Re: float bug? perl 5.8, DBI and oracle 10.2.0

On Wed, Jul 18, 2007 at 10:58:58AM -0400, Rutherdale, Will wrote:
Tim, I have a couple of feedback comments on your text.

A) I would not characterise 32-bit signed integers as giving 10 digits
of precision as you did. They give log10( 2^31 ) ~= 9.3319 digits of
precision. Since you can't count on the full 10th digit, I would
truncate and tell people you get 9 digits of precision. Similarly a
signed 64-bit integer gives 18.9649 or really just 18 digits of
precision to be safe, but 128-bit signed integers give you a full 38
digits.

B) long double is not usually 96 bits, but rather 80 bits. Most
machines that people use follow the IEEE 754 standard, which says >=79
bits but is normally implemented as 80 bits.

Okay.

Good explanation generally.

Thanks for the feedback Will. I'll make some changes.

For the general list, I'm still interested in the issue of alternative
representations for financial work. Has anyone had much experience with
the ( NUMERIC , Math::BigFloat ) combination? Is that used generally by
people in the field?

Might be worth meditating on that question over at http://perlmonks.org

Tim.


-Will


-----Original Message-----
From: Tim Bunce [mailto:Tim.Bunce@xxxxxxxxx]
Sent: Tuesday 17 July 2007 19:02
To: Christopher Sarnowski
Cc: erwan@xxxxxxxxxxxx; dbi-users@xxxxxxxx
Subject: Re: float bug? perl 5.8, DBI and oracle 10.2.0

. . .

Funnily enough I wrote a section on this topic back in May
2006 for the
2nd edition of DBI book (which is currently shelved, by the way).
I've appended the relevant chunk of the rough draft. Comments welcome.

Tim.

. . .

=head3 Perl Integer Values

Integers are typically stored as 32 or 64 bit (4 or 8 byte) values
depending on how perl was configured. You can check the size
of integers
in your perl by running C<perl -V> and looking for C<ivsize=>
in the output.
The range of a 32 bit integer is -2,147,483,648 to 2,147,483,647
(10 digits of precision). The range of a 64 bit integer is
-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 (19 digits
of precision).
. . .
=head3 Perl Floating Point Values

Floating point values are typically stored in 64 bits or sometimes 96
bits (that's 8, or 12 bytes) depending on how your perl was
configured.
You can check the size used in your perl by running C<perl -V> and
looking for C<nvsize=> in the output. The 64 bit floats are known as
I<doubles> and have approximately 15 digits of precision
between 1e-307
to 1e+308, and the 96 bit floats are known as I<long doubles> and have
approximately 18 digits of precision between 1e-4931 to 1e+4932. Some
systems support 128 bit I<quad doubles> with even greater
precision and
scale.

It's becoming more common for perl to be configured with 64 bit
integers but still using 64 bit floating point values. But a 64 bit
integer has 19 digits of precision whereas a 64 bit floating point
value only has approximately 18. This is important to know because it
means that a large integer may loose precision if it's involved in a
calculation that causes it to be converted to a floating point value
(which is basically anything more involved that addition or
subtraction
of another integer).
. . .

Some databases support very large integers. Oracle, for
example, supports
integers with 38 digits of precision. That's far beyond the
10 digits of a
simple 32bit integer and even the 19 digits of a 64bit
integer. Because of
this the DBD::Oracle driver returns integers, and indeed all
other numberic
types, as strings.

The DBI will probably gain a way to hook into the fetching of a
value from the database so that the use of Math::BigInt, for example,
can be made transparent and not clutter up the code. But that hasn't
happened yet. XXX

Many databases support multiple sizes of integer types from 1 to 8
bytes in size with INTEGER (4 bytes) and SMALLINT (2 bytes) being
the most common.N<Oracle is a little unusual in that it only has
one underlying numeric type which is variable width and all other
numeric types are aliases for it.>




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