Known Limitations of MySQL Cluster
1526
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 primary key
lookups or unique key lookups to retrieve
BLOB
or
TEXT
columns, or, in cases where such
queries are not avoidable, by committing transactions as soon as possible afterward.
We are working on overcoming this limitation in a future MySQL Cluster release (see Bug
#49190); however, we do not plan to backport any fix for this issue to MySQL 5.0.
• Rollbacks.
There are no partial transactions, and no partial rollbacks of transactions. A duplicate
key or similar error aborts the entire transaction, and subsequent statements raise
ERROR 1296
(HY000): Got error 4350 'Transaction already aborted' from NDBCLUSTER
. In
such cases, you must issue an explicit
ROLLBACK
and retry the entire transaction.
This behavior differs from that of other transactional storage engines such as
InnoDB
that may roll
back individual statements.
• Transactions and memory usage.
As noted elsewhere in this chapter, MySQL Cluster does not handle large transactions well; it
is better to perform a number of small transactions with a few operations each than to attempt a
single large transaction containing a great many operations. Among other considerations, large
transactions require very large amounts of memory. Because of this, the transactional behavior of a
number of MySQL statements is effected as described in the following list:
•
TRUNCATE TABLE
is not transactional when used on
NDB
tables. If a
TRUNCATE TABLE
fails to
empty the table, then it must be re-run until it is successful.
•
DELETE FROM
(even with no
WHERE
clause) is transactional. For tables containing a great
many rows, you may find that performance is improved by using several
DELETE FROM ...
LIMIT ...
statements to “chunk” the delete operation. If your objective is to empty the table, then
you may wish to use
TRUNCATE TABLE
instead.
•
LOAD DATA
statements.
LOAD DATA INFILE
is not transactional when used on
NDB
tables.
Important
When executing a
LOAD DATA INFILE
statement, the
NDB
engine
performs commits at irregular intervals that enable better utilization of the
communication network. It is not possible to know ahead of time when such
commits take place.
LOAD DATA FROM MASTER
is not supported in MySQL Cluster.
•
ALTER TABLE
and transactions.
When copying an
NDB
table as part of an
ALTER TABLE
, the
creation of the copy is nontransactional. (In any case, this operation is rolled back when the copy
is deleted.)
17.1.5.4. MySQL Cluster Error Handling
Starting, stopping, or restarting a node may give rise to temporary errors causing some transactions to
fail. These include the following cases:
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