Michael Paquier <mich...@paquier.xyz> writes:
> On Thu, Feb 22, 2018 at 08:00:45PM -0500, Demitri Muna wrote:
>> Out of curiosity, how does one read a numeric type from within a C
>> extension (i.e. get a number value out of the Datum type)?


I think the core point here is that PG's "numeric" type *isn't* any
primitive C type; it's a variable-length BCD encoding.  If you want
to deal with it you can, but you must spend some time reading
src/backend/utils/adt/numeric.c, as Michael suggests.  If you just
want a convenient C approximation, then work with float8 and let
the implicit-coercion machinery do the conversion for you.

                        regards, tom lane

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