fix(mysql): preserve functional index expressions

crates/dbx-core/src/table_structure_sql/indexes.rs

@@ -116,6 +116,16 @@ pub(super) fn mysql_index_parts(index_type: &str) -> (String, String) {
     }
 }
 
+fn mysql_index_column_sql(column: &str) -> String {
+    let trimmed = column.trim();
+    // Keep the wrapped expression from MySQL metadata instead of quoting it as a column identifier.
+    if trimmed.starts_with("((") && trimmed.ends_with("))") {
+        trimmed.to_string()
+    } else {
+        quote_ident(StructureDialect::Mysql, column)
+    }
+}
+
 pub(super) fn build_drop_index_sql(
     database_type: Option<DatabaseType>,
     dialect: StructureDialect,
@@ -160,7 +170,17 @@ pub(super) fn build_create_index_statements(
     }
 
     let unique = if index.is_unique { "UNIQUE " } else { "" };
-    let cols = columns.iter().map(|column| quote_ident(dialect, column)).collect::<Vec<_>>().join(", ");
+    let cols = columns
+        .iter()
+        .map(|column| {
+            if dialect == StructureDialect::Mysql {
+                mysql_index_column_sql(column)
+            } else {
+                quote_ident(dialect, column)
+            }
+        })
+        .collect::<Vec<_>>()
+        .join(", ");
     let idx_type = normalized_index_type(index);
     let mut type_prefix = String::new();
     let mut using_clause = String::new();

crates/dbx-core/src/table_structure_sql/tests.rs

@@ -1240,6 +1240,31 @@ fn mysql_create_unique_index_with_comment_and_btree() {
     );
 }
 
+#[test]
+fn mysql_create_functional_index_preserves_key_part_syntax() {
+    let functional_key_part = "((case when (`STATUS` = _utf8mb4'online') then _utf8mb4'online' else NULL end))";
+    let mut idx = index("test_UNIQUE", &["attr", "attr2", functional_key_part]);
+    idx.is_unique = true;
+
+    let result = build_table_structure_change_sql(TableStructureSqlOptions {
+        database_type: Some(DatabaseType::Mysql),
+        schema: None,
+        table_name: "test".to_string(),
+        columns: Vec::new(),
+        indexes: vec![idx],
+        foreign_keys: Vec::new(),
+        triggers: Vec::new(),
+        table_comment: None,
+        original_table_comment: None,
+    });
+
+    assert_eq!(result.warnings, Vec::<String>::new());
+    assert_eq!(
+        result.statements,
+        vec![format!("CREATE UNIQUE INDEX `test_UNIQUE` ON `test` (`attr`, `attr2`, {functional_key_part});")]
+    );
+}
+
 #[test]
 fn mysql_add_timestamp_column_drops_invalid_precision() {
     let mut created_at = column("created_at");