improve sql parser performance

src/main/java/com/alibaba/druid/sql/parser/Lexer.java

@@ -79,10 +79,6 @@ public class Lexer {
         return text.charAt(index);
     }
     
-    public final int indexOf(char ch, int fromIndex) {
-        return text.indexOf(ch, fromIndex);
-    }
-
     public String addSymbol() {
         return subString(mark, bufPos);
     }
@@ -248,6 +244,11 @@ public class Lexer {
             return;
         }
         
+        if (isFirstIdentifierChar(ch) && ch != 'N') {
+            scanIdentifier();
+            return;
+        }
+        
         nextToken();
     }