Bug #65086 DML queries block SELECTs using unique index lookups

Description:
A SELECT that has a WHERE clause like "WHERE unique_column = constant_value"
is blocked by INSERTs or UPDATEs touching the row that has "unique_column = constant_value"

How to repeat:
   Connection #1                 |   Connection #2
                                 |  
CREATE TABLE t1 (                | 
 id INT PRIMARY KEY,             | 
 val INT UNIQUE,                 |
 val2 INT                        |
) ENGINE=ndb;                    |
                                 |
BEGIN;                           |
INSERT INTO t1 VALUES(23,42,0);  |
                                 |
                                 |   SELECT * FROM t1 WHERE val=42;
                                 |    -> Lock wait timeout exceeded
                                 | 
                                 |   SELECT * FROM t1 IGNORE INDEX(val) WHERE val=42;
                                 |    -> returns empty result set without blocking
                                 |
COMMIT;                          |
                                 |
BEGIN;                           |
UPDATE t1 SET val2=1 WHERE id=23;|
                                 |
                                 |   SELECT * FROM t1 WHERE val=42;
                                 |    -> Lock wait timeout exceeded
                                 | 
                                 |   SELECT * FROM t1 IGNORE INDEX(val) WHERE val=42;
                                 |    -> returns empty result set without blocking
 

Suggested fix:
As the isolation level is READ COMMITTED i do not expect pure reads to block due to uncommitted changes in a different transaction ...

Fixed in NDB 7.3+ documentation in mysqldoc rev 65075.

See e.g. https://dev.mysql.com/doc/refman/8.0/en/mysql-cluster-limitations-transactions.html

Closed.