I've compared Debug log between correct and wrong activity:
Good activity:
2006-03-24 16:30:36 2632 Debug String written: select * from
getvideodata(2::int4, 1::int4, '2006-03-24 16:30:31.400000'::timestamp,
1::int4) AS (idvideodate timestamp, mediabinary bytea).
2006-03-24 16:30:36 2632 Debug Entering
NpgsqlState.ProcessBackendResponses()
2006-03-24 16:30:36 2632 Debug RowDescription message received from
server.
2006-03-24 16:30:36 2632 Debug Entering
NpgsqlRowDescription.ReadFromStream_Ver_3()
2006-03-24 16:30:36 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:36 2632 Debug String read: idvideodate.
2006-03-24 16:30:36 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:36 2632 Debug String read: mediabinary.
2006-03-24 16:30:36 2632 Debug DataRow message received from server.
2006-03-24 16:30:36 2632 Debug Entering NpgsqlAsciiRow.NpgsqlAsciiRow()
2006-03-24 16:30:36 2632 Debug Entering
NpgsqlAsciiRow.ReadFromStream_Ver_3()
2006-03-24 16:30:36 2632 Debug Entering
NpgsqlTypesHelper.ConvertBackendStringToSystemType()
2006-03-24 16:30:37 2632 Debug Entering
NpgsqlTypesHelper.ConvertBackendStringToSystemType()
2006-03-24 16:30:38 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:38 2632 Debug String read: SELECT.
2006-03-24 16:30:38 2632 Debug CompletedResponse message from Server:
SELECT.
2006-03-24 16:30:38 2632 Debug ReadyForQuery message received from
server.
2006-03-24 16:30:38 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:38 2632 Debug String read: I.
...
Wrong activity:
2006-03-24 16:30:38 2632 Debug String written: select * from
getvideodata(2::int4, 1::int4, '2006-03-24 16:30:35.400000'::timestamp,
1::int4) AS (idvideodate timestamp, mediabinary bytea).
2006-03-24 16:30:38 2632 Debug Entering
NpgsqlState.ProcessBackendResponses()
2006-03-24 16:30:38 2632 Debug RowDescription message received from
server.
2006-03-24 16:30:38 2632 Debug Entering
NpgsqlRowDescription.ReadFromStream_Ver_3()
2006-03-24 16:30:38 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:38 2632 Debug String read: idvideodate.
2006-03-24 16:30:38 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:38 2632 Debug String read: mediabinary.
2006-03-24 16:30:38 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:38 2632 Debug String read: SELECT.
2006-03-24 16:30:38 2632 Debug CompletedResponse message from Server:
SELECT.
2006-03-24 16:30:38 2632 Debug ReadyForQuery message received from
server.
2006-03-24 16:30:38 2632 Debug Entering PGUtil.ReadString()
2006-03-24 16:30:38 2632 Debug String read: I.
2006-03-24 16:30:38 2632 Debug Entering NpgsqlState.ChangeState()
2006-03-24 16:30:38 2632 Debug Entering
NpgsqlCommand.UpdateOutputParameters()
2006-03-24 16:30:38 2632 Debug Get NpgsqlCommand.Connection
2006-03-24 16:30:48 2632 Debug Entering NpgsqlDataReader.Read()
2006-03-24 16:30:48 2632 Debug Entering NpgsqlDataReader.Dispose()
2006-03-24 16:30:48 2632 Debug Entering NpgsqlConnection.Close()