| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
| Cc: | Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Gather Merge |
| Date: | 2016-10-17 13:48:07 |
| Message-ID: | CA+TgmoYAmHV5MKefwj=2Eg5MrS7Nqe5xwBLiVy520DUdRWtBwg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Oct 17, 2016 at 4:56 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> + node->nreaders < 2)
...
> I see there are small discrepancies in both the codes like I don't see
> the use of single_copy flag, as it is present in gather node.
single_copy doesn't make sense for GatherMerge, because the whole
point is to merge a bunch of individually-sorted output streams into a
single stream. If you have only one stream of tuples, you don't need
to merge anything: you could have just used Gather for that.
It does, however, make sense to merge one worker's output with the
leader's output.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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