On 21-1-2018 21:48, Mark Rotteveel wrote:
I think there are still a number of issues or deficiencies with the
DECFLOAT implementation that I think need to be addressed and fixed.
I'm not sure of the best way to address these. Please let me know how to
move forward with these points (create/update tickets in Jira, further
discussion, etc)?
Below is an inventory of the problems I think need addressing; I might
come up with others later.
[..]
2. Underflow should yield zero, not an error
Currently, underflow conditions raise an error, instead it should round
to zero.
The SQL:2016 specification in 5.3 <literal> and 6.13 <cast
specification> explicitly mention that underflow with literals or casts
should yield zero, not an exception, so for example a literal 1.0E-7000
or cast('1.0E-7000' as decfloat(34)) (which is too small to be
represented) should not raise an errors, but instead yield 0.
The wording in section 6.29 <numeric value expression> at first glance
would seem to suggest that we would need to raise an exception if the
result of an expression is an underflow, but I don't think that is how
it is intended and instead we should yield zero as well. Similar to what
happens with calculations involving real and double precision.
This would be allowed because of the phrasing "If the most specific type
MST of the result of an arithmetic operation is the decimal floating
point type, then the result is an implementation-defined value IV of MST
such that no other value of MST is strictly between IV and the
mathematical result MR of the operation."
In the case of calculations that yield such a small value, there is no
other value than 0 that is "strictly between IV and the mathematical
result MR of the operation". This would also nicely fit with the
behaviour described for literals and cast
Tickets:
CORE-5700 - DECFLOAT underflow should yield zero instead of an error
The definition in 4.4.2 Characteristics of numbers is probably clearer
support for the fact this also applies to the result of calculations:
"""
An approximation obtained by truncation or rounding of a numeric value N
for an approximate numeric type T or the decimal floating-point type T
is a value V in T such that there is no numeric value in T distinct from
that of V that lies between the numeric value of V and N, inclusive.
If there is more than one such value V then it is implementation-defined
which one is taken. It is implementation-defined which numeric values
have approximations obtained by rounding or truncation for a given
approximate numeric type or a given decimal floating-point type.
Whenever a numeric value is assigned to an approximate numeric value
site or a decimal floating-point value site, an approximation of its
value is represented in the declared type of the target. The value is
converted to have the precision of the target.
"""
Mark
--
Mark Rotteveel
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