revert pr444(#444).

sql/src/ast.rs

@@ -3,7 +3,7 @@
 //! SQL statement
 
 use sqlparser::ast::{
-    ColumnDef, Ident, ObjectName, SqlOption, Statement as SqlStatement, TableConstraint,
+    ColumnDef, ObjectName, SqlOption, Statement as SqlStatement, TableConstraint,
 };
 
 /// Statement representations
@@ -33,22 +33,14 @@ impl TableName {
     pub fn is_empty(&self) -> bool {
         self.0 .0.is_empty()
     }
-
-    // Normalize an identifer to a lowercase string unless the identifier is quoted.
-    fn normalize_ident(id: &Ident) -> String {
-        match id.quote_style {
-            Some(_) => id.value.clone(),
-            None => id.value.to_ascii_lowercase(),
-        }
-    }
 }
 
 impl ToString for TableName {
     fn to_string(&self) -> String {
         self.0
              .0
             .iter()
-            .map(Self::normalize_ident)
+            .map(|ident| ident.value.as_str())
             .collect::<Vec<_>>()
             .join(".")
     }

sql/src/parser.rs

@@ -544,14 +544,12 @@ fn build_timestamp_key_constraint(col_defs: &[ColumnDef], constraints: &mut Vec<
     }
 }
 
-/// Add quotes in table name, for example:
-///     
-///     table --> `table`
+/// Add quotes in table name (for example: convert table to `table`).
 ///
 /// It is used to process table name in `SELECT`, for preventing `datafusion`
 /// converting the table name to lowercase, because `CeresDB` only support
 /// case-sensitive in sql.
-// TODO: other items(such as: alias, column name) are not normalized now.
+// TODO: maybe other items(such as: alias, column name) need to be normalized, too.
 pub fn maybe_normalize_table_name(statement: SqlStatement) -> SqlStatement {
     let original_statement = statement.clone();
     if let SqlStatement::Query(query) = statement {