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移除书签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。
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:
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如下例所示，Q1 会被改写成 Q2 的形式，其中 Q2 中的谓词 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。
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。
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| 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),
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。
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 = 2ORDER 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 = 2ORDER 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)]),limit(10), offset(nil)
