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When a PL/Python procedure is executed, the python code is executed by a Python interpreter. My question is, is the Python interpreter running as a separate process, or is it a shared library that gets linked to the calling databases process?

I'm concerned about what happens when we call something like plpy.execute(...). If the python interpreter is running as a separate process I imagine there would be a lot of overhead involved in passing the result of the sql query back to the python interpreter, which would require reading from a file or pipe.

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The language handler function (plpython3_call_handler()) loads the plpython3.so library into the PostgreSQL process, which is linked to libpython3.so. So the interpreter is loaded into the backend, it is not executed as a separate process (multiprocessing/multithreading is not allowed in PostgreSQL client backends, with the exception of parallel workers).

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Thanks that answers my question. Are there other sources of overhead when using PL/Python other than those specific to executing python code in the first place?
There is always overhead. Every function call is overhead, and there is definitely an overhead in loading and initializing the Python interpreter the first time you call a PL/Python function in a database session.
I played around with PL/Python a bit and I can see executing queries from within pl/python functions is particularly slow. Makes me wonder how data is passed between the backend and python interpreter. I posted a separate question if you are willing to look: stackoverflow.com/questions/73657548/…

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