In #2366, I added NpgsqlSetOperationTypeResolutionCompensatingExpressionVisitor, which adds an explicit convert node on the leftmost set operation operand, to work around problematic PG type inference behavior.

At the moment, this visitor needs to visit the EF-internal TpcTablesExpression. @smitpatel will be looking at changes on the EF side to obviate this. This is non-critical.