Minimal example of using select… for update to isolate rows (1 reply)

I want concurrent transactions to select the next "free" row from a table, marking it as "dirty" so that other transactions **cannot select it**. For example, I want a transaction T1 to select row 1 and while it continues, T2 must select row 2 and proceed. It must not select row 1 as it is dirty.

I had trouble using `select... for update` as the second transaction causes a lock wait as it contends for same row.

Please provide a minimal example for **different transactions to select distinct rows**.

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Here is my failed approach:

My data is:

mysql> select * from SolrCoresPreallocated;
+----+-------------+-----+-----+
| id | used_status | sid | cid |
+----+-------------+-----+-----+
| 1 | 0 | 0 | 400 |
| 2 | 0 | 0 | 401 |
| 3 | 0 | 0 | 402 |
| 4 | 0 | 0 | 403 |
| 5 | 0 | 0 | 404 |
| 6 | 0 | 0 | 405 |
+----+-------------+-----+-----+
6 rows in set (0.00 sec)

And T1 gets it's read:

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from SolrCoresPreallocated order by id limit 1 for update;
+----+-------------+-----+-----+
| id | used_status | sid | cid |
+----+-------------+-----+-----+
| 1 | 0 | 0 | 400 |
+----+-------------+-----+-----+
1 row in set (0.00 sec)


...but T2 fails, it gives me a lock wait timeout. How do I make it select the next free row cleanly?

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from SolrCoresPreallocated order by id limit 1 for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)