InnoDB can avoid the overhead associated with
setting up the transaction
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 DML 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
queries by grouping them inside
START TRANSACTION READ
COMMIT, or by
turning on the
setting before running the
or simply by avoiding any DML
statements interspersed with the queries.
For information about
Section 14.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.
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
INNODB STATUS output.