分布式连接 ========================== 本节主要介绍 OceanBase 数据库支持的分布式连接的几种典型场景。 PARTITION-WISE JOIN 场景 ------------------------------- PARTITION-WISE JOIN 是指两表的连接条件包含了两表的分区键，并且两表的分区方式是一样的。 在如下例所示，t2 和 t3 都是在 c1 这个列上进行了 key 分区，分区总数为 4 个分区，分区模式完全相同。当查询的条件为 `t2.c1=t3.c1 and t2.c2=t3.c2` 时，查询条件完全包含了分区键，查询可以以分区为单位在每个分区内进行。如果并行度为 4 的话，该查询可以同时做 4 个分区的JOIN 并且将最后结果输出。 ```javascript create table t2 (c1 int, c2 int) partition by key(c1) partitions 4; create table t3 (c1 int, c2 int) partition by key(c1) partitions 4; ``` ```javascript explain select * from t2, t3 where t2.c1 = t3.c1 and t2.c2=t3.c2\G *************************** 1. row *************************** Query Plan: ============================================ |ID|OPERATOR |NAME|EST. ROWS|COST| -------------------------------------------- |0 |EXCHANGE IN DISTR | |63 |8374| |1 | EXCHANGE OUT DISTR| |63 |8362| |2 | HASH JOIN | |63 |8362| |3 | TABLE SCAN |t2 |4000 |499 | |4 | TABLE SCAN |t3 |4000 |499 | ============================================ Outputs & filters: ------------------------------------- 0 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil) 1 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil) 2 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil), equal_conds([t2.c1 = t3.c1], [t2.c2 = t3.c2]), other_conds(nil) 3 - output([t2.c1], [t2.c2]), filter(nil), access([t2.c1], [t2.c2]), partitions(p[0-3]) 4 - output([t3.c1], [t3.c2]), filter(nil), access([t3.c1], [t3.c2]), partitions(p[0-3]) ``` 以一边分区表的分区模式重新分布另一个表的场景 ------------------------------- 同样以 t2 和 t3 表为例，如下例所示，查询 JOIN 条件覆盖了 t2 的分区键而没有覆盖 t3 的分区键，所以执行计划将会按照 t2 的分区模式，将 t3 的数据进行以 partition key 为目标的分组。在将 t3 的数据依照 c2 的值和 t2 的 c1 列的分区方式打散重分布后，可以以 t2 的分区为单位进行分区间的并行连接。 ```javascript explain select * from t2, t3 where t2.c1 = t3.c2\G *************************** 1. row *************************** Query Plan: ======================================================= |ID|OPERATOR |NAME|EST. ROWS|COST | ------------------------------------------------------- |0 |EXCHANGE IN DISTR | |31680 |35454| |1 | EXCHANGE OUT DISTR | |31680 |29456| |2 | HASH JOIN | |31680 |29456| |3 | TABLE SCAN |t2 |4000 |499 | |4 | EXCHANGE IN DISTR | |4000 |878 | |5 | EXCHANGE OUT DISTR (PKEY)| |4000 |499 | |6 | TABLE SCAN |t3 |4000 |499 | ======================================================= Outputs & filters: ------------------------------------- 0 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil) 1 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil) 2 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil), equal_conds([t2.c1 = t3.c2]), other_conds(nil) 3 - output([t2.c1], [t2.c2]), filter(nil), access([t2.c1], [t2.c2]), partitions(p[0-3]) 4 - output([t3.c1], [t3.c2]), filter(nil) 5 - (#keys=1, [t3.c2]), output([t3.c1], [t3.c2]), filter(nil) 6 - output([t3.c1], [t3.c2]), filter(nil), access([t3.c1], [t3.c2]), partitions(p[0-3]) ``` 两个表都重新分布的场景 -------------------- 对于同样的 t2 和 t3 表，如果连接条件既没有包含 t2 的分区键也没有包含 t3 的分区键，会生成如下例所示的计划。注意这个计划的 JOIN 部分是在主线程完成。在后续的 OceanBase 数据库版本中，这样的计划的 JOIN 部分也会在工作线程以并行的方式执行。 ```javascript explain select * from t2, t3 where t2.c2 = t3.c2\G *************************** 1. row *************************** Query Plan: ============================================== |ID|OPERATOR |NAME|EST. ROWS|COST | ---------------------------------------------- |0 |HASH JOIN | |31680 |29835| |1 | EXCHANGE IN DISTR | |4000 |878 | |2 | EXCHANGE OUT DISTR| |4000 |499 | |3 | TABLE SCAN |t2 |4000 |499 | |4 | EXCHANGE IN DISTR | |4000 |878 | |5 | EXCHANGE OUT DISTR| |4000 |499 | |6 | TABLE SCAN |t3 |4000 |499 | ============================================== Outputs & filters: ------------------------------------- 0 - output([t2.c1], [t2.c2], [t3.c1], [t3.c2]), filter(nil), equal_conds([t2.c2 = t3.c2]), other_conds(nil) 1 - output([t2.c1], [t2.c2]), filter(nil) 2 - output([t2.c1], [t2.c2]), filter(nil) 3 - output([t2.c1], [t2.c2]), filter(nil), access([t2.c1], [t2.c2]), partitions(p[0-3]) 4 - output([t3.c1], [t3.c2]), filter(nil) 5 - output([t3.c1], [t3.c2]), filter(nil) 6 - output([t3.c1], [t3.c2]), filter(nil), access([t3.c1], [t3.c2]), partitions(p[0-3]) ```