On Thu, Dec 18, 2014 at 11:51:12PM -0500, Tom Lane wrote:
> If you're doing any sort of higher math or statistics, I stand by my
> statement that you'd better think rather than just blindly assume that
> numeric is going to be better for you. A moment's fooling about finds
> this example, which is pretty relevant to the formula we started this
> thread with:
>
> regression=# select (1234::numeric/1235) * 1235;
> ?column?
> ---------------------------
> 1234.00000000000000000100
> (1 row)
>
> regression=# select (1234::float8/1235) * 1235;
> ?column?
> ----------
> 1234
> (1 row)
>
> What it boils down to is that numeric is great for storing given decimal
> inputs exactly, and it can do exact addition/subtraction/multiplication
> on those too, but as soon as you get into territory where the result is
> fundamentally inexact it is *not* promised to be better than float8.
> In fact, it's designed to be more or less the same as float8; see the
> comments in select_div_scale.
Based on the analysis above, I have written the attached patch to the
NUMERIC docs to mention this.
--
Bruce Momjian <[email protected]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
new file mode 100644
index edf636b..4a65971
*** a/doc/src/sgml/datatype.sgml
--- b/doc/src/sgml/datatype.sgml
***************
*** 498,508 ****
<para>
The type <type>numeric</type> can store numbers with a
! very large number of digits and perform calculations exactly. It is
! especially recommended for storing monetary amounts and other
! quantities where exactness is required. However, arithmetic on
! <type>numeric</type> values is very slow compared to the integer
! types, or to the floating-point types described in the next section.
</para>
<para>
--- 498,510 ----
<para>
The type <type>numeric</type> can store numbers with a
! very large number of digits. It is especially recommended for
! storing monetary amounts and other quantities where exactness is
! required. Calculations with <type>numeric</type> values yield exact
! results where possible, e.g. addition, subtraction, multiplication.
! However, calculations on <type>numeric</type> values is very slow
! compared to the integer types, or to the floating-point types
! described in the next section.
</para>
<para>
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