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: | 2013-05-15 18:49:00 |
Message-ID: | 20130515184900.GA22783@awork2.anarazel.de |
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Thread: | |
Lists: | pgsql-hackers |
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. This will
> also be an essential building block for any parallelism project that consults
> user tables. Implementing this means copying the subtransaction stack and the
> combocid hash to each worker. For the sake of completeness, we should also
> copy the global MVCC snapshot data (sorting probably won't care). It also
> means forbidding, while a parallel task is in flight, operations that affect
> the transaction state:
Btw, if you assume you can simply copy a snapshot from the normal
backend to the worker backend to make visibility decisions in the
general case: You're wrong. Unfortunately you need in-memory state to
make sense of combocids...
Not impossible to solve, but you should be aware of the issue.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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