8.5.3 Optimizing InnoDB Read-Only Transactions
InnoDB can avoid the overhead associated with setting up the transaction ID (
TRX_ID field) for transactions that are known to be read-only. A transaction ID is only needed for a transaction that might perform write operations or locking reads such as
SELECT ... FOR UPDATE. Eliminating unnecessary transaction IDs reduces the size of internal data structures that are consulted each time a query or data change statement constructs a read view.
InnoDB detects read-only transactions when:
The transaction is started with the
START TRANSACTION READ ONLYstatement. In this case, attempting to make changes to the database (for
MyISAM, or other types of tables) causes an error, and the transaction continues in read-only state:
ERROR 1792 (25006): Cannot execute statement in a READ ONLY transaction.
You can still make changes to session-specific temporary tables in a read-only transaction, or issue locking queries for them, because those changes and locks are not visible to any other transaction.
autocommitsetting is turned on, so that the transaction is guaranteed to be a single statement, and the single statement making up the transaction is a “non-locking”
SELECTstatement. That is, a
SELECTthat does not use a
LOCK IN SHARED MODEclause.
The transaction is started without the
READ ONLYoption, but no updates or statements that explicitly lock rows have been executed yet. Until updates or explicit locks are required, a transaction stays in read-only mode.
Thus, for a read-intensive application such as a report generator, you can tune a sequence of
InnoDB queries by grouping them inside
START TRANSACTION READ ONLY and
COMMIT, or by turning on the
autocommit setting before running the
SELECT statements, or simply by avoiding any data change statements interspersed with the queries.
For information about
START TRANSACTION and
autocommit, see Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Statements”.
Transactions that qualify as auto-commit, non-locking, and read-only (AC-NL-RO) are kept out of certain internal
InnoDB data structures and are therefore not listed in
SHOW ENGINE INNODB STATUS output.