Numeric value does not fit in a System.Decimal (value less than decimal.MaxValue)

I first looked at posts with a similar issues, for example

https://github.com/npgsql/efcore.pg/issues/438 https://github.com/npgsql/npgsql/issues/3238 https://github.com/npgsql/npgsql/issues/448

and didn’t find a solution to my issue

Introductory information

There are many type fields in our database: numeric (56,28) and numeric (55,19), everything worked fine in Npgsql 3.7 But when we updated the Npgsql version to 4.0/4.1/5 then some!!! fields have ceased to be correctly retrieved from the database below is an example of such a value

Steps to reproduce

for example:

1. database field: numeric (55,19)
2. save the value in the field: 99999999999
3. if we make a select from this field, we get: 99999999999.0000000000000000000

simple code for reproduce:

using (var dbConn = new NpgsqlConnection("..."))
{
	dbConn.Open();
	var cmd = new NpgsqlCommand("select 99999999999.0000000000000000000::numeric", dbConn);
	var reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess);
	reader.Read();
	reader.GetValue(0);
}

The issue

We get an exception in any Npgsql above version 3.7 In Npgsql version 3.7 everything works fine

Exception message: System.OverflowException: 'Numeric value does not fit in a System.Decimal'
Stack trace:
Stack trace: 
This exception was originally thrown at this call stack:
    Npgsql.Internal.TypeHandlers.NumericHandlers.DecimalRaw.Multiply(ref Npgsql.Internal.TypeHandlers.NumericHandlers.DecimalRaw, uint) in DecimalRaw.cs
    Npgsql.Internal.TypeHandlers.NumericHandlers.NumericHandler.Read(Npgsql.Internal.NpgsqlReadBuffer, int, Npgsql.BackendMessages.FieldDescription) in NumericHandler.cs
    Npgsql.Internal.TypeHandling.NpgsqlSimpleTypeHandler<TDefault>.Read<TAny>(Npgsql.Internal.NpgsqlReadBuffer, int, Npgsql.BackendMessages.FieldDescription) in NpgsqlSimpleTypeHandler.cs
    Npgsql.Internal.TypeHandling.NpgsqlSimpleTypeHandler<TDefault>.Read<TAny>(Npgsql.Internal.NpgsqlReadBuffer, int, bool, Npgsql.BackendMessages.FieldDescription) in NpgsqlSimpleTypeHandler.cs
    [External Code]
    Npgsql.Internal.TypeHandling.NpgsqlTypeHandler<TDefault>.ReadAsObject(Npgsql.Internal.NpgsqlReadBuffer, int, bool, Npgsql.BackendMessages.FieldDescription) in NpgsqlTypeHandler`.cs
    [External Code]
    Npgsql.NpgsqlDataReader.GetValue(int) in NpgsqlDataReader.cs
    ConsoleApp1.Program.Main(string[]) in Program.cs

Further technical details

Npgsql version: Npgsql 4.0/4.1/5.0 PostgreSQL version: PostgreSQL 10.8 Operating system: Windows 10

https://github.com/npgsql/npgsql/blob/v4.1.4/src/Npgsql/TypeHandlers/NumericHandlers/DecimalRaw.cs Screenshot with error:

[roji]

Thanks @YohDeadfall!

[YohDeadfall]

@SomeSpy, you have to change the following method:

https://github.com/npgsql/npgsql/blob/0f80008ce02415553a0a9b82f449afb45a04acb6/src/Npgsql/Internal/TypeHandlers/NumericHandlers/NumericHandler.cs#L42-L104

As you can see it throws on an overflow, but should not in your case. Therefore, tweak the logic in a such way that reading fractional part after reaching the maximum capacity of decimal will just take the first overflowing group, check that the digit at the overflowing position is less or equal to 5 and add 1 to the result in that case. Otherwise, do nothing. Read left groups till the end, but discard values.

You should find in other threads how PostgreSQL stores and handles numerics, but to be short, they are stored in groups of 4 decimal digits. Each group is multiplied by 10^(4 * g) where g is the number of groups minus the current group index. So 1234567 can be represented as 0123 and 4567. The scale complicates the logic a bit, but no so much.