Horton

Re: Partition-wise join for join between (declaratively) partitioned tables

Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>

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From: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
To: Rafia Sabih <rafia.sabih@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Rajkumar Raghuwanshi <rajkumar.raghuwanshi@enterprisedb.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-03-21T11:41:25Z
Lists: pgsql-hackers

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On Mon, Mar 20, 2017 at 10:17 PM, Ashutosh Bapat
<ashutosh.bapat@enterprisedb.com> wrote:
>>
>> On a further testing of this patch I find another case when it is
>> showing regression, the time taken with patch is around 160 secs and
>> without it is 125 secs.
>> Another minor thing to note that is planning time is almost twice with
>> this patch, though I understand that this is for scenarios with really
>> big 'big data' so this may not be a serious issue in such cases, but
>> it'd be good if we can keep an eye on this that it doesn't exceed the
>> computational bounds for a really large number of tables.
>
> Right, planning time would be proportional to the number of partitions
> at least in the first version. We may improve upon it later.
>
>> Please find the attached .out file to check the output I witnessed and
>> let me know if anymore information is required
>> Schema and data was similar to the preciously shared schema with the
>> addition of more data for this case, parameter settings used were:
>> work_mem = 1GB
>> random_page_cost = seq_page_cost = 0.1

this doesn't look good. Why do you set both these costs to the same value?

>
> The patch does not introduce any new costing model. It costs the
> partition-wise join as sum of costs of joins between partitions. The
> method to create the paths for joins between partitions is same as
> creating the paths for joins between regular tables and then the
> method to collect paths across partition-wise joins is same as
> collecting paths across child base relations. So, there is a large
> chance that the costing for joins between partitions might have a
> problem which is showing up here. There may be some special handling
> for regular tables versus child tables that may be the root cause. But
> I have not seen that kind of code till now.
>
> Can you please provide the outputs of individual partition-joins? If
> the plans for joins between partitions are same as the ones chosen for
> partition-wise joins, we may need to fix the existing join cost
> models.

Offlist, Rafia shared the outputs of joins between partitions and join
between partitioned table. The joins between partitions look similar
to those pick up by the partition-wise join. So, it looks that some
costing error in regular joins is resulting in an costing error in
partition-wise join as suspected. Attached the SQL and the output.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company

Commits

  1. Basic partition-wise join functionality.

  2. Assorted preparatory refactoring for partition-wise join.

  3. Teach adjust_appendrel_attrs(_multilevel) to do multiple translations.

  4. Stamp 10beta2.

  5. Eat XIDs more efficiently in recovery TAP test.

  6. Abstract logic to allow for multiple kinds of child rels.

  7. Implement SortSupport for macaddr data type

  8. Attempt to stabilize grouping sets regression test plans.

  9. Teach xlogreader to follow timeline switches

  10. Don't scan partitioned tables.

  11. Fix grammar.

  12. postgres_fdw: Push down FULL JOINs with restriction clauses.

  13. Some preliminary refactoring towards partitionwise join.

  14. contrib/amcheck needs RecentGlobalXmin to be PGDLLIMPORT'ified.

  15. Print test parameters like "foo: 123", and results like "foo = 123".