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Re: Money casting too liberal?

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On 30/03/13 11:30, Gavan Schneider wrote:
On 29/3/13 at 3:32 AM, D'Arcy J.M. Cain wrote:

On Fri, 29 Mar 2013 11:46:40 -0400 Tom Lane wrote:
Well, this has been discussed before, and the majority view every
time has been that MONEY is a legacy thing that most people would
rather rip out than sink a large amount of additional effort into.

The only reason I have tried to explore these ideas is that the type is currently too quirky for most use cases. So I must agree that remove/ignore is the least work option. An argument for making the type more useful can be made by analogy to the geolocation add-in type. Most never go there but those who need to do so seem to prefer the builtin functionality over hand coding the same behaviour with columns of arrays that just happen to contain location data.

It has some use-cases but they are narrow, and it's not clear how
much wider the use-cases would be if we tried to generalize it.

A well designed and specific tool can be worth the effort.

The use cases include:

Financial data, accounts in a single currency, i.e., the money column in a transaction

Multi currency data, i.e., keeping track of transactions across several currencies. specifically we are NOT doing conversions, what arrives/leaves as $ or ¥ stays that way, this implies the dB has tables for each area of operation or columns for each currency

One thing the type should not attempt or allow any implicit transforming of alues. Mostly a currency change is a transaction and whenever it happens it has to be recored as such, e.g., so many ¥ leave their column, appropriate $ are added to their column, and commission $/¥ is added to its column, also included will be: exchange rate reference time-stamp journal reference, etc. A constraint could be constructed to ensure the double entry book keeping zero sum convention has been maintained across the whole transaction.

One time this might not be so detailed is for a VIEW where something akin to total worth is being reported. In cases like this the exchange rates would usually be in their table and the business rules would dictate which one is to be used to build the VIEW, e.g., end of month report, and it might be shown with all values in a single currency depending on the company's HQ.


I wonder if our vision isn't a little tunneled here.  Using this type
for money is, perhaps, a specialized use and the type should really be
called something else and modified to remove all connotations of money
from it.  So...


- Drop the currency symbol
- Allow number of decimals to be defined once for the column
- Don't use locale except to specify decimal separator (',' vs. '.')

Mostly this is cosmetic and only relevant for parsing text on data entry or default formatting with SELECT on the command line. The power of the class is that none of this is in the data other than as dB column flags. The values themselves are integer. The class is meant to keep the books moving right along.

- Allow operations against numeric

Whatever else is done this should happen.

Not sure what to rename it to.  Decimal would be good if it wasn't
already in use.  Maybe DecimalInt.

I don't think there is much use for another fixed precision integral type. NUMERIC does a good job when INTEGER isn't suitable. If this exercise is worth anything then MONEY should just do its job better so people who track money (and there is an awful lot of them) will find it useful.

My own experience with this sort of thing leads me to think that
real applications dealing with a variety of currencies will be
needing to store additional details, such as the exact exchange
rate that applied to a particular transaction.  So while merely

Seems like something that can be stored in a different column.

Exactly. We to think this through as would a real user.

If the business is receiving money from multiple regions then there will be rows which show the currency, number of units (numeric type since the column is not devoted to a specific currency), transaction tracing data, exchange reference (another table), amt_received::MONEY('USD','D2'), amt_transaction_fee::MONEY('USD','D3'), etc.

Within the accounts of the organisation the MONEY columns are likely to be in a single currency with movements between ledgers in the time honoured fashion of adding to this while removing the same from other(s) so all money entries add to zero across the row. Movements between currencies are just another transaction as detailed above.

I have sketched something of a notation for MONEY columns along these lines:

    amt_received MONEY (    CURRENCY    -- e.g., 'USD' 'AUD' 'YEN' ...
[,SCALE -- default as per currency, e.g. USD 2 decimals -- but could be used to see money in bigger units -- such as '000s (e.g., that end-of-month view) [,ROUND -- need to allow for multiple rules here, sometimes -- cents are just dropped, otherwise it can be -- required that rounding is up or down
                            [,OTHER?
                        ]]])

I have left the display characteristics out (they could be there as a default) but column values are going to be displayed however the application wants them, and this only applies at the time of reporting. Each currency can carry the conventional defaults and the application should have formatting tools to alter this during output.

Inputting money values, i.e., text to MONEY should follow the conventions of the target currency. Specifically the input conversion routine should handle the symbol (or no symbol) and all the usual conventions for negative values, decimals and separators. It should throw an error if asked to add a value to a USD column but finds a yen symbol in the text. (There is no such help for all of us sharing the $ symbol. :) Also it should parse such things as 123.456,00 (Europe) and 123,456.00 (Anglo) properly. Errors need to be thrown when it looks wrong 123,456.789.00 -- since this is likely to be corrupted data, and finally gets me back to the issue raised by OP. :)

Hope this hasn't been too much of a ramble.

Regards, and happy (Western) Easter to all,
Gavan Schneider



While for my current project I don't need this, I've dealt with money many times, so it would be great to have a proper money type when I next needed to handle money in a database (which is bound to happen sooner or later!)!

How about a picture clause for display - encodes details about how to handle negative numbers.

QUESTION:
How best to handle the difference between English and European conventions for thousand separators:
 English convention: NZ$1,000,005.34
European convention: NZ$1.000.005,34
note the use of '.' & ',' are swapped!

QUESTION:
Should the type of $ (NZ$ vs US$) be dropped if it matches the locale?


Cheers,
Gavin





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