fix(perf): Fixed performance

* Performance are better when you write columns byte by
byte rather than string generation

qb/select.go

@@ -10,7 +10,6 @@ package qb
 import (
 	"bytes"
 	"fmt"
-	"strings"
 )
 
 // Order specifies sorting order.
@@ -54,7 +53,11 @@ func (b *SelectBuilder) ToCql() (stmt string, names []string) {
 		cql.WriteString("DISTINCT ")
 		b.distinct.writeCql(&cql)
 	case len(b.groupBy) > 0:
-		cql.WriteString(strings.Join(append(b.groupBy, b.columns...), ","))
+		b.groupBy.writeCql(&cql)
+		if len(b.columns) != 0 {
+			cql.WriteByte(',')
+			b.columns.writeCql(&cql)
+		}
 	case len(b.columns) == 0:
 		cql.WriteByte('*')
 	default: