauses and then append is performed.
Else we do a partial aggregation on each partition, append them and then add
finalization step over it.

I have observed that cost estimated for partition-wise aggregation and cost
for the plans without partition-wise aggregation is almost same. However,
execution time shows significant improvement (as explained my in the very
first email) with partition-wise aggregates. Planner chooses a plan
to the costs, and thus most of the time plan without partition-wise
aggregation is chosen. Hence, to force partition-wise plans and for the
regression runs, I have added a GUC named partition_wise_agg_cost_factor to
adjust the costings.

This feature is only used when enable_partition_wise_agg GUC is set to on.

Here are the details of the patches in the patch-set:

0001 - Refactors sort and hash final grouping paths into separate functions.
Since partition-wise aggregation too builds paths same as that of
create_grouping_paths(), separated path creation for sort and hash agg into
separate functions. These functions later used by main partition-wise
aggregation/grouping patch.

0002 - Passes targetlist to get_number_of_groups().
We need to estimate groups for individual child relations and thus need to
pass targetlist corresponding to the child rel.

0003 - Adds enable_partition_wise_agg and partition_wise_agg_cost_factor

0004 - Implements partition-wise aggregation.

0005 - Adds test-cases.

0006 - postgres_fdw changes which enable pushing aggregation for other upper

Since this patch is highly dependent on partition-wise join [1], one needs
apply all those patches on HEAD (my repository head was at:
66ed3829df959adb47f71d7c903ac59f0670f3e1) before applying these patches in

Suggestions / feedback / inputs ?


On Tue, Mar 21, 2017 at 12:47 PM, Jeevan Chalke <
jeevan(dot)chalke(at)enterprisedb(dot)com> wrote:

> Hi all,
> Declarative partitioning is supported in PostgreSQL 10 and work is already
> in
> progress to support partition-wise joins. Here is a proposal for
> partition-wise
> aggregation/grouping. Our initial performance measurement has shown 7
> times
> performance when partitions are on foreign servers and approximately 15%
> when
> partitions are local.
> Partition-wise aggregation/grouping computes aggregates for each partition
> separately. If the group clause contains the partition key, all the rows
> belonging to a given group come from one partition, thus allowing
> aggregates
> to be computed completely for each partition. Otherwise, partial
> aggregates
> computed for each partition are combined across the partitions to produce
> the
> final aggregates. This technique improves performance because:
> i. When partitions are located on foreign server, we can push down the
> aggregate to the foreign server.
> ii. If hash table for each partition fits in memory, but that for the whole
> relation does not, each partition-wise aggregate can use an in-memory hash
> table.
> iii. Aggregation at the level of partitions can exploit properties of
> partitions like indexes, their storage etc.
> Attached an experimental patch for the same based on the partition-wise
> join
> patches posted in [1].
> This patch currently implements partition-wise aggregation when group
> clause
> contains the partitioning key. A query below, involving a partitioned
> table
> with 3 partitions containing 1M rows each, producing total 30 groups showed
> 15% improvement over non-partition-wise aggregation. Same query showed 7
> times
> improvement when the partitions were located on the foreign servers.
> Here is the sample plan:
> postgres=# set enable_partition_wise_agg to true;
> postgres=# EXPLAIN ANALYZE SELECT a, count(*) FROM plt1 GROUP BY a;
> ------------------------------------------------------------
> --------------------------------------------------
> Append (cost=5100.00..61518.90 rows=30 width=12) (actual
> time=324.837..944.804 rows=30 loops=1)
> -> Foreign Scan (cost=5100.00..20506.30 rows=10 width=12) (actual
> time=324.837..324.838 rows=10 loops=1)
> Relations: Aggregate on (public.fplt1_p1 plt1)
> -> Foreign Scan (cost=5100.00..20506.30 rows=10 width=12) (actual
> time=309.954..309.956 rows=10 loops=1)
> Relations: Aggregate on (public.fplt1_p2 plt1)
> -> Foreign Scan (cost=5100.00..20506.30 rows=10 width=12) (actual
> time=310.002..310.004 rows=10 loops=1)
> Relations: Aggregate on (public.fplt1_p3 plt1)
> Planning time: 0.370 ms
> Execution time: 945.384 ms
> (9 rows)
> postgres=# set enable_partition_wise_agg to false;
> postgres=# EXPLAIN ANALYZE SELECT a, count(*) FROM plt1 GROUP BY a;
> ------------------------------------------------------------
> ------------------------------------------------------------
> ---------------
> HashAggregate (cost=121518.01..121518.31 rows=30 width=12) (actual
> time=6498.452..6498.459 rows=30 loops=1)
> Group Key: plt1.a
> -> Append (cost=0.00..106518.00 rows=3000001 width=4) (actual
> time=0.595..5769.592 rows=3000000 loops=1)
> -> Seq Scan on plt1 (cost=0.00..0.00 rows=1 width=4) (actual
> time=0.007..0.007 rows=0 loops=1)
> -> Foreign Scan on fplt1_p1 (cost=100.00..35506.00 rows=1000000
> width=4) (actual time=0.587..1844.506 rows=1000000 loops=1)
> -> Foreign Scan on fplt1_p2 (cost=100.00..35506.00 rows=1000000
> width=4) (actual time=0.384..1839.633 rows=1000000 loops=1)
> -> Foreign Scan on fplt1_p3 (cost=100.00..35506.00 rows=1000000
> width=4) (actual time=0.402..1876.505 rows=1000000 loops=1)
> Planning time: 0.251 ms
> Execution time: 6499.018 ms
> (9 rows)
> Patch needs a lot of improvement including:
> 1. Support for partial partition-wise aggregation
> 2. Estimating number of groups for every partition
> 3. Estimating cost of partition-wise aggregation based on sample partitions
> similar to partition-wise join
> and much more.
> In order to support partial aggregation on foreign partitions, we need
> support
> to fetch partially aggregated results from the foreign server. That can be
> handled as a separate follow-on patch.
> Though is lot of work to be done, I would like to get suggestions/opinions
> from
> hackers.
> I would like to thank Ashutosh Bapat for providing a draft patch and
> helping
> me off-list on this feature while he is busy working on partition-wise join
> feature.
> [1]
> Thanks
> --
> Jeevan Chalke
> Principal Software Engineer, Product Development
> EnterpriseDB Corporation
> The Enterprise PostgreSQL Company

Jeevan Chalke
Principal Software Engineer, Product Development
EnterpriseDB Corporation
The Enterprise PostgreSQL Company

Attachment Content-Type Size
partition-wise-agg-v1.tar.gz application/x-gzip 24.8 KB

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