树叶云OceanBase教程：OceanBase FOR UPDATE

FOR UPDATE 算子用于对表中的数据进行加锁操作。

OceanBase 数据库支持的 FOR UPDATE 算子包括 FOR UPDATE 和 MULTI FOR UPDATE。

FOR UPDATE 算子执行查询的一般流程如下：

1. 首先执行 ​SELECT​ 语句部分，获得查询结果集。
2. 对查询结果集相关的记录进行加锁操作。

FOR UPDATE

FOR UPDATE 用于对单表（或者单个分区）进行加锁。

如下示例中，Q1 查询是对 t1 表中满足 c1 = 1 的行进行加锁。这里 t1 表是一张单分区的表，所以 1 号算子生成了一个 FOR UPDATE 算子。

obclient>CREATE TABLE t1(c1 INT, c2 INT);
Query OK, 0 rows affected (0.12 sec)

obclient>CREATE TABLE t2(c1 INT, c2 INT);
Query OK, 0 rows affected (0.12 sec)

obclient>INSERT INTO t1 VALUES(1, 1);
Query OK, 0 rows affected (0.12 sec)

obclient>INSERT INTO t1 VALUES(2, 2);
Query OK, 1 rows affected (0.12 sec)

obclient>INSERT INTO t1 VALUES(3, 3);
Query OK, 1 rows affected (0.12 sec)

obclient>INSERT INTO t2 VALUES(1, 1);
Query OK, 1 rows affected (0.12 sec)

obclient>INSERT INTO t2 VALUES(2, 2);
Query OK, 1 rows affected (0.12 sec)

obclient>INSERT INTO t2 VALUES(3, 3);
Query OK, 1 rows affected (0.12 sec)

Q1: 
obclient> EXPLAIN SELECT * FROM t1 WHERE c1 = 1 FOR UPDATE\G;
*************************** 1. row ***************************
Query Plan:
=====================================
|ID|OPERATOR    |NAME|EST. ROWS|COST|
-------------------------------------
|0 |MATERIAL    |    |10       |856 |
|1 | FOR UPDATE |    |10       |836 |
|2 |  TABLE SCAN|T1  |10       |836 |
=====================================
Outputs & filters:
-------------------------------------
  0 - output([T1.C1], [T1.C2]), filter(nil)
  1 - output([T1.C1], [T1.C2]), filter(nil), lock tables(T1)
  2 - output([T1.C1], [T1.C2], [T1.__pk_increment]), filter([T1.C1 = 1]),
      access([T1.C1], [T1.C2], [T1.__pk_increment]), partitions(p0)

上述示例中，Q1 查询的执行计划展示中的 outputs & filters 详细列出了 FOR UPDATE 算子的输出信息如下：

MULTI FOR UPDATE

MULTI FOR UPDATE 用于对多表（或者多个分区）进行加锁操作。

如下示例中，Q2 查询是对 t1 和 t2 两张表的数据进行加锁，加锁对象是满足 c1 = 1 AND c1 = d1 的行。由于需要对多个表的行进行加锁，因此 1 号算子是 MULTI FOR UPDATE。

obclient>CREATE TABLE t1 (c1 INT, c2 INT);
Query OK, 0 rows affected (0.12 sec)

obclient>CREATE TABLE t2 (d1 INT, d2 INT);
Query OK, 0 rows affected (0.12 sec)

obclient>EXPLAIN SELECT * FROM t1, t2 WHERE c1 = 1 AND c1 = d1 
                FOR UPDATE\G;
*************************** 1. row ***************************
Query Plan:
=====================================================
|ID|OPERATOR                    |NAME|EST. ROWS|COST|
-----------------------------------------------------
|0 |MATERIAL                    |    |10       |931 |
|1 | MULTI FOR UPDATE           |    |10       |895 |
|2 |  NESTED-LOOP JOIN CARTESIAN|    |10       |895 |
|3 |   TABLE GET                |T2  |1        |52  |
|4 |   TABLE SCAN               |T1  |10       |836 |
=====================================================
Outputs & filters:
-------------------------------------
  0 - output([T1.C1], [T1.C2], [T2.D1], [T2.D2]), filter(nil)
  1 - output([T1.C1], [T1.C2], [T2.D1], [T2.D2]), filter(nil), lock tables(T1, T2)
  2 - output([T1.C1], [T1.C2], [T2.D1], [T2.D2], [T1.__pk_increment]), filter(nil),
      conds(nil), nl_params_(nil)
  3 - output([T2.D1], [T2.D2]), filter(nil),
      access([T2.D1], [T2.D2]), partitions(p0)
  4 - output([T1.C1], [T1.C2], [T1.__pk_increment]), filter([T1.C1 = 1]),
      access([T1.C1], [T1.C2], [T1.__pk_increment]), partitions(p0)

上述示例中，Q2 查询的执行计划展示中的 outputs & filters 详细列出了 MULTI FOR UPDATE 算子的信息如下：