Npgsql如何处理演员类型

Question

I've noticed the following behavior:

In pgAdmin I perform this query and I get this result:

SELECT 
CAST(COALESCE(c.latitude, '-1') as varchar),
CAST(COALESCE(c.longitude, '-1') as varchar) 
FROM mytable c WHERE c.acolumn = 'S' AND c.anothercolumn=1

---------------------------------
coalesce            coalesce
character varying   character varying
"-25.30089"        |  "-57.625866"

I use Npgsql version 2.2.4.3 in a C# project, I perform the same query using IDbCommand and IDataReader and I get these results:

IDbCommand cmd = con.CreateCommand();
cmd.Connection = con;
IDataReader reader = null;
cmd.CommandText = "SELECT CAST(COALESCE(c.latitude, '-1') as varchar), CAST(COALESCE(c.longitude, '-1') as varchar) FROM mytable c WHERE c.acolumn = 'S' AND c.anothercolumn=1";
cmd.CommandType = CommandType.Text;
reader = cmd.ExecuteReader();
while (reader.Read())
{
    string latitude_r= reader.GetValue(0).ToString();
    string longitude_r= reader.GetValue(1).ToString();
}

---------------------------------------------
latitude_r  "-25.300889999999999"   string
longitude_r "-57.625866000000002"   string

Can someone explain to me why? is because I'm using the .NET IDbCommand interface? is there a way to get the exact value without those extra decimal values? this is the postgresql definition of the columns:

ALTER TABLE mytable ADD COLUMN longitude double precision;
ALTER TABLE mytable ADD COLUMN latitude double precision;

Answer 1

Npgsql 2 is very old by now, you should use the latest stable version (3.0.5).

The explanation for what you're seeing is simple - Npgsql 2 set the extra_float_digits parameter to 3 (see http://www.postgresql.org/docs/current/static/runtime-config-client.html ) - this causes many more digits to be returned, and was necessary because of the way Npgsql transferred data to/from the server.

Npgsql 3 no longer sets this parameter (although you can set it yourself), so you should see the same behavior as in pgAdmin.