Map property to column xmin

[asymetrixs]

The column "xmin" is a default hidden column for every table in PostgreSQL. In order to use the concurrency feature, the best is to map a property to this auto-updating column. If it is just done as described in the docs now, with a random column name mapped to type 'xid', the column value remains at its default 0 on insert and upate.

[asymetrixs]

I noticed, that doing this leads to the following error message when using .Distinct() since apparently the DISTINCT operation is not supported on columns of type xid:

      Unhandled exception rendering component: 0A000: could not implement DISTINCT
      
      DETAIL: Some of the datatypes only support hashing, while others only support sorting.
      Npgsql.PostgresException (0x80004005): 0A000: could not implement DISTINCT
      
      DETAIL: Some of the datatypes only support hashing, while others only support sorting.
         at Npgsql.Internal.NpgsqlConnector.ReadMessageLong(Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications, Boolean isReadingPrependedMessage)
         at System.Runtime.CompilerServices.PoolingAsyncValueTaskMethodBuilder`1.StateMachineBox`1.System.Threading.Tasks.Sources.IValueTaskSource<TResult>.GetResult(Int16 token)
         at Npgsql.NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken)
         at Npgsql.NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken)
         at Npgsql.NpgsqlCommand.ExecuteReader(Boolean async, CommandBehavior behavior, CancellationToken cancellationToken)
         at Npgsql.NpgsqlCommand.ExecuteReader(Boolean async, CommandBehavior behavior, CancellationToken cancellationToken)
         at Npgsql.NpgsqlCommand.ExecuteDbDataReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken)
        [...]
        Exception data:
          Severity: ERROR
          SqlState: 0A000
          MessageText: could not implement DISTINCT
          Detail: Some of the datatypes only support hashing, while others only support sorting.
          File: planner.c
          Line: 4787
          Routine: create_distinct_paths