Example : LOCKING Request Modifier
The following LOCKING clause can be used to select data from the employee table while it is being modified:
LOCKING TABLE personnel.employee FOR ACCESS
SELECT name, salary
FROM employee
WHERE salary < 25000 ;
The query results may:
Example : LOCKING Request Modifier and Secondary Indexes
The system synchronizes base data rows and index subtable rows. However, an ACCESS lock can allow inconsistent results even when secondary indexes are used in conditional expressions because index constraints are not always rechecked against the data row.
For example, a column named qualify_accnt is defined as a secondary index for a base table named accnt_rec, as in the following request:
LOCKING TABLE accnt_rec FOR ACCESS
SELECT accnt_no, qualify_accnt
FROM accnt_rec
WHERE qualify_accnt = 1587;
The request could return:
accnt_no qualify_accnt
‑‑‑‑-‑‑‑ -‑‑‑‑‑‑‑‑‑‑‑‑
1761 4214
In this case, the value 1587 was found in the secondary index subtable and the corresponding data row was selected and returned. However, the data for account 1761 had been changed by another user while the retrieval was in process. This is referred to as a dirty read. See SQL Request and Transaction Processing.
Anomalous results like these are possible even if the data is changed only momentarily by a transaction that is ultimately aborted. The ACCESS lock is most useful to those who simply want an overview of data and are not concerned with consistency.
Example : LOCKING ROW
This example shows the proper use of a rowhash lock.
CREATE TABLE customer (
cust_id INTEGER,
phone INTEGER,
fax INTEGER,
telex INTEGER)
PRIMARY INDEX (cust_id),
UNIQUE INDEX(fax),
INDEX(telex):
CREATE TABLE sales (
custcode INTEGER,
zip INTEGER,
salesvol INTEGER);
User A:
BEGIN TRANSACTION;
LOCKING ROW EXCLUSIVE
SELECT phone
FROM customer
WHERE cust_id=12345;
UPDATE customer
SET phone=3108292488
WHERE cust_id=12345;
The User A EXCLUSIVE rowhash lock prevents another user from accessing the same row.
In the following, the user A rowhash WRITE lock, in conjunction with LOCKING TABLE, prevents user B from accessing the same row:
User A:
BEGIN TRANSACTION;
LOCKING TABLE sales FOR READ,
LOCKING ROW FOR WRITE
SELECT telex
FROM customer
WHERE fax=0;
UPDATE customer
SET telex=0
WHERE fax=0;
SELECT zip
FROM sales
WHERE custcode=111;
SELECT salesvol
FROM sales
WHERE custcode=222;
...
END TRANSACTION;
User B:
BEGIN TRANSACTION;
LOCKING ROW FOR WRITE
SELECT *
FROM customer
WHERE cust_id=12345
INSERT INTO customer (12345, 3108284422, 3108684231, 5555);
END TRANSACTION;
The User B LOCKING ROW FOR WRITE modifier waits until the User A transaction ends before it can be completed.
Example : NOWAIT Option
You have a request that you do not want to be placed in the lock queue if it cannot be serviced immediately. In this case, use the NOWAIT option. For example:
LOCKING employee FOR READ NOWAIT
SELECT employee_number, last_name
FROM employee
WHERE department_number=401;
*** Failure 7423 Object already locked and NOWAIT.
Transaction Aborted. Statement# 1, Info =0
Another request had employee locked, so you must resubmit the request.
LOCKING employee FOR READ NOWAIT
SELECT employee_number, last_name
FROM employee
WHERE department_number=401;
*** Query completed. 7 rows found. 2 columns returned.
*** Total elapsed time was 1 second.
employee_number last_name
--------------- --------------------
1003 Trader
1004 Johnson
1013 Phillips
1002 Brown
1010 Rogers
1022 Machado
1001 Hoover
This time, no locks were being held on employee, so the request completed successfully.