| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | Noah Misch <noah(at)leadboat(dot)com> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Parallel Sort |
| Date: | 2013-05-13 14:39:01 |
| Message-ID: | 20130513143901.GC27618@awork2.anarazel.de |
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| Thread: | |
| Lists: | pgsql-hackers |
Hi,
Interesting! Need to think about most, but one piece immediately came to
mind:
On 2013-05-13 10:28:59 -0400, Noah Misch wrote:
> Each worker needs to make SnapshotNow visibility decisions coherent with the
> master. For sorting, this allows us to look up comparison functions, even
> when the current transaction created or modified those functions.
I don't really see how you can achieve that given how SnapshotNow
works. There's nothing protecting you against one backend seeing changes
made by another transaction while another doesn't see them. SnapshotNow
doesn't even guarantee consistency within a single backend during a
single scan...
If you are meaning the above to just apply to changes made by the local
"master" backend, sure I can see that.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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