fix invalid condition used in QueryFirst and QuerySingle

[neuecc]

I think (row & Row.Single) != 0 is row is not Row.First so should swap condition.

By the way, in MySQL if use CommandBehavior.SingleRow, MySQL Connector execute SET SQL_SELECT_LIMIT=1 on each query. (at command.cs - HandleCommandBeehaviors)
It causes performance issue so I need disable command behavior option...

[mgravell]

I think the code is correct as written.

(row & Row.Single) != 0 is true for Single and SingleOrDefault. In those cases we have to read multiple rows in order to validate that we have actually only got one, hence not passing SingleRow.

Conversely, (row & Row.Single) != 0 is false for First and FirstOrDefault. In that scenario we can use the "single row" optimization, precisely because we don't care whether multiple rows would have otherwise been returned.

[mgravell]

Re the performance thing; we're discussing options as to whether we should disable those flags.

[mgravell]

Closing; I think the code change is invalid (please let me know if I've misunderstood - it happens); the performance thing should be addressed by #602