Re: [HACKERS] Partition-wise aggregation/grouping

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
Cc: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Rajkumar Raghuwanshi <rajkumar(dot)raghuwanshi(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Partition-wise aggregation/grouping
Date: 2018-03-21 21:56:52
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On Wed, Mar 21, 2018 at 11:33 AM, Jeevan Chalke
<jeevan(dot)chalke(at)enterprisedb(dot)com> wrote:
> Let me try to explain this:
> 2. extra->is_partial_aggregation
> 3. extra->perform_partial_partitionwise_aggregation

Please find attached an incremental patch that attempts to refactor
this logic into a simpler form. What I've done is merged all three of
the above Booleans into a single state variable called 'patype', which
When create_ordinary_grouping_paths() is called, extra.patype is the
value for the parent relation; that function computes a new value and
passes it down to create_partitionwise_grouping_paths(), which inserts
into the new 'extra' structure for the child.

Essentially, in your system, extra->is_partial_aggregation and
extra->perform_partial_partitionwise_aggregation both corresponded to
whether or not patype == PARTITIONWISE_AGGREGATE_PARTIAL, but the
former indicated whether the *parent* was doing partition-wise
aggregation (and thus we needed to generate only partial paths) while
the latter indicated whether the *current* relation was doing
partition-wise aggregation (and thus we needed to force creation of
partially_grouped_rel). This took me a long time to understand
because of the way the fields were named; they didn't indicate that
one was for the parent and one for the current relation. Meanwhile,
GROUPING_CAN_PARTITIONWISE_AGG indicated whether partition-wise
aggregate should be tried at all for the current relation; there was
no analogous indicator for the parent relation because we can't be
processing a child at all if the parent didn't decide to do
partition-wise aggregation. So to know what was happening for the
current relation you had to look at GROUPING_CAN_PARTITIONWISE_AGG +
extra->perform_partial_partitionwise_aggregation, and to know what was
happening for the parent relation you just looked at
extra->is_partial_aggregation. With this proposed refactoring patch,
there's just one patype value at each level, which at least to me
seems simpler. I tried to improve the comments somewhat, too.

You have some long lines that it would be good to break, like this:

child_extra.targetList = (List *) adjust_appendrel_attrs(root,

(Node *) extra->targetList,

If you put a newline after (List *), the formatting will come out
nicer -- it will fit within 80 columns. Please go through the patches
and make these kinds of changes for lines over 80 columns where

I guess we'd better move the GROUPING_CAN_* constants to a header
file, if they're going to be exposed through GroupPathExtraData. That
can go in some refactoring patch.

Is there a good reason not to use input_rel->relids as the input to
fetch_upper_rel() in all cases, rather than just at subordinate

Robert Haas
The Enterprise PostgreSQL Company

Attachment Content-Type Size
0001-Refactor-partitonwise-aggregate-signalling.patch application/octet-stream 16.2 KB

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