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. In some ways, Postgres-XC has faced (and is still facing) similar challenges and they have been partially solved.
I'm not sure what the specific answer here should look like. Simply having a CREATE FUNCTION ... PARALLEL_IS_FINE flag is not entirely satisfying, because the rules are liable to loosen over time.
Having a flag would be enough to control parallelism, but cannot we also determine if the execution of a function can be shipped safely to a worker based on its volatility only? Immutable functions are presumably safe as they do not modify the database state and give always the same result, volatile and stable functions are definitely not safe.
For such reasons, it would be better to keep things simple and rely on simple rules to
determine if a given expression can be executed safely on a backend worker.
