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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 ONLY
statement. In this case, attempting to make changes to the database (forInnoDB
,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.
The
autocommit
setting 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”SELECT
statement. That is, aSELECT
that does not use aFOR UPDATE
orLOCK IN SHARED MODE
clause.The transaction is started without the
READ ONLY
option, 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.