Re: Jsonb transform for pl/python

Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com> writes:
> On 6/23/18 01:44, Nikita Glukhov wrote:
>> We are simply trying first to convert numeric to int64 if is does not have
>> digits after the decimal point, and then construct Python Int instead of
>> Decimal. Standard Python json.loads() does the same for exact integers.

> We just had a very similar but not identical discussion in the thread
> about the PL/Perl transforms, where we rejected the proposal. Other
> comments on this one?

I can sympathize with the speed concern, but the proposed patch produces
a functional change (ie, you get a different kind of Python object, with
different behaviors, in some cases). That seems to me to be a good reason
to reject it. The fact that it makes the behavior vary depending on the
local width of "long" is also a problem, although I think that could be
fixed.

More generally, I'd be happier with a patch that sped things up for
non-integers too. I don't know whether Python exposes enough internals
of type Decimal to make that practical, though. One idea for doing it at
arm's length is to compute the Decimal value using numeric-digit-at-a-time
arithmetic, roughly

x := 0;
foreach(digit, numeric)
x.fma(10000, digit);
x.scaleb(appropriate-exponent);

In principle that's probably faster than string conversion, but not
sure by how much.

regards, tom lane