树叶云OceanBase教程：OceanBase 联接顺序

在多表联接的场景中，优化器的一个很重要的任务是决定各个表之间的联接顺序（Join Order），因为不同的联接顺序会影响中间结果集的大小，进而影响到计划整体的执行代价。

为了减少执行计划的搜索空间和计划执行的内存占用，OceanBase 数据库优化器在生成联接顺序时主要考虑左深树的联接形式。下图展示了左深树、右深树和多支树的计划形状。

OceanBase 数据库联接顺序的生成采用了 System-R 的动态规划算法，考虑的因素包括每一个表可能的访问路径、Interesting Order、联接算法（NESTED-LOOP、BLOCK-BASED NESTED-LOOP 或者 SORT-MERGE 等）以及不同表之间的联接选择率等。

给定 N 个表的联接，OceanBase 数据库生成联接顺序的方法如下:

1. 为每一个基表生成访问路径，保留代价最小的访问路径以及有所有有 Interesting Order 的路径。一个路径 如果具有 Interesting Order，它的序能够被后续的算子使用。

2. 生成所有表集合的大小为 i (1 < i <= N) 的计划。 OceanBase 数据库一般只考虑左深树，表集合大小为 i 的计划可以由一个表集合大小为 i 的计划和一个基表的计划组成。OceanBase 数据库按照这种策略，考虑了所有的联接算法以及 Interesting Order 的继承等因素把所有表集合大小为 i 的计划生成。这里也只是保留代价最小的计划以及所有具有 Interesting Order 的计划。

同时，OceanBase 数据库提供了 HINT 机制 /*+LEADING(table_name_list)*/去控制多表联接的顺序。

如下例所示，开始选择的联接顺序是先做 t1、t2 的 JOIN 联接，然后再和 t3 做 JOIN 联接；如果用户希望先做 t2、t3 的 JOIN 联接，然后再和 t1做 JOIN 联接，则可以使用 HINT /*+LEADING(t2,t3,t1)*/去控制；如果用户希望先做 t1、t3 的 JOIN 联接，然后再和 t2 做 JOIN 联接，则可以使用 HINT /*+LEADING(t1,t3,t2)*/去控制。

obclient>CREATE TABLE t1(c1 INT, c2 INT, PRIMARY KEY(c1));
Query OK, 0 rows affected (0.31 sec)

obclient>CREATE TABLE t2(c1 INT, c2 INT, PRIMARY KEY(c1));
Query OK, 0 rows affected (0.33 sec)

obclient>CREATE TABLE t3(c1 INT, c2 INT, PRIMARY KEY(c1));
Query OK, 0 rows affected (0.44 sec)

obclient>EXPLAIN SELECT * FROM t1,t2,t3 WHERE t1.c1 = t2.c2 AND t2.c1 = t3.c2;
+-----------------------------------------------------------------+
| Query Plan                                                                              |
+-----------------------------------------------------------------+
| =======================================
|ID|OPERATOR    |NAME|EST. ROWS|COST  |
---------------------------------------
|0 |HASH JOIN   |    |98010    |926122|
|1 | TABLE SCAN |T3  |100000   |61860 |
|2 | HASH JOIN  |    |99000    |494503|
|3 |  TABLE SCAN|T1  |100000   |61860 |
|4 |  TABLE SCAN|T2  |100000   |61860 |
=======================================

Outputs & filters: 
-------------------------------------
  0 - output([T1.C1], [T1.C2], [T2.C1], [T2.C2], [T3.C1], [T3.C2]), filter(nil), 
      equal_conds([T2.C1 = T3.C2]), other_conds(nil)
  1 - output([T3.C2], [T3.C1]), filter(nil), 
      access([T3.C2], [T3.C1]), partitions(p0)
  2 - output([T1.C1], [T1.C2], [T2.C1], [T2.C2]), filter(nil), 
      equal_conds([T1.C1 = T2.C2]), other_conds(nil)
  3 - output([T1.C1], [T1.C2]), filter(nil), 
      access([T1.C1], [T1.C2]), partitions(p0)
  4 - output([T2.C2], [T2.C1]), filter(nil), 
      access([T2.C2], [T2.C1]), partitions(p0)

obclient>EXPLAIN SELECT /*+LEADING(t2,t3,t1)*/* FROM t1,t2,t3 WHERE t1.c1 = t2.c2
        AND t2.c1 = t3.c2;
+-----------------------------------------------------------------+
| Query Plan                                                                              |
+-----------------------------------------------------------------+
| ========================================
|ID|OPERATOR    |NAME|EST. ROWS|COST   |
----------------------------------------
|0 |HASH JOIN   |    |98010    |1096613|
|1 | HASH JOIN  |    |99000    |494503 |
|2 |  TABLE SCAN|T2  |100000   |61860  |
|3 |  TABLE SCAN|T3  |100000   |61860  |
|4 | TABLE SCAN |T1  |100000   |61860  |
========================================

Outputs & filters: 
-------------------------------------
  0 - output([T1.C1], [T1.C2], [T2.C1], [T2.C2], [T3.C1], [T3.C2]), filter(nil), 
      equal_conds([T1.C1 = T2.C2]), other_conds(nil)
  1 - output([T2.C1], [T2.C2], [T3.C1], [T3.C2]), filter(nil), 
      equal_conds([T2.C1 = T3.C2]), other_conds(nil)
  2 - output([T2.C2], [T2.C1]), filter(nil), 
      access([T2.C2], [T2.C1]), partitions(p0)
  3 - output([T3.C2], [T3.C1]), filter(nil), 
      access([T3.C2], [T3.C1]), partitions(p0)
  4 - output([T1.C1], [T1.C2]), filter(nil), 
      access([T1.C1], [T1.C2]), partitions(p0)

obclient>EXPLAIN SELECT /*+LEADING(t1,t3,t2)*/* FROM t1,t2,t3 WHERE t1.c1 = t2.c2 
       AND t2.c1 = t3.c2;
+-----------------------------------------------------------------+
| Query Plan                                                                              |
+-----------------------------------------------------------------+
| =============================================================
|ID|OPERATOR                   |NAME|EST. ROWS  |COST       |
-------------------------------------------------------------
|0 |HASH JOIN                  |    |98010      |53098071243|
|1 | NESTED-LOOP JOIN CARTESIAN|    |10000000000|7964490204 |
|2 |  TABLE SCAN               |T1  |100000     |61860      |
|3 |  MATERIAL                 |    |100000     |236426     |
|4 |   TABLE SCAN              |T3  |100000     |61860      |
|5 | TABLE SCAN                |T2  |100000     |61860      |
=============================================================

Outputs & filters: 
-------------------------------------
  0 - output([T1.C1], [T1.C2], [T2.C1], [T2.C2], [T3.C1], [T3.C2]), filter(nil), 
      equal_conds([T1.C1 = T2.C2], [T2.C1 = T3.C2]), other_conds(nil)
  1 - output([T1.C1], [T1.C2], [T3.C1], [T3.C2]), filter(nil), 
      conds(nil), nl_params_(nil)
  2 - output([T1.C1], [T1.C2]), filter(nil), 
      access([T1.C1], [T1.C2]), partitions(p0)
  3 - output([T3.C1], [T3.C2]), filter(nil)
  4 - output([T3.C2], [T3.C1]), filter(nil), 
      access([T3.C2], [T3.C1]), partitions(p0)
  5 - output([T2.C2], [T2.C1]), filter(nil), 
      access([T2.C2], [T2.C1]), partitions(p0)