Re: Let plan_cache_mode to be a little less strict

On Thu, Jul 31, 2025 at 11:52:59AM -0500, Sami Imseih wrote:
> > This means that the configuration parameter (GUC) currently overrides
> > any explicit request for a plan type. The documentation does not clearly
> > explain the rationale behind this, at least to me.
>
> The documentation [0] mentions " If this default behavior is
> unsuitable, you can alter it by
> passing the CURSOR_OPT_GENERIC_PLAN or CURSOR_OPT_CUSTOM_PLAN flag to
> SPI_prepare_cursor". The default behavior here is plan_cache_mode = AUTO.
>
> But the docs leave out the fact that is plan_cache_mode is not AUTO, that the
> GUC settings take precedence over the cursor_options. I agree as well that is
> wrong. I also agree with your fix.
>
> [0] https://www.postgresql.org/docs/current/spi-spi-prepare.html

It seems to me that if we want to keep track of the priority of the
GUC versus the options given to the SPI call, then we should at least
have some tests for this purpose.  I would imagine a test module with
a set of SQL functions that act as wrappers of the SPI calls we want
to test, and arguments that can be given directly in input, using
PG_GETARG_POINTER and PG_RETURN_POINTER for some of them.  We could
also have a function in regress.c, which may be simpler here.  These
functions should be designed to be generic enough, so as they could be
reused for more tests than the ones we'd look at here.
--
Michael