For any SELECT from a MySQL Cluster table: If the SELECT includes a BLOB or TEXT column, the READ COMMITTED transaction isolation level is converted to a read with read lock. This is done to guarantee consistency.

Prior to MySQL Cluster NDB 7.0.12, for any SELECT which used a primary key lookup or unique key lookup to retrieve any columns that used any of the BLOB or TEXT data types and that was executed within a transaction, a shared read lock was held on the table for the duration of the transaction — that is, until the transaction was either committed or aborted.

In MySQL Cluster NDB 7.0.12 and later, for primary key lookups, the lock is released as soon as all BLOB or TEXT data has been read. (Bug#49190) However, for unique key lookups, the shared lock continues to be held for the lifetime of the transaction.

This issue does not occur for queries that use index or table scans, even against NDB tables having BLOB or TEXT columns.

For example, consider the table t defined by the following CREATE TABLE statement:

CREATE TABLE t (
    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    b INT NOT NULL,
    c INT NOT NULL,
    d TEXT,
    INDEX i(b),
    UNIQUE KEY u(c)
) ENGINE = NDB,

Either of the following queries on t causes a shared read lock, because the first query uses a primary key lookup and the second uses a unique key lookup:

SELECT * FROM t WHERE a = 1;
SELECT * FROM t WHERE c = 1;

However, none of the four queries shown here causes a shared read lock:

SELECT * FROM t WHERE b 1;
SELECT * FROM t WHERE d = '1';
SELECT * FROM t;
SELECT b,c WHERE a = 1; 

This is because, of these four queries, the first uses an index scan, the second and third use table scans, and the fourth, while using a primary key lookup, does not retrieve the value of any BLOB or TEXT columns.

You can help minimize issues with shared read locks by avoiding queries that use unique key lookups (or primary key lookups in MySQL Cluster NDB 7.0.11 and earlier) that retrieve BLOB or TEXT columns, or, in cases where such queries are not avoidable, by committing transactions as soon as possible afterwards.