Consistent Nonlocking Reads

A consistent read means that InnoDB uses multi-versioning to present to a query a snapshot of the database at a point in time. The query sees the changes made by transactions that committed before that point of time, and no changes made by later or uncommitted transactions. The exception to this rule is that the query sees the changes made by earlier statements within the same transaction. This exception causes the following anomaly: If you update some rows in a table, a SELECT sees the latest version of the updated rows, but it might also see older versions of any rows. If other sessions simultaneously update the same table, the anomaly means that you might see the table in a state that never existed in the database.

If the transaction isolation level is REPEATABLE READ (the default level), all consistent reads within the same transaction read the snapshot established by the first such read in that transaction. You can get a fresher snapshot for your queries by committing the current transaction and after that issuing new queries.

With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot.

Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels. A consistent read does not set any locks on the tables it accesses, and therefore other sessions are free to modify those tables at the same time a consistent read is being performed on the table.

Suppose that you are running in the default REPEATABLE READ isolation level. When you issue a consistent read (that is, an ordinary SELECT statement), InnoDB gives your transaction a timepoint according to which your query sees the database. If another transaction deletes a row and commits after your timepoint was assigned, you do not see the row as having been deleted. Inserts and updates are treated similarly.

You can advance your timepoint by committing your transaction and then doing another SELECT or START TRANSACTION WITH CONSISTENT SNAPSHOT.

This is called multi-versioned concurrency control.

In the following example, session A sees the row inserted by B only when B has committed the insert and A has committed as well, so that the timepoint is advanced past the commit of B.

             Session A              Session B

           SET autocommit=0;      SET autocommit=0;
|          SELECT * FROM t;
|          empty set
|                                 INSERT INTO t VALUES (1, 2);
v          SELECT * FROM t;
           empty set

           SELECT * FROM t;
           empty set


           SELECT * FROM t;
           |    1    |    2    |
           1 row in set

If you want to see the freshest state of the database, you should use either the READ COMMITTED isolation level or a locking read:


With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot. With LOCK IN SHARE MODE, a locking read occurs instead: A SELECT blocks until the transaction containing the freshest rows ends (see Section, “SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads”).

Consistent read does not work over certain DDL statements:

  • Consistent read does not work over DROP TABLE, because MySQL cannot use a table that has been dropped and InnoDB destroys the table.

  • Consistent read does not work over ALTER TABLE, because that statement makes a temporary copy of the original table and deletes the original table when the temporary copy is built. When you reissue a consistent read within a transaction, rows in the new table are not visible because those rows did not exist when the transaction's snapshot was taken.

InnoDB uses a consistent read for select in clauses like INSERT INTO ... SELECT, UPDATE ... (SELECT), and CREATE TABLE ... SELECT that do not specify FOR UPDATE or LOCK IN SHARE MODE if the innodb_locks_unsafe_for_binlog option is set and the isolation level of the transaction is not set to SERIALIZABLE. Thus, no locks are set on rows read from the selected table. Otherwise, InnoDB uses stronger locks and the SELECT part acts like READ COMMITTED, where each consistent read, even within the same transaction, sets and reads its own fresh snapshot.

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User Comments
  Posted by A Ramos on October 24, 2009
Programmer beware, "Consistent Nonlocking Reads" are not automatic or even very easy to accomplish reliably. In my own experience I've found blocking reads (and the resulting lock timeouts) to be quite common with innodb.

The following very simple test locks out a reader in innodb. Tested in 5.0.67. I reported it as a bug and was told it's normal, expected behavior:

Session #1
mysql> create table t1(a int) engine=innodb;
Query OK, 0 rows affected (0.05 sec)

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values (3);
Query OK, 1 row affected (0.02 sec)

Session #2 (fire up a separate window without closing the above)
mysql> select * from t1;
^CQuery aborted by Ctrl+C
ERROR 1317 (70100): Query execution was interrupted
(I had to kill the query after several seconds)

Setting "set session transaction isolation level read committed;" did not help.

Setting "set session transaction isolation level read uncommitted;" did not help.

Using lock tables on either side did not help.

Setting innodb_locks_unsafe_for_binlog=1 in my.cnf fixes the problem, but I guess the downside is that now I will not be able to use replication in this database.

If you change the simple select statement to a CREATE TEMPORARY TABLE ... AS SELECT ..., and the target involves an index range, then even innodb_locks_unsafe_for_binlog does not help.
  Posted by Asaf M on November 18, 2009
Dude, someone answered this on the forum:
  Posted by Pradeep Gupta on October 10, 2012
When we are selecting from any table from 1 session then we cant alter that table from another session.

Session A :

mysql> select * from person_test where id=1;
| id | name |
| 1 | Antonio Paz |
1 row in set (0.00 sec)

Session B :

mysql> alter table person_test add (city char(20));
Ctrl-C -- sending "KILL QUERY 4" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted

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