Re: Parallel Sort

On Tue, May 14, 2013 at 01:51:42PM +0900, Michael Paquier wrote:
> On Mon, May 13, 2013 at 11:28 PM, Noah Misch <noah@leadboat.com> wrote:
>
> > * Identifying Parallel-Compatible Functions
> >
> > Not all functions can reasonably run on a worker backend.  We should not
> > presume that a VOLATILE function can tolerate the unstable execution order
> > imposed by parallelism, though a function like clock_timestamp() is
> > perfectly
> > reasonable to run that way.  STABLE does not have that problem, but neither
> > does it constitute a promise that the function implementation is compatible
> > with parallel execution.  Consider xid_age(), which would need code
> > changes to
> > operate correctly in parallel.  IMMUTABLE almost guarantees enough; there
> > may
> > come a day when all IMMUTABLE functions can be presumed parallel-safe.  For
> > now, an IMMUTABLE function could cause trouble by starting a (read-only)
> > subtransaction.  The bottom line is that parallel-compatibility needs to be
> > separate from volatility classes for the time being.
> >
> I am not sure that this problem is only limited to functions, but to all
> the expressions
> and clauses of queries that could be shipped and evaluated on the worker
> backends when
> fetching tuples that could be used to accelerate a parallel sort. Let's
> imagine for example
> the case of a LIMIT clause that can be used by worker backends to limit the
> number of tuples
> to sort as final result.

It's true that the same considerations apply to other plan tree constructs;
however, every such construct is known at build time, so we can study each one
and decide how it fits with parallelism.

Since functions are user-definable, it's preferable to reason about classes of functions.