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.