OceanBase 数据库目前只支持基于代价的查询改写—或展开（OR-EXPANSION）。数据库中很多高级的改写规则(比如 complex view merge 和窗口函数改写)都需要基于代价进行改写，OceanBase 数据库后续会支持这些复杂的改写规则。

或展开（OR-EXPANSION）

OR-EXPANSION 把一个查询改写成若干个用 UNION 组成的子查询，这个改写可能会给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。通常来说，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。

  1. Q1: select * from t1 where t1.a = 1 or t1.b = 1;
  2. Q2: select * from t1 where t1.a = 1 union all select * from t1.b = 1 and lnnvl(t1.a = 1);
  1. obclient> create table t1(a int, b int, c int, d int, e int, index idx_a(a), index idx_b(b));
  2. --- 如果不进行or-expansion的改写，该查询只能使用主表访问路径
  3. obclient> explain select/*+NO_REWRITE()*/ * from t1 where t1.a = 1 or t1.b = 1;
  4. +-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  5. | Query Plan                                                                                                                                                                                                                                                                                                                                                                                                  |
  6. +-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  7. | ===================================
  8. |ID|OPERATOR  |NAME|EST. ROWS|COST|
  9. -----------------------------------
  10. |0 |TABLE SCAN|t1  |4        |649 |
  11. ===================================
  13. Outputs & filters:
  14. -------------------------------------
  15.   0 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter([t1.a = 1 OR t1.b = 1]),
  16.       access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p0)
  17. --- 改写之后，每个子查询能使用不同的索引访问路径
  18. obclient> explain select * from t1 where t1.a = 1 or t1.b = 1;
  19. +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  20. | Query Plan                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
  21. +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  22. | =========================================
  23. |ID|OPERATOR   |NAME     |EST. ROWS|COST|
  24. -----------------------------------------
  25. |0 |UNION ALL  |         |3        |190 |
  26. |1 | TABLE SCAN|t1(idx_a)|2        |94  |
  27. |2 | TABLE SCAN|t1(idx_b)|1        |95  |
  28. =========================================
  30. Outputs & filters:
  31. -------------------------------------
  32.   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)
  33.   1 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter(nil),
  34.       access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p0)
  35.   2 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter([lnnvl(t1.a = 1)]),
  36.       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，避免使用笛卡尔连接。如下例所示，Q1 会被改写成 Q2 的形式。对于 Q1 来说，它的连接方式只能是 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。
  1. Q1: select * from t1, t2 where t1.a = t2.a or t1.b = t2.b;
  2. Q2: select * from t1, t2 where t1.a = t2.a union all
  3.     select * from t1, t2 where t1.b = t2.b and lnnvl(t1.a = t2.a)
  1. obclient> create table t1(a int, b int);
  2. Query OK, 0 rows affected (0.17 sec)
  3. obclient> create table t2(a int, b int);
  4. Query OK, 0 rows affected (0.13 sec)
  5. ---如果不进行改写，只能使用nested loop join
  6. obclient> explain select/*+NO_REWRITE()*/ * from t1, t2 where t1.a = t2.a or t1.b = t2.b;
  7. | ===========================================
  8. |ID|OPERATOR        |NAME|EST. ROWS|COST  |
  9. -------------------------------------------
  10. |0 |NESTED-LOOP JOIN|    |3957     |585457|
  11. |1 | TABLE SCAN     |t1  |1000     |499   |
  12. |2 | TABLE SCAN     |t2  |4        |583   |
  13. ===========================================
  15. Outputs & filters:
  16. -------------------------------------
  17.   0 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
  18.       conds(nil), nl_params_([t1.a], [t1.b])
  19.   1 - output([t1.a], [t1.b]), filter(nil),
  20.       access([t1.a], [t1.b]), partitions(p0)
  21.   2 - output([t2.a], [t2.b]), filter([? = t2.a OR ? = t2.b]),
  22.       access([t2.a], [t2.b]), partitions(p0)
  23. ---被改写之后，每个子查询都使用了hash join
  24. obclient> explain select * from t1, t2 where t1.a = t2.a or t1.b = t2.b;
  25. |ID|OPERATOR    |NAME|EST. ROWS|COST|
  26. -------------------------------------
  27. |0 |UNION ALL   |    |2970     |9105|
  28. |1 | HASH JOIN  |    |1980     |3997|
  29. |2 |  TABLE SCAN|t1  |1000     |499 |
  30. |3 |  TABLE SCAN|t2  |1000     |499 |
  31. |4 | HASH JOIN  |    |990      |3659|
  32. |5 |  TABLE SCAN|t1  |1000     |499 |
  33. |6 |  TABLE SCAN|t2  |1000     |499 |
  34. =====================================
  36. Outputs & filters:
  37. -------------------------------------
  38.   0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)], [UNION(t2.a, t2.a)], [UNION(t2.b, t2.b)]), filter(nil)
  39.   1 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
  40.       equal_conds([t1.a = t2.a]), other_conds(nil)
  41.   2 - output([t1.a], [t1.b]), filter(nil),
  42.       access([t1.a], [t1.b]), partitions(p0)
  43.   3 - output([t2.a], [t2.b]), filter(nil),
  44.       access([t2.a], [t2.b]), partitions(p0)
  45.   4 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
  46.       equal_conds([t1.b = t2.b]), other_conds([lnnvl(t1.a = t2.a)])
  47.   5 - output([t1.a], [t1.b]), filter(nil),
  48.       access([t1.a], [t1.b]), partitions(p0)
  49.   6 - output([t2.a], [t2.b]), filter(nil),
  50.       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。
  1. Q1: select * from t1 where t1.a = 1 or t1.a = 2 order by b limit 10;
  2. Q2: select * from 
  3.     (select * from t1 where t1.a = 1 order by b limit 10 union all
  4.      select * from t1 where t1.a = 2 order by b limit 10) as temp
  5.     order by temp.b limit 10;
  1. obclient> create table t1(a int, b int, index idx_a(a, b));
  2. Query OK, 0 rows affected (0.20 sec)
  3. ---不改写的话，需要排序最终获取top-k结果
  4. obclient> explain select/*+NO_REWRITE()*/ * from t1 where t1.a = 1 or t1.a = 2 order by b limit 10;
  5. | ==========================================
  6. |ID|OPERATOR    |NAME     |EST. ROWS|COST|
  7. ------------------------------------------
  8. |0 |LIMIT       |         |4        |77  |
  9. |1 | TOP-N SORT |         |4        |76  |
  10. |2 |  TABLE SCAN|t1(idx_a)|4        |73  |
  11. ==========================================
  13. Outputs & filters:
  14. -------------------------------------
  15.   0 - output([t1.a], [t1.b]), filter(nil), limit(10), offset(nil)
  16.   1 - output([t1.a], [t1.b]), filter(nil), sort_keys([t1.b, ASC]), topn(10)
  17.   2 - output([t1.a], [t1.b]), filter(nil),
  18.       access([t1.a], [t1.b]), partitions(p0)
  19. --- 进行改写的话，排序算子可以被消除，最终获取top-k结果
  20. obclient> explain select * from t1 where t1.a = 1 or t1.a = 2 order by b limit 10;
  21. | ===========================================
  22. |ID|OPERATOR     |NAME     |EST. ROWS|COST|
  23. -------------------------------------------
  24. |0 |LIMIT        |         |3        |76  |
  25. |1 | TOP-N SORT  |         |3        |76  |
  26. |2 |  UNION ALL  |         |3        |74  |
  27. |3 |   TABLE SCAN|t1(idx_a)|2        |37  |
  28. |4 |   TABLE SCAN|t1(idx_a)|1        |37  |
  29. ===========================================
  31. Outputs & filters:
  32. -------------------------------------
  33.   0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil), limit(10), offset(nil)
  34.   1 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil), sort_keys([UNION(t1.b, t1.b), ASC]), topn(10)
  35.   2 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil)
  36.   3 - output([t1.a], [t1.b]), filter(nil),
  37.       access([t1.a], [t1.b]), partitions(p0),
  38.       limit(10), offset(nil)
  39.   4 - output([t1.a], [t1.b]), filter([lnnvl(t1.a = 1)]),
  40.       access([t1.a], [t1.b]), partitions(p0),
  41.       limit(10), offset(nil)