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
Data in Trouble? Get it in Treble! http://BlueTreble.com
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