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Money data type in PostgreSQL?

What do people recommend for storing money amounts? I've seen people use
NUMERIC(18,3) and other use NUMERIC(18,4). Which one is more appropriate
and why? This is considering various existing currencies, some having
low rates (like IDR, in which you can have large amount up to hundreds
of trillions) and some high rates (like USD, in which you can have small
amount like 0.1 cent). Are there places/industries which involve values
lower than 0.1 cent?

And what about 'factor' field in currency conversion table? Should I use
REAL, or DOUBLE PRECISION (do we need 15-16 digit precision?) or NUMERIC
(exact numbers). The factor should range between 1E-3 (e.g. converting
IDR to USD) to 1E4 (e.g. converting IDR to pounds/euros).

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dave


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Nov 12 '05
12 17920
Jan Wieck wrote:
You won't get any rounding errors in NUMERIC either. What people should
be concerned of is to find an arbitrary precision package for the
frontend programming language they're using.


What is the definition of a numeric number? I understand (from studying
numeric methods all those years ago) that the base 10 decimal number 0.1
cannot be stored exactly in base 2 floating point, thus my use of
integers - is numeric an arbitrary precision concept?

Regards,
Graham
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Nov 12 '05 #11
It's probably in the documentation, but numeric is stored as a string of
digits (or was it mibbles). In any case, if you wanted to list the weight of
the earth to 20 decimal places, numeric is for you. If you like you can
consider it integer arithmatic except the scaling is handled for you.

The format is NUMERIC(x,y) where x is the total number of digits and y is
the number of decimal places. There are rules about the results of
multiplication and division and such.

Hope this helps,

On Fri, Dec 05, 2003 at 02:39:42PM +0200, Graham Leggett wrote:
Jan Wieck wrote:
You won't get any rounding errors in NUMERIC either. What people should
be concerned of is to find an arbitrary precision package for the
frontend programming language they're using.
What is the definition of a numeric number? I understand (from studying
numeric methods all those years ago) that the base 10 decimal number 0.1
cannot be stored exactly in base 2 floating point, thus my use of
integers - is numeric an arbitrary precision concept?

Regards,
Graham
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Nov 12 '05 #12
Graham Leggett wrote:
Jan Wieck wrote:
You won't get any rounding errors in NUMERIC either. What people should
be concerned of is to find an arbitrary precision package for the
frontend programming language they're using.


What is the definition of a numeric number? I understand (from studying
numeric methods all those years ago) that the base 10 decimal number 0.1
cannot be stored exactly in base 2 floating point, thus my use of
integers - is numeric an arbitrary precision concept?


The PostgreSQL datatype NUMERIC is performing decimal arithmetic. The
original I wrote used to do it string based, with one digit per byte but
stored the number as some sort of BCD, one digit per nibble. Tom Lane
changed that a while back into base 10,000 storage and calculation,
which has the advantages of doing 4 digits per loop and no need to
convert back and forth between the storage and the computational
representation.
Jan

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Nov 12 '05 #13

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