OceanBase 数据库目前只支持一种基于代价的查询改写——或展开（OR-EXPANSION）。

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数据库中很多高级的改写规则（例如 complex view merge 和窗口函数改写）都需要基于代价进行改写，OceanBase 数据库后续版本会支持这些复杂的改写规则。

或展开（OR-EXPANSION）

OR-EXPANSION 是将一个查询改写成若干个用 UNION 组成的子查询，可以为每个子查询提供更优的优化空间，但是也会导致多个子查询的执行，所以这个改写需要基于代价去判断。

OR-EXPANSION 的改写主要有如下三个作用:

• 允许每个分支使用不同的索引来加速查询。

  如下例所示，Q1 会被改写成 Q2 的形式，其中 Q2 中的谓词 LNNVL(t1.a = 1) 保证了这两个子查询不会生成重复的结果。如果不进行改写，Q1 一般来说会选择主表作为访问路径，对于 Q2 来说，如果 t1 上存在索引（a）和索引（b），那么该改写可能会让 Q2 中的每一个子查询选择索引作为访问路径。

  Q1: 
  obclient>SELECT * FROM t1 WHERE t1.a = 1 OR t1.b = 1;
  Q2: 
  obclient>SELECT * FROM t1 WHERE t1.a = 1 UNION ALL SELECT * FROM t1.b = 1 
         AND LNNVL(t1.a = 1);

  完整示例如下：

  obclient>CREATE TABLE t1(a INT, b INT, c INT, d INT, e INT, INDEX IDX_a(a), 
           INDEX IDX_b(b));
  Query OK, 0 rows affected (0.17 sec)
  
  /*如果不进行 OR-EXPANSION 的改写，该查询只能使用主表访问路径*/
  obclient> EXPLAIN SELECT/*+NO_REWRITE()*/ * FROM t1 WHERE t1.a = 1 OR t1.b = 1;
  +--------------------------------------------------------------+
  | Query Plan                                                                         |
  +--------------------------------------------------------------+
  | ===================================
  |ID|OPERATOR  |NAME|EST. ROWS|COST|
  -----------------------------------
  |0 |TABLE SCAN|t1  |4        |649 |
  ===================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter([t1.a = 1 OR t1.b = 1]),
        access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p0)
  
  /*改写之后，每个子查询能使用不同的索引访问路径*/
  obclient>EXPLAIN SELECT * FROM t1 WHERE t1.a = 1 OR t1.b = 1;
  +------------------------------------------------------------------------+
  | Query Plan                                                                                         |
  +------------------------------------------------------------------------+
  | =========================================
  |ID|OPERATOR   |NAME     |EST. ROWS|COST|
  -----------------------------------------
  |0 |UNION ALL  |         |3        |190 |
  |1 | TABLE SCAN|t1(idx_a)|2        |94  |
  |2 | TABLE SCAN|t1(idx_b)|1        |95  |
  =========================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)], [UNION(t1.c, t1.c)], [UNION(t1.d, t1.d)], [UNION(t1.e, t1.e)]), filter(nil)
    1 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter(nil),
        access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p0)
    2 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter([lnnvl(t1.a = 1)]),
        access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p02

• 允许每个分支使用不同的连接算法来加速查询，避免使用笛卡尔联接。

  如下例所示，Q1 会被改写成 Q2 的形式。对于 Q1 来说，它的联接方式只能是 NESTED LOOP JOIN (笛卡尔乘积)， 但是被改写之后，每个子查询都可以选择 NESTED LOOP JOIN、HASH JOIN 或者 MERGE JOIN，这样会有更多的优化空间。

  Q1: 
  obclient>SELECT * FROM t1, t2 WHERE t1.a = t2.a OR t1.b = t2.b;
  
  Q2: 
  obclient>SELECT * FROM t1, t2 WHERE t1.a = t2.a UNION ALL
       SELECT * FROM t1, t2 WHERE t1.b = t2.b AND LNNVL(t1.a = t2.a);

  完整示例如下：

  obclient> CREATE TABLE t1(a INT, b INT);
  Query OK, 0 rows affected (0.17 sec)
  
  obclient> CREATE TABLE t2(a INT, b INT);
  Query OK, 0 rows affected (0.13 sec)
  
  /*如果不进行改写，只能使用 NESTED LOOP JOIN*/
  obclient> EXPLAIN SELECT/*+NO_REWRITE()*/ * FROM t1, t2 
         WHERE t1.a = t2.a OR t1.b = t2.b;
  +--------------------------------------------------------------------------+
  | Query Plan                                                                                          |
  +--------------------------------------------------------------------------+
  | ===========================================
  |ID|OPERATOR        |NAME|EST. ROWS|COST  |
  -------------------------------------------
  |0 |NESTED-LOOP JOIN|    |3957     |585457|
  |1 | TABLE SCAN     |t1  |1000     |499   |
  |2 | TABLE SCAN     |t2  |4        |583   |
  ===========================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
        conds(nil), nl_params_([t1.a], [t1.b])
    1 - output([t1.a], [t1.b]), filter(nil),
        access([t1.a], [t1.b]), partitions(p0)
    2 - output([t2.a], [t2.b]), filter([? = t2.a OR ? = t2.b]),
        access([t2.a], [t2.b]), partitions(p0)
  
  /*被改写之后，每个子查询都使用了 HASH JOIN*/
  obclient>  EXPLAIN SELECT * FROM t1, t2 WHERE t1.a = t2.a OR t1.b = t2.b;
  +--------------------------------------------------------------------------+
  | Query Plan                                                                                         |
  +--------------------------------------------------------------------------+
  |ID|OPERATOR    |NAME|EST. ROWS|COST|
  -------------------------------------
  |0 |UNION ALL   |    |2970     |9105|
  |1 | HASH JOIN  |    |1980     |3997|
  |2 |  TABLE SCAN|t1  |1000     |499 |
  |3 |  TABLE SCAN|t2  |1000     |499 |
  |4 | HASH JOIN  |    |990      |3659|
  |5 |  TABLE SCAN|t1  |1000     |499 |
  |6 |  TABLE SCAN|t2  |1000     |499 |
  =====================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)], [UNION(t2.a, t2.a)], [UNION(t2.b, t2.b)]), filter(nil)
    1 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
        equal_conds([t1.a = t2.a]), other_conds(nil)
    2 - output([t1.a], [t1.b]), filter(nil),
        access([t1.a], [t1.b]), partitions(p0)
    3 - output([t2.a], [t2.b]), filter(nil),
        access([t2.a], [t2.b]), partitions(p0)
    4 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
        equal_conds([t1.b = t2.b]), other_conds([lnnvl(t1.a = t2.a)])
    5 - output([t1.a], [t1.b]), filter(nil),
        access([t1.a], [t1.b]), partitions(p0)
    6 - output([t2.a], [t2.b]), filter(nil),
        access([t2.a], [t2.b]), partitions(p0)

• 允许每个分支分别消除排序，更加快速的获取 TOP-K 结果。

  如下例所示，Q1 会被改写成 Q2。对于 Q1 来说，执行方式是只能把满足条件的行数找出来，然后进行排序，最终取 TOP-10 结果。对于 Q2 来说，如果存在索引（a,b）, 那么 Q2 中的两个子查询都可以使用索引把排序消除，每个子查询取 TOP-10 结果，然后最终对这 20 行数据排序一下取出最终的 TOP-10 行。

  Q1: 
  obclient>SELECT * FROM t1 WHERE t1.a = 1 OR t1.a = 2 ORDER BY b LIMIT 10;
  
  Q2: 
  obclient>SELECT * FROM  
      (SELECT * FROM  t1 WHERE t1.a = 1 ORDER BY b LIMIT 10 UNION ALL
       SELECT * FROM  t1 WHERE t1.a = 2 ORDER BY b LIMIT 10) AS TEMP
      ORDER BY temp.b LIMIT 10;

  完整示例如下：

  obclient> CREATE TABLE t1(a INT, b INT, INDEX IDX_a(a, b));
  Query OK, 0 rows affected (0.20 sec)
  
  /*不改写的话，需要排序最终获取 TOP-K 结果*/
  obclient> EXPLAIN SELECT/*+NO_REWRITE()*/ * FROM t1 WHERE t1.a = 1 OR t1.a = 2 
          ORDER BY b LIMIT 10;
  +-------------------------------------------------------------------------+
  | Query Plan                                                                                         |
  +-------------------------------------------------------------------------+
  | ==========================================
  |ID|OPERATOR    |NAME     |EST. ROWS|COST|
  ------------------------------------------
  |0 |LIMIT       |         |4        |77  |
  |1 | TOP-N SORT |         |4        |76  |
  |2 |  TABLE SCAN|t1(idx_a)|4        |73  |
  ==========================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t1.a], [t1.b]), filter(nil), limit(10), offset(nil)
    1 - output([t1.a], [t1.b]), filter(nil), sort_keys([t1.b, ASC]), topn(10)
    2 - output([t1.a], [t1.b]), filter(nil),
        access([t1.a], [t1.b]), partitions(p0)
  
  /* 进行改写的话，排序算子可以被消除，最终获取 TOP-K 结果*/
  obclient>EXPLAIN SELECT * FROM t1 WHERE t1.a = 1 OR t1.a = 2 
          ORDER BY b LIMIT 10;
  +-------------------------------------------------------------------------+
  | Query Plan                                                                                          |
  +-------------------------------------------------------------------------+
  | ===========================================
  |ID|OPERATOR     |NAME     |EST. ROWS|COST|
  -------------------------------------------
  |0 |LIMIT        |         |3        |76  |
  |1 | TOP-N SORT  |         |3        |76  |
  |2 |  UNION ALL  |         |3        |74  |
  |3 |   TABLE SCAN|t1(idx_a)|2        |37  |
  |4 |   TABLE SCAN|t1(idx_a)|1        |37  |
  ===========================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil), limit(10), offset(nil)
    1 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil), sort_keys([UNION(t1.b, t1.b), ASC]), topn(10)
    2 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil)
    3 - output([t1.a], [t1.b]), filter(nil),
        access([t1.a], [t1.b]), partitions(p0),
        limit(10), offset(nil)
    4 - output([t1.a], [t1.b]), filter([lnnvl(t1.a = 1)]),
        access([t1.a], [t1.b]), partitions(p0),
        limit(10), offset(nil)

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