Improve performance for BaseShowTablesWithSizes query.

Signed-off-by: Arthur Schreiber <[email protected]>

go/mysql/flavor_mysql.go

@@ -329,41 +329,25 @@ GROUP BY t.table_name, t.table_type, t.create_time, t.table_comment`
 
 // TablesWithSize80 is a query to select table along with size for mysql 8.0
 //
-// We join with a subquery that materializes the data from `information_schema.innodb_sys_tablespaces`
-// early for performance reasons. This effectively causes only a single read of `information_schema.innodb_tablespaces`
-// per query.
 // Note the following:
-//   - We use UNION ALL to deal differently with partitioned tables vs. non-partitioned tables.
-//     Originally, the query handled both, but that introduced "WHERE ... OR" conditions that led to poor query
-//     optimization. By separating to UNION ALL we remove all "OR" conditions.
+//   - We use a single query to fetch both partitioned and non-partitioned tables. This is because
+//     accessing `information_schema.innodb_tablespaces` is expensive on servers with many tables,
+//     and every query that loads the table needs to perform full table scans on it. Doing a single
+//     table scan is more efficient than doing more than one.
 //   - We utilize `INFORMATION_SCHEMA`.`TABLES`.`CREATE_OPTIONS` column to do early pruning before the JOIN.
 //   - `TABLES`.`TABLE_NAME` has `utf8mb4_0900_ai_ci` collation.  `INNODB_TABLESPACES`.`NAME` has `utf8mb3_general_ci`.
 //     We normalize the collation to get better query performance (we force the casting at the time of our choosing)
-//   - `create_options` is NULL for views, and therefore we need an additional UNION ALL to include views
 const TablesWithSize80 = `SELECT t.table_name,
-		t.table_type,
-		UNIX_TIMESTAMP(t.create_time),
-		t.table_comment,
-		i.file_size,
-		i.allocated_size
-	FROM information_schema.tables t
-		LEFT JOIN information_schema.innodb_tablespaces i
-	ON i.name = CONCAT(t.table_schema, '/', t.table_name) COLLATE utf8mb3_general_ci
-	WHERE
-		t.table_schema = database() AND not t.create_options <=> 'partitioned'
-UNION ALL
-	SELECT
-		t.table_name,
 		t.table_type,
 		UNIX_TIMESTAMP(t.create_time),
 		t.table_comment,
 		SUM(i.file_size),
 		SUM(i.allocated_size)
 	FROM information_schema.tables t
 		LEFT JOIN information_schema.innodb_tablespaces i
-	ON i.name LIKE (CONCAT(t.table_schema, '/', t.table_name, '#p#%') COLLATE utf8mb3_general_ci )
+	ON i.name LIKE CONCAT(t.table_schema, '/', t.table_name, IF(t.create_options <=> 'partitioned', '#p#%', '')) COLLATE utf8mb3_general_ci
 	WHERE
-		t.table_schema = database() AND t.create_options <=> 'partitioned'
+		t.table_schema = database()
 	GROUP BY
 		t.table_schema, t.table_name, t.table_type, t.create_time, t.table_comment
 `