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. OceanBase (root@test)> 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. OceanBase (root@test)> 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. ===================================
  12. Outputs & filters:
  13. -------------------------------------
  14. 0 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter([t1.a = 1 OR t1.b = 1]),
  15. access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p0)
  16. --- 改写之后,每个子查询能使用不同的索引访问路径
  17. OceanBase (root@test)> explain select * from t1 where t1.a = 1 or t1.b = 1;
  18. +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  19. | Query Plan |
  20. +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  21. | =========================================
  22. |ID|OPERATOR |NAME |EST. ROWS|COST|
  23. -----------------------------------------
  24. |0 |UNION ALL | |3 |190 |
  25. |1 | TABLE SCAN|t1(idx_a)|2 |94 |
  26. |2 | TABLE SCAN|t1(idx_b)|1 |95 |
  27. =========================================
  28. Outputs & filters:
  29. -------------------------------------
  30. 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)
  31. 1 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter(nil),
  32. access([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), partitions(p0)
  33. 2 - output([t1.a], [t1.b], [t1.c], [t1.d], [t1.e]), filter([lnnvl(t1.a = 1)]),
  34. 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. OceanBase (root@test)> create table t1(a int, b int);
  2. Query OK, 0 rows affected (0.17 sec)
  3. OceanBase (root@test)> 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. OceanBase (root@test)> 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. ===========================================
  14. Outputs & filters:
  15. -------------------------------------
  16. 0 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
  17. conds(nil), nl_params_([t1.a], [t1.b])
  18. 1 - output([t1.a], [t1.b]), filter(nil),
  19. access([t1.a], [t1.b]), partitions(p0)
  20. 2 - output([t2.a], [t2.b]), filter([? = t2.a OR ? = t2.b]),
  21. access([t2.a], [t2.b]), partitions(p0)
  22. ---被改写之后,每个子查询都使用了hash join
  23. OceanBase (root@test)> explain select * from t1, t2 where t1.a = t2.a or t1.b = t2.b;
  24. |ID|OPERATOR |NAME|EST. ROWS|COST|
  25. -------------------------------------
  26. |0 |UNION ALL | |2970 |9105|
  27. |1 | HASH JOIN | |1980 |3997|
  28. |2 | TABLE SCAN|t1 |1000 |499 |
  29. |3 | TABLE SCAN|t2 |1000 |499 |
  30. |4 | HASH JOIN | |990 |3659|
  31. |5 | TABLE SCAN|t1 |1000 |499 |
  32. |6 | TABLE SCAN|t2 |1000 |499 |
  33. =====================================
  34. Outputs & filters:
  35. -------------------------------------
  36. 0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)], [UNION(t2.a, t2.a)], [UNION(t2.b, t2.b)]), filter(nil)
  37. 1 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
  38. equal_conds([t1.a = t2.a]), other_conds(nil)
  39. 2 - output([t1.a], [t1.b]), filter(nil),
  40. access([t1.a], [t1.b]), partitions(p0)
  41. 3 - output([t2.a], [t2.b]), filter(nil),
  42. access([t2.a], [t2.b]), partitions(p0)
  43. 4 - output([t1.a], [t1.b], [t2.a], [t2.b]), filter(nil),
  44. equal_conds([t1.b = t2.b]), other_conds([lnnvl(t1.a = t2.a)])
  45. 5 - output([t1.a], [t1.b]), filter(nil),
  46. access([t1.a], [t1.b]), partitions(p0)
  47. 6 - output([t2.a], [t2.b]), filter(nil),
  48. 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. OceanBase (root@test)> 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. OceanBase (root@test)> 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. ==========================================
  12. Outputs & filters:
  13. -------------------------------------
  14. 0 - output([t1.a], [t1.b]), filter(nil), limit(10), offset(nil)
  15. 1 - output([t1.a], [t1.b]), filter(nil), sort_keys([t1.b, ASC]), topn(10)
  16. 2 - output([t1.a], [t1.b]), filter(nil),
  17. access([t1.a], [t1.b]), partitions(p0)
  18. --- 进行改写的话,排序算子可以被消除,最终获取top-k结果
  19. OceanBase (root@test)> explain select * from t1 where t1.a = 1 or t1.a = 2 order by b limit 10;
  20. | ===========================================
  21. |ID|OPERATOR |NAME |EST. ROWS|COST|
  22. -------------------------------------------
  23. |0 |LIMIT | |3 |76 |
  24. |1 | TOP-N SORT | |3 |76 |
  25. |2 | UNION ALL | |3 |74 |
  26. |3 | TABLE SCAN|t1(idx_a)|2 |37 |
  27. |4 | TABLE SCAN|t1(idx_a)|1 |37 |
  28. ===========================================
  29. Outputs & filters:
  30. -------------------------------------
  31. 0 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil), limit(10), offset(nil)
  32. 1 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil), sort_keys([UNION(t1.b, t1.b), ASC]), topn(10)
  33. 2 - output([UNION(t1.a, t1.a)], [UNION(t1.b, t1.b)]), filter(nil)
  34. 3 - output([t1.a], [t1.b]), filter(nil),
  35. access([t1.a], [t1.b]), partitions(p0),
  36. limit(10), offset(nil)
  37. 4 - output([t1.a], [t1.b]), filter([lnnvl(t1.a = 1)]),
  38. access([t1.a], [t1.b]), partitions(p0),
  39. limit(10), offset(nil)