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:
-
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。
-
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)