MySQL Manual
7 MySQL Table Types
- 7.5
InnoDBTables- 7.5.9 InnoDB Transaction Model
- 7.5.9.1 Consistent Read
- 7.5.9.2 Locking Reads
- 7.5.9.3 Next-key Locking: Avoiding the Phantom Problem
- 7.5.9.4 Locks Set by Different SQL Statements in
InnoDB - 7.5.9.5 Deadlock Detection and Rollback
- 7.5.9.6 An Example of How the Consistent Read Works in
InnoDB - 7.5.9.7 How to cope with deadlocks?
- 7.5.9.8 Performance Tuning Tips
- 7.5.9.9 The
InnoDBMonitor
7.5.9.1 Consistent Read
A consistent read means that InnoDB uses its multi-versioning to present to a query a snapshot of the database at a point in time. The query will see the changes made by exactly those 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 will see the changes made by the transaction itself which issues the query.
If you are running with the default REPEATABLE READ isolation level,
then 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.
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 users are free to modify those tables at
the same time a consistent read is being performed on the table.
