shopspring/decimal support

[El-76]

Is your feature request related to a problem? Please describe.
We widely use shopspring/decimal as an internal standard to handle fixed-point numbers, so would like to have it supported in the driver.

Describe the solution you'd like
The driver supports Decimal and NullDecimal from the library. I'm ready to provide impementation while need some guidance. If the library couldn't be integrated into the driver as a 3rd party dependency (considered non-standard or whatever), I'll likely fork the driver, so it also would be great if you provide some notes on how to properly implement in such case.

Describe alternatives you've considered
Using Decimal as is - it returns a string in its Value() func, plus implementing our own NullDecimal that mimics NULL by passing some typed value for NULL parameters - "NULL" string for example. Looks ugly and maybe slow due to extra conversions.

Additional context
Despite the library structs support Scanner / Valuer abstraction, straightforward use of NullDecimal from the library breaks for NULL values likely due to type information loss. Also TVPs do not work.

Looks like makeParam requires some knowledge as it has, say, for UniqueIdentifier / NullUniqueIdentifier.

[shueybubbles]

Considering shopspring/decimal doesn't seem to bring in other dependencies, I think it'd be ok to bring it in as another source of parameters and scan output choice. One potential controversy is whether you'd want to have a way to make it the default output type for decimal when scanning to interface{}. Such a model would require some refactoring for an app to configure the default scan type.

[El-76]

In fact we don't have a requirement to change default scan behavior. So, if I prepare a patch that alters serialization only (for various cases - In / Out parameters, TVP, etc) and leaves deserialization unchanged, would it be considered correct / accepted? It would be sort of inconsistent since (de)serialization would be defined in two different places (the driver itself, makeParam likely and Scan func of the decimal library structs), but should be simple.

[shueybubbles]

I don't know how other people in Go development would like it, but if it solves a problem without breaking other people I have no issue with it.

[El-76]

Long story short,

I prepared some preliminary solution and the following questions arise:

1. What strategy to choose for serialization / deserialization?

There are 2 options that I considered:

• Conversion through string - looks like this is the best option available - you don't have to guess / match target type scale / precision for OUT / TVP parameters, the only drawback I think about is potential performance issues. AE won't work for OUT params.
• Native - using native decimal type encoding - will use maximum precision of 38 and scale taken from the Decimal structs themselves (exp field). The only advantage is AE will work for DECIMAL(38, N) parameters. However, comparing to the string conversion method, it may require more knowledge about DB target type from the client.

So now I've chosen the first way.

2. What to do with MONEY type?

Unfortunately, string conversions introduce data loss for OUT MONEY parameters. 0.005 MONEY is converted to "0.00" NVARCHAR(MAX) by default. It seems this (OUT MONEY) is the only case when the data is lost.

The way to overcome is quite easy - just encode OUT MONEY parameters ans typeMoneyN, not strings. However it's unclear how to distinguish MONEY from DECIMAL on the client. I introduced wrapper type struct Money with Decimal field, but the way is quite ugly.

So as a result, I'd like to have your thoughts before I proceed on:

1. Decimal encoding - native vs string
2. MONEY type - how to mark it on the client

Regarding MONEY - I'm aware its discouraged to be used, but we have significant amount of code that uses it.

Thank you!

[El-76]

Made a PR, see: #287

Resembles a bit denisenkom#430 in bulk copy part.

[sblackstone]

FWIW, shopspring/decimal is no longer being actively developed and they suggest alternatives..

shopspring/decimal#324

[El-76]

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