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15.7.2.1 Transaction Isolation Levels
Transaction isolation is one of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.
InnoDB
offers all four transaction isolation
levels described by the SQL:1992 standard:
READ UNCOMMITTED
,
READ COMMITTED
,
REPEATABLE READ
, and
SERIALIZABLE
. The default
isolation level for InnoDB
is
REPEATABLE READ
.
A user can change the isolation level for a single session or
for all subsequent connections with the SET
TRANSACTION
statement. To set the server's default
isolation level for all connections, use the
--transaction-isolation
option on
the command line or in an option file. For detailed information
about isolation levels and level-setting syntax, see
Section 13.3.7, “SET TRANSACTION Syntax”.
InnoDB
supports each of the transaction
isolation levels described here using different
locking strategies. You can
enforce a high degree of consistency with the default
REPEATABLE READ
level, for
operations on crucial data where
ACID compliance is important.
Or you can relax the consistency rules with
READ COMMITTED
or even
READ UNCOMMITTED
, in
situations such as bulk reporting where precise consistency and
repeatable results are less important than minimizing the amount
of overhead for locking.
SERIALIZABLE
enforces even
stricter rules than REPEATABLE
READ
, and is used mainly in specialized situations,
such as with XA transactions and
for troubleshooting issues with concurrency and
deadlocks.
The following list describes how MySQL supports the different transaction levels. The list goes from the most commonly used level to the least used.
This is the default isolation level for
InnoDB
. Consistent reads within the same transaction read the snapshot established by the first read. This means that if you issue several plain (nonlocking)SELECT
statements within the same transaction, theseSELECT
statements are consistent also with respect to each other. See Section 15.7.2.3, “Consistent Nonlocking Reads”.For locking reads (
SELECT
withFOR UPDATE
orFOR SHARE
),UPDATE
, andDELETE
statements, locking depends on whether the statement uses a unique index with a unique search condition, or a range-type search condition.For a unique index with a unique search condition,
InnoDB
locks only the index record found, not the gap before it.For other search conditions,
InnoDB
locks the index range scanned, using gap locks or next-key locks to block insertions by other sessions into the gaps covered by the range. For information about gap locks and next-key locks, see Section 15.7.1, “InnoDB Locking”.
Each consistent read, even within the same transaction, sets and reads its own fresh snapshot. For information about consistent reads, see Section 15.7.2.3, “Consistent Nonlocking Reads”.
For locking reads (
SELECT
withFOR UPDATE
orFOR SHARE
),UPDATE
statements, andDELETE
statements,InnoDB
locks only index records, not the gaps before them, and thus permits the free insertion of new records next to locked records. Gap locking is only used for foreign-key constraint checking and duplicate-key checking.Because gap locking is disabled, phantom problems may occur, as other sessions can insert new rows into the gaps. For information about phantoms, see Section 15.7.4, “Phantom Rows”.
Only row-based binary logging is supported with the
READ COMMITTED
isolation level. If you useREAD COMMITTED
withbinlog_format=MIXED
, the server automatically uses row-based logging.Using
READ COMMITTED
has additional effects:For
UPDATE
orDELETE
statements,InnoDB
holds locks only for rows that it updates or deletes. Record locks for nonmatching rows are released after MySQL has evaluated theWHERE
condition. This greatly reduces the probability of deadlocks, but they can still happen.For
UPDATE
statements, if a row is already locked,InnoDB
performs a “semi-consistent” read, returning the latest committed version to MySQL so that MySQL can determine whether the row matches theWHERE
condition of theUPDATE
. If the row matches (must be updated), MySQL reads the row again and this timeInnoDB
either locks it or waits for a lock on it.
Consider the following example, beginning with this table:
In this case, the table has no indexes, so searches and index scans use the hidden clustered index for record locking (see Section 15.6.2.1, “Clustered and Secondary Indexes”) rather than indexed columns.
Suppose that one session performs an
UPDATE
using these statements:- # Session A
Suppose also that a second session performs an
UPDATE
by executing these statements following those of the first session:As
InnoDB
executes eachUPDATE
, it first acquires an exclusive lock for each row, and then determines whether to modify it. IfInnoDB
does not modify the row, it releases the lock. Otherwise,InnoDB
retains the lock until the end of the transaction. This affects transaction processing as follows.When using the default
REPEATABLE READ
isolation level, the firstUPDATE
acquires an x-lock on each row that it reads and does not release any of them:x-lock(1,2); retain x-lock x-lock(2,3); update(2,3) to (2,5); retain x-lock x-lock(3,2); retain x-lock x-lock(4,3); update(4,3) to (4,5); retain x-lock x-lock(5,2); retain x-lock
The second
UPDATE
blocks as soon as it tries to acquire any locks (because first update has retained locks on all rows), and does not proceed until the firstUPDATE
commits or rolls back:x-lock(1,2); block and wait for first UPDATE to commit or roll back
If
READ COMMITTED
is used instead, the firstUPDATE
acquires an x-lock on each row that it reads and releases those for rows that it does not modify:x-lock(1,2); unlock(1,2) x-lock(2,3); update(2,3) to (2,5); retain x-lock x-lock(3,2); unlock(3,2) x-lock(4,3); update(4,3) to (4,5); retain x-lock x-lock(5,2); unlock(5,2)
For the second
UPDATE
,InnoDB
does a “semi-consistent” read, returning the latest committed version of each row that it reads to MySQL so that MySQL can determine whether the row matches theWHERE
condition of theUPDATE
:x-lock(1,2); update(1,2) to (1,4); retain x-lock x-lock(2,3); unlock(2,3) x-lock(3,2); update(3,2) to (3,4); retain x-lock x-lock(4,3); unlock(4,3) x-lock(5,2); update(5,2) to (5,4); retain x-lock
However, if the
WHERE
condition includes an indexed column, andInnoDB
uses the index, only the indexed column is considered when taking and retaining record locks. In the following example, the firstUPDATE
takes and retains an x-lock on each row where b = 2. The secondUPDATE
blocks when it tries to acquire x-locks on the same records, as it also uses the index defined on column b.- # Session A
- # Session B
The effects of using the
READ COMMITTED
isolation level are the same as enabling the deprecatedinnodb_locks_unsafe_for_binlog
configuration option, with these exceptions:Enabling
innodb_locks_unsafe_for_binlog
is a global setting and affects all sessions, whereas the isolation level can be set globally for all sessions, or individually per session.innodb_locks_unsafe_for_binlog
can be set only at server startup, whereas the isolation level can be set at startup or changed at runtime.
READ COMMITTED
therefore offers finer and more flexible control thaninnodb_locks_unsafe_for_binlog
.SELECT
statements are performed in a nonlocking fashion, but a possible earlier version of a row might be used. Thus, using this isolation level, such reads are not consistent. This is also called a dirty read. Otherwise, this isolation level works likeREAD COMMITTED
.This level is like
REPEATABLE READ
, butInnoDB
implicitly converts all plainSELECT
statements toSELECT ... FOR SHARE
ifautocommit
is disabled. Ifautocommit
is enabled, theSELECT
is its own transaction. It therefore is known to be read only and can be serialized if performed as a consistent (nonlocking) read and need not block for other transactions. (To force a plainSELECT
to block if other transactions have modified the selected rows, disableautocommit
.)
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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