I've been looking at the performance of SPI calls within plpython.
There's a roughly 1.5x difference from equivalent python code just in
pulling data out of the SPI tuplestore. Some of that is due to an
inefficiency in how plpython is creating result dictionaries, but fixing
that is ultimately a dead-end: if you're dealing with a lot of results
in python, you want a tuple of arrays, not an array of tuples.
While we could just brute-force a tuple of arrays by plowing through the
SPI tuplestore (this is what pl/r does), there's still a lot of extra
work involved in doing that. AFAICT there's at least 2 copies that
happen between the executor producing a tuple and it making it into the
tuplestore, plus the tuplestore is going to consume a potentially very
large amount of memory for a very short period of time, before all the
data gets duplicated (again) into python objects.
It would be a lot more efficient if we could just grab datums from the
executor and make a single copy into plpython (or R), letting the PL
deal with all the memory management overhead.
I briefly looked at using SPI cursors to do just that, but that looks
even worse: every fetch is executed in a subtransaction, and every fetch
creates an entire tuplestore even if it's just going to return a single
value. (But hey, we never claimed cursors were fast...)
Is there any way to avoid all of this? I'm guessing one issue might be
that we don't want to call an external interpreter while potentially
holding page pins, but even then couldn't we just copy a single tuple at
a time and save a huge amount of palloc overhead?
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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