fix: handle missing EXPRESSION column in get_indexes_sql()

The EXPRESSION column in information_schema.STATISTICS only exists in
MySQL 8.0.13+. Add a fallback query without EXPRESSION for MySQL < 8.0.13
and MariaDB. heading.py catches the error and retries with the fallback.

Fixes #1436

Author: dimitri-yatsenko

src/datajoint/adapters/mysql.py

@@ -737,8 +737,10 @@ def parse_foreign_key_error(self, error_message: str) -> dict[str, str | list[st
     def get_indexes_sql(self, schema_name: str, table_name: str) -> str:
         """Query to get index definitions.
 
-        Note: For MySQL 8.0+, EXPRESSION column contains the expression for
+        Note: For MySQL 8.0.13+, EXPRESSION column contains the expression for
         functional indexes. COLUMN_NAME is NULL for such indexes.
+        On MySQL < 8.0.13 and MariaDB, the EXPRESSION column does not exist;
+        heading.py falls back to get_indexes_sql_fallback() in that case.
         """
         return (
             f"SELECT INDEX_NAME as index_name, "
@@ -751,6 +753,19 @@ def get_indexes_sql(self, schema_name: str, table_name: str) -> str:
             f"ORDER BY index_name, seq_in_index"
         )
 
+    def get_indexes_sql_fallback(self, schema_name: str, table_name: str) -> str:
+        """Fallback index query for MySQL < 8.0.13 and MariaDB (no EXPRESSION column)."""
+        return (
+            f"SELECT INDEX_NAME as index_name, "
+            f"COLUMN_NAME as column_name, "
+            f"NON_UNIQUE as non_unique, SEQ_IN_INDEX as seq_in_index "
+            f"FROM information_schema.statistics "
+            f"WHERE table_schema = {self.quote_string(schema_name)} "
+            f"AND table_name = {self.quote_string(table_name)} "
+            f"AND index_name != 'PRIMARY' "
+            f"ORDER BY index_name, seq_in_index"
+        )
+
     def parse_column_info(self, row: dict[str, Any]) -> dict[str, Any]:
         """
         Parse MySQL SHOW FULL COLUMNS output into standardized format.

src/datajoint/heading.py

@@ -551,10 +551,22 @@ def _init_from_database(self) -> None:
 
         # Read and tabulate secondary indexes
         keys = defaultdict(dict)
-        for item in conn.query(
-            adapter.get_indexes_sql(database, table_name),
-            as_dict=True,
-        ):
+        try:
+            index_rows = conn.query(
+                adapter.get_indexes_sql(database, table_name),
+                as_dict=True,
+            )
+        except Exception:
+            # Fall back for MySQL < 8.0.13 / MariaDB (no EXPRESSION column)
+            index_rows = (
+                conn.query(
+                    adapter.get_indexes_sql_fallback(database, table_name),
+                    as_dict=True,
+                )
+                if hasattr(adapter, "get_indexes_sql_fallback")
+                else []
+            )
+        for item in index_rows:
             # Note: adapter.get_indexes_sql() already filters out PRIMARY key
             # MySQL/PostgreSQL adapters return: index_name, column_name, non_unique
             index_name = item.get("index_name") or item.get("Key_name")