MySQL Server (version 3.23-max and all versions 4.0 and above)
supports transactions with the
BDB transactional storage engines.
InnoDB provides full
ACID compliance. See Chapter 14, Storage Engines. For
InnoDB differences from
standard SQL with regard to treatment of transaction errors,
see Section 14.2.12, “InnoDB Error Handling”.
The other nontransactional storage engines in MySQL Server
MyISAM) follow a different
paradigm for data integrity called “atomic
operations.” In transactional terms,
MyISAM tables effectively always operate in
autocommit = 1 mode. Atomic
operations often offer comparable integrity with higher
Because MySQL Server supports both paradigms, you can decide whether your applications are best served by the speed of atomic operations or the use of transactional features. This choice can be made on a per-table basis.
As noted, the tradeoff for transactional versus
nontransactional storage engines lies mostly in performance.
Transactional tables have significantly higher memory and disk
space requirements, and more CPU overhead. On the other hand,
transactional storage engines such as
InnoDB also offer many significant
features. MySQL Server's modular design enables the concurrent
use of different storage engines to suit different
requirements and deliver optimum performance in all
But how do you use the features of MySQL Server to maintain
rigorous integrity even with the nontransactional
MyISAM tables, and how do these features
compare with the transactional storage engines?
If your applications are written in a way that is dependent on being able to call
COMMITin critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.
If you use nontransactional tables, MySQL Server in almost all cases enables you to resolve potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. You can normally fix tables perfectly with no data integrity loss just by using the MySQL log or even adding one extra log.
More often than not, critical transactional updates can be rewritten to be atomic. Generally speaking, all integrity problems that transactions solve can be done with
LOCK TABLESor atomic updates, ensuring that there are no automatic aborts from the server, which is a common problem with transactional database systems.
To be safe with MySQL Server, regardless of whether you use transactional tables, you only need to have backups and have binary logging turned on. When that is true, you can recover from any situation that you could with any other transactional database system. It is always good to have backups, regardless of which database system you use.
The transactional paradigm has its advantages and disadvantages. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be necessary, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that nontransactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.
In situations where integrity is of highest importance, MySQL
Server offers transaction-level reliability and integrity even
for nontransactional tables. If you lock tables with
LOCK TABLES, all updates stall
until integrity checks are made. If you obtain a
LOCAL lock (as opposed to a write lock) for a table
that enables concurrent inserts at the end of the table, reads
are permitted, as are inserts by other clients. The newly
inserted records are not be seen by the client that has the
read lock until it releases the lock. With
INSERT DELAYED, you can write
inserts that go into a local queue until the locks are
released, without having the client wait for the insert to
complete. See Section 8.11.3, “Concurrent Inserts”, and
Section 220.127.116.11, “INSERT DELAYED Syntax”.
“Atomic,” in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there can never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there are no dirty reads.
Following are some techniques for working with nontransactional tables:
Loops that need transactions normally can be coded with the help of
LOCK TABLES, and you don't need cursors to update records on the fly.
To avoid using
ROLLBACK, you can employ the following strategy:
This is usually a much faster method than using transactions with possible rollbacks, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In that case, all locks are released but some of the updates may not have been executed.
You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:
Modify columns relative to their current value.
Update only those columns that actually have changed.
For example, when we are updating customer information, we update only the customer data that has changed and test only that none of the changed data, or data that depends on the changed data, has changed compared to the original row. The test for changed data is done with the
WHEREclause in the
UPDATEstatement. If the record wasn't updated, we give the client a message: “Some of the data you have changed has been changed by another user.” Then we show the old row versus the new row in a window so that the user can decide which version of the customer record to use.
This gives us something that is similar to column locking but is actually even better because we only update some of the columns, using values that are relative to their current values. This means that typical
UPDATEstatements look something like these:
UPDATE tablename SET pay_back=pay_back+125; UPDATE customer SET customer_date='current_date', address='new address', phone='new phone', money_owed_to_us=money_owed_to_us-125 WHERE customer_id=id AND address='old address' AND phone='old phone';
This is very efficient and works even if another client has changed the values in the
In many cases, users have wanted
ROLLBACKfor the purpose of managing unique identifiers. This can be handled much more efficiently without locking or rolling back by using an
AUTO_INCREMENTcolumn and either the
LAST_INSERT_ID()SQL function or the
mysql_insert_id()C API function. See Section 12.13, “Information Functions”, and Section 18.104.22.168, “mysql_insert_id()”.
You can generally code around the need for row-level locking. Some situations really do need it, and
InnoDBtables support row-level locking. Otherwise, with
MyISAMtables, you can use a flag column in the table and do something like the following:
tbl_nameSET row_flag=1 WHERE id=ID;
1for the number of affected rows if the row was found and
1in the original row. You can think of this as though MySQL Server changed the preceding statement to:
tbl_nameSET row_flag=1 WHERE id=ID AND row_flag <> 1;