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plpgsql has always executed the query given in a RETURN QUERY command by opening it as a cursor and then fetching a few rows at a time, which it turns around and dumps into the function's result tuplestore. The point of this was to keep from blowing out memory with an oversized SPITupleTable result (note that while a tuplestore can spill tuples to disk, SPITupleTable cannot). However, it's rather inefficient, both because of extra data copying and because of executor entry/exit overhead. In recent versions, a new performance problem has emerged: use of a cursor prevents use of a parallel plan for the executed query. We can improve matters by skipping use of a cursor and having the executor push result tuples directly into the function's result tuplestore. However, a moderate amount of new infrastructure is needed to make that idea work: * We can use the existing tstoreReceiver.c DestReceiver code to funnel executor output to the tuplestore, but it has to be extended to support plpgsql's requirement for possibly applying a tuple conversion map. * SPI needs to be extended to allow use of a caller-supplied DestReceiver instead of its usual receiver that puts tuples into a SPITupleTable. Two new API calls are needed to handle both the RETURN QUERY and RETURN QUERY EXECUTE cases. I also felt that I didn't want these new API calls to use the legacy method of specifying query parameter values with "char" null flags (the old ' '/'n' convention); rather they should accept ParamListInfo objects containing the parameter type and value info. This required a bit of additional new infrastructure since we didn't yet have any parse analysis callback that would interpret $N parameter symbols according to type data supplied in a ParamListInfo. There seems to be no harm in letting makeParamList install that callback by default, rather than leaving a new ParamListInfo's parserSetup hook as NULL. (Indeed, as of HEAD, I couldn't find anyplace that was using the parserSetup field at all; plpgsql was using parserSetupArg for its own purposes, but parserSetup seemed to be write-only.) We can actually get plpgsql out of the business of using legacy null flags altogether, and using ParamListInfo instead of its ad-hoc PreparedParamsData structure; but this requires inventing one more SPI API call that can replace SPI_cursor_open_with_args. That seems worth doing, though. SPI_execute_with_args and SPI_cursor_open_with_args are now unused anywhere in the core PG distribution. Perhaps someday we could deprecate/remove them. But cleaning up the crufty bits of the SPI API is a task for a different patch. Per bug #16040 from Jeremy Smith. This is unfortunately too invasive to consider back-patching. Patch by me; thanks to Hamid Akhtar for review. Discussion: https://postgr.es/m/16040-eaacad11fecfb198@postgresql.org |
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