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1 Overview
- Alone
select 语句is not going to add any locks - By
for update 语句locking data or row data table
2 Grammar
select * from table_name for update [of column1, columnN][wait n|nowait];
Syntax analysis:
1. 表级锁: 锁住表中所有的记录
select * from table_name for update; -- 锁住表中所有的记录
select * from table_name where column_name = 1 for update; -- 锁住表中列名为 1 的记录
2. 行级锁: 锁住某列的记录（单表时，加不加效果一样。多表时，锁定 of column 列对应的表）
select * from table_name for update of column1;
3. wait n | nowait
(1) wait 5: 当锁冲突时,等待 5s 后报错（资源正忙...）
(2) nowait: 当锁冲突时,立即报错（资源正忙...）
3 Examples
3.1 for update of column
Overall conclusion:
| Number of connected tables | Explanation |
|---|---|
| Single table | for update 和 for update of it's the same |
| Multi-table | Lock only of columnthe corresponding table (same table to lock a column) |
other:
- Without the where condition, the entire table is locked
- Added where condition is a row-level lock
Single table: there is no difference between for update and for update of column
Conclusion: All dml statements in session 2 must wait for session 1 commit or rollback
Please note:
1.scott.emp and scott.dept are built-in Oracle tables,
2.scott.emp used for testing is a data table, and scott.dept is a type table (foreign key)
Scenario 1: for update
session 1:
SELECT * FROM scott.dept t WHERE t.deptno = 10 FOR UPDATE;
session 2:
UPDATE scott.dept t SET t.loc = 'test_lock' WHERE t.deptno = 10;
场景2：for update of column
session 1:
SELECT * FROM scott.dept t WHERE t.deptno = 10 FOR UPDATE OF t.loc, t.dname; -- 仅多了 OF t.loc, t.dname
session 2:
UPDATE scott.dept t SET t.loc = 'test_lock' WHERE t.deptno = 10;
Conclusion: The table corresponding to the non-of column column in the dml statements in session 2 and 3 does not need to wait for session 1 commit or rollback
Scenario 1: for update
session 1:
SELECT e.empno, e.ename, d.deptno, d.dname
FROM scott.emp e, scott.dept d
WHERE d.deptno = e.deptno
AND d.deptno = 10
FOR UPDATE;
session 2:
UPDATE scott.emp e SET t.ename = 'test_ename' WHERE e.deptno = 10;
session 3:
UPDATE scott.dept d SET t.dname = 'test_dname' WHERE d.deptno = 10;
场景2：for update of column
session 1:
SELECT e.empno, e.ename, d.deptno, d.dname
FROM scott.emp e, scott.dept d
WHERE d.deptno = e.deptno
AND d.deptno = 10
FOR UPDATE OF d.dname; -- 仅多了 OF d.dname
session 2:
UPDATE scott.emp e SET t.ename = 'test_ename' WHERE e.deptno = 10;
session 3:
UPDATE scott.dept d SET t.dname = 'test_dname' WHERE d.deptno = 10;
3.2 wait n | nowait
in conclusion:
- wait 5: When the lock conflicts, an error will be reported after waiting 5s (resources are busy...)
- nowait: When the lock conflicts, an error is reported immediately (resources are busy...)
session 1:
SELECT * FROM scott.dept t WHERE t.deptno = 10 FOR UPDATE;
session 2:
SELECT * FROM scott.dept t WHERE t.deptno = 10 FOR UPDATE NOWAIT;
4 Extension: rowid
1. 通过 for update 的方式修改表记录会产生 锁
SELECT * FROM scott.dept t WHERE t.deptno = 10 FOR UPDATE;
2. 通过 rowid 的方式修改表记录是不会产生 锁 的
SELECT t.*, ROWID FROM scott.dept t WHERE t.deptno = 10;