18.104.22.168 Locking Reads
If you query data and then insert or update related data within the same transaction, the regular
SELECT statement does not give enough protection. Other transactions can update or delete the same rows you just queried.
InnoDB supports two types of locking reads that offer extra safety:
Sets a shared mode lock on any rows that are read. Other sessions can read the rows, but cannot modify them until your transaction commits. If any of these rows were changed by another transaction that has not yet committed, your query waits until that transaction ends and then uses the latest values.
For index records the search encounters, locks the rows and any associated index entries, the same as if you issued an
UPDATEstatement for those rows. Other transactions are blocked from updating those rows, from doing
SELECT ... LOCK IN SHARE MODE, or from reading the data in certain transaction isolation levels. Consistent reads ignore any locks set on the records that exist in the read view. (Old versions of a record cannot be locked; they are reconstructed by applying undo logs on an in-memory copy of the record.)
These clauses are primarily useful when dealing with tree-structured or graph-structured data, either in a single table or split across multiple tables. You traverse edges or tree branches from one place to another, while reserving the right to come back and change any of these “pointer” values.
All locks set by
LOCK IN SHARE MODE and
FOR UPDATE queries are released when the transaction is committed or rolled back.
A locking read clause in an outer statement does not lock the rows of a table in a nested subquery unless a locking read clause is also specified in the subquery. For example, the following statement does not lock rows in table
SELECT * FROM t1 WHERE c1 = (SELECT c1 FROM t2) FOR UPDATE;
To lock rows in table
t2, add a locking read clause to the subquery:
SELECT * FROM t1 WHERE c1 = (SELECT c1 FROM t2 FOR UPDATE) FOR UPDATE;
Suppose that you want to insert a new row into a table
child, and make sure that the child row has a parent row in table
parent. Your application code can ensure referential integrity throughout this sequence of operations.
First, use a consistent read to query the table
PARENT and verify that the parent row exists. Can you safely insert the child row to table
CHILD? No, because some other session could delete the parent row in the moment between your
SELECT and your
INSERT, without you being aware of it.
To avoid this potential issue, perform the
LOCK IN SHARE MODE:
SELECT * FROM parent WHERE NAME = 'Jones' LOCK IN SHARE MODE;
LOCK IN SHARE MODE query returns the parent
'Jones', you can safely add the child record to the
CHILD table and commit the transaction. Any transaction that tries to acquire an exclusive lock in the applicable row in the
PARENT table waits until you are finished, that is, until the data in all tables is in a consistent state.
For another example, consider an integer counter field in a table
CHILD_CODES, used to assign a unique identifier to each child added to table
CHILD. Do not use either consistent read or a shared mode read to read the present value of the counter, because two users of the database could see the same value for the counter, and a duplicate-key error occurs if two transactions attempt to add rows with the same identifier to the
LOCK IN SHARE MODE is not a good solution because if two users read the counter at the same time, at least one of them ends up in deadlock when it attempts to update the counter.
To implement reading and incrementing the counter, first perform a locking read of the counter using
FOR UPDATE, and then increment the counter. For example:
SELECT counter_field FROM child_codes FOR UPDATE; UPDATE child_codes SET counter_field = counter_field + 1;
The preceding description is merely an example of how
SELECT ... FOR UPDATE works. In MySQL, the specific task of generating a unique identifier actually can be accomplished using only a single access to the table:
UPDATE child_codes SET counter_field = LAST_INSERT_ID(counter_field + 1); SELECT LAST_INSERT_ID();
SELECT statement merely retrieves the identifier information (specific to the current connection). It does not access any table.