static void TestDataSet()
{
// register c#'s sqrt as an external function
string sqroot(double d) => Math.Sqrt(d).ToString("#.###");
SQLContext sqlContext = new SQLContext();
SQLContext.Register <double, string>("sqroot", sqroot);
var a = sqlContext.Read("a");
var b = sqlContext.Read("b");
a.filter("a1>1").join(b, "b2=a2").select("a1", "sqroot(b1*a1+2)").show();
string s = a.physicPlan_.Explain();
Console.WriteLine(s);
var sql = "SELECT a1, sqroot(b1*a1+2) from a join b on b2=a2 where a1>1";
var rows = SQLStatement.ExecSQL(sql, out string plan, out _);
}
