Re: Parallel Sort

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 &
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