Conversion error of floating point numbers in pl/pgsql

Hello. I found that 9.5 has an undocumented difference from 9.4
in type cast in pl/pgsql and I think it might better be mentioned
as a change of behavior in release notes.

Whether do you think it is worth mentioning or not in release notes?

=====
9.4 and 9.5 has difference in casting between floating point
numbers.

CREATE OR REPLACE FUNCTION hoge () RETURNS text AS $$ DECLARE vr real; vf8 float8; BEGIN vr := 0.1; vf8 = vr; RETURN 'hoge = '|| vf8 ; END; $$ LANGUAGE plpgsql;

9.5=# select hoge();
hoge
--------------------------
hoge = 0.100000001490116

9.4=# select hoge();
hoge
------------
hoge = 0.1

This is stemming from the difference between '0.1'::real::float8
and '0.1'::real::text::float8, made in exec_cast_value().

=# select '0.1'::real::float8, '0.1'::real::text::float8;
float8 | float8
-------------------+--------
0.100000001490116 | 0.1

This example itself looks somewhat artifitial but it would be
rather common to load real values in a table into float8
variables in a function for further calculations.

This is a side effect of the commit
1345cc67bbb014209714af32b5681b1e11eaf964 and the relase notes has
the following discription corresponds to this commit in the
Migration section, with no mention of this.

> Use assignment cast behavior for data type conversions in
> PL/pgSQL assignments, rather than converting to and from text
> (Tom Lane)
>
> This change causes conversions of Booleans to strings to produce
> true or false, not t or f. Other type conversions may succeed in
> more cases than before; for example, assigning a numeric value
> 3.9 to an integer variable will now assign 4 rather than
> failing. If no assignment-grade cast is defined for the
> particular source and destination types, PL/pgSQL will fall back
> to its old I/O conversion behavior.

Whether do you think it is worth mentioning or not?

Though the attached patch adds a description for that, it should
be rewritten even if this is worth mentioning.

+ This change also may bring different results of type casts
+ between floating point numbers having different conversion
+ errors.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center