When streaming large volumes of data out of PostgreSQL into C#, using Npgsql, does the command default to Single Row Mode or will it extract the entire result set before returning? I can find no mention of Single Row Mode in the Npgsql documentation, and nothing in the source code to suggest that it is optional one way or the other.
When Npgsql sends the SQL query you give it, PostgreSQL will immediately send back all the rows. If you pass CommandBehavior.SingleRow (or SingleResult) to NpgsqlCommand.ExecuteReader, Npgsql will simply not return those rows to the user; it will consume them internally, but they are still sent from the server. In other words, if you expect these options to reduce the network bandwidth used, that won't work; your only way to do that is to limit the resultset in the SQL itself, via a LIMIT clause. This is in general a better idea anyway.
See https://github.com/npgsql/npgsql/issues/410 for a bit more detail on why we didn't implement something more aggressive.