On 11/13/16 18:06, Dmitry Yemanov wrote:
> 11.11.2016 18:26, Dimitry Sibiryakov wrote:
>
>>> - Added new datatypes: DECFLOAT(16) and DECFLOAT(34), using 64/128 bits
>>> for numbers representation.
>> What is the point of these new types? Cannot you just expand list of
>> back-end storage
>> for standard DECIMAL?
> This is my concern too. What is the user-visible difference between
> NUMERIC(15) based on blr_int64 and DECFLOAT(15) based on blr_dec64? Are
> both arithmetics compatible and if not then why? I bet we have a demand
> for longer standard NUMERICs rather than for non-standard DECFLOAT.
Decimal float is certainly more than numeric. It's much closer to
floating point number - but free from binary exponent disadvantage:
SQL> CREATE TABLE TESTDECFLOAT
CON> (
CON> FEE_DECFLOAT DECFLOAT(16),
CON> FEE_REAL REAL,
CON> PERCENTAGE DECFLOAT(16)
CON> );
SQL>
SQL> INSERT INTO TESTDECFLOAT VALUES ( 0.70, 0.70, 0.05);
SQL>
SQL> SELECT * FROM TESTDECFLOAT;
FEE_DECFLOAT FEE_REAL PERCENTAGE
======================== ============== ========================
0.70 0.69999999 0.05
SQL>
> Moreover, what are we going to do when people ask as for precisions
> beyond the 34 decimal digits? Introduce blr_dec256/blr_dec512/etc or
> switch to blr_varydec backed by decNumber (and probably stored as packed
> BCD)? Are there any reasons why the current implementation doesn't
> follow this way other than hardware accelerated computations for 64/128
> bits?
>
Use of unlimited length fields is certainly great but I suppose we
should switch to it in future versions (including unlimited length
strings). Better precision calculations are needed right now, in v.4.
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