Re: Parallel Append implementation

From: Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Rafia Sabih <rafia(dot)sabih(at)enterprisedb(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Parallel Append implementation
Date: 2017-09-20 06:02:36
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On 11 September 2017 at 18:55, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> 1.
>>> + else if (IsA(subpath, MergeAppendPath))
>>> + {
>>> + MergeAppendPath *mpath = (MergeAppendPath *) subpath;
>>> +
>>> + /*
>>> + * If at all MergeAppend is partial, all its child plans have to be
>>> + * partial : we don't currently support a mix of partial and
>>> + * non-partial MergeAppend subpaths.
>>> + */
>>> + if (is_partial)
>>> + return list_concat(partial_subpaths, list_copy(mpath->subpaths));
>>> In which situation partial MergeAppendPath is generated? Can you
>>> provide one example of such path?
>> Actually currently we don't support partial paths for MergeAppendPath.
>> That code just has that if condition (is_partial) but currently that
>> condition won't be true for MergeAppendPath.
> I think then it is better to have Assert for MergeAppend. It is
> generally not a good idea to add code which we can never hit.


> One more thing, I think you might want to update comment atop
> add_paths_to_append_rel to reflect the new type of parallel-aware
> append path being generated. In particular, I am referring to below
> part of the comment:
> * Similarly it collects partial paths from
> * non-dummy children to create partial append paths.
> */
> static void
> add_paths_to_append_rel()

Added comments.

Attached revised patch v15. There is still the open point being
discussed : whether to have non-parallel-aware partial Append path, or
always have parallel-aware Append paths.

-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company

Attachment Content-Type Size
ParallelAppend_v15.patch application/octet-stream 58.5 KB

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