PolarDB:Enable X-Engine

Feature limitations

X-Engine does not support all InnoDB features. Any feature not listed in the following table behaves the same as in InnoDB.

Large-transaction limitations

X-Engine does not support large transactions. When a transaction modifies 10,000 or more rows, X-Engine automatically triggers the commit in middle mechanism, which internally commits the data in batches and starts sub-transactions to continue. This behavior does not guarantee transaction atomicity.

The following examples show how commit in middle affects different operations.

Partial rollback after large inserts

If a large INSERT transaction triggers commit in middle, the committed rows cannot be rolled back. Only the remaining uncommitted rows are rolled back.

DROP TABLE t1;
CREATE TABLE t1(c1 int primary key, c2 int) ENGINE=xengine;
BEGIN;
CALL insert_data(12000); -- Inserts 12,000 rows; commit in middle commits the first 10,000
ROLLBACK;               -- Only the last 2,000 rows are rolled back
SELECT COUNT(*) FROM t1;

Expected output:

+----------+
| COUNT(*) |
+----------+
|    10000 |
+----------+
1 row in set (0.00 sec)

DELETE cannot read rows committed mid-transaction

If DELETE is preceded by INSERT in the same large transaction, the commit in middle mechanism may cause the DELETE to miss newly committed rows.

DROP TABLE t1;
CREATE TABLE t1(c1 int primary key, c2 int) ENGINE=xengine;
CALL insert_data(10000);
BEGIN;
INSERT INTO t1 VALUES(10001, 10001), (10002, 10002);
DELETE FROM t1 WHERE c1 >= 0; -- Triggers commit in middle; the two inserted rows are not deleted
COMMIT;
SELECT * FROM t1;

Expected output:

+-------+-------+
| c1    | c2    |
+-------+-------+
| 10001 | 10001 |
| 10002 | 10002 |
+-------+-------+
2 rows in set (0.00 sec)