On 12/20/16 10:14 PM, Jim Nasby wrote:
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?

AFAICT that's exactly how DestRemote works: it grabs a raw slot from the executor, makes sure it's fully expanded, and sends it on it's way via pq_send*(). So presumably the same could be done for SPI, by creating a new CommandDest (ISTM none of the existing ones would do what we want).

I'm not sure what the API for this should look like. One possibility is to have SPI_execute and friends accept a flag that indicates not to build a tupletable. I don't think a query needs to be read-only to allow for no tuplestore, so overloading read_only seems like a bad idea.

Another option is to treat this as a "lightweight cursor" that only allows forward fetches. One nice thing about that option is it leaves open the possibility of using a small tuplestore for each "fetch", without all the overhead that a full blown cursor has. This assumes there are some use cases where you want to operate on relatively small sets of tuples at a time, but you don't need to materialize the whole thing in one shot.
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
855-TREBLE2 (855-873-2532)

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