云数据库OceanBase入门教程 OceanBase 基于规则的查询改写

2024-02-26 开发教程云数据库OceanBase入门教程匿名 5℃

基于规则的查询改写方式主要包括子查询相关改写、外联接消除、简化条件改写和非 SPJ（SELECT PROJECT JOIN）的改写等。

子查询相关改写

优化器对于子查询一般使用嵌套执行的方式，也就是父查询每生成一行数据后，都需要执行一次子查询。使用这种方式需要多次执行子查询，执行效率很低。对于子查询的优化方式，一般会将其改写为联接操作，可大大提高执行效率，主要优点如下：

可避免子查询多次执行。
优化器可根据统计信息选择更优的联接顺序和联接方法。
子查询的联接条件、过滤条件改写为父查询的条件后，优化器可以进行进一步优化，比如条件下压等。

子查询改写的方式主要包括视图合并、子查询展开和将 ANY/ALL 使用 MAX/MIN 改写等。

视图合并

视图合并是指将代表一个视图的子查询合并到包含该视图的查询中，视图合并后，有助于优化器增加联接顺序的选择、访问路径的选择以及进一步做其他改写操作，从而选择更优的执行计划。

OceanBase 数据库支持对 SPJ 视图进行合并。如下示例为 Q1 改写为 Q2：

obclient>CREATE TABLE t1 (c1 INT, c2 INT);
Query OK, 0 rows affected (0.00 sec)
obclient>CREATE TABLE t2 (c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.00 sec)
obclient>CREATE TABLE t3 (c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.00 sec)
Q1:
obclient>SELECT t1.c1, v.c1
                FROM t1, (SELECT t2.c1, t3.c2
                FROM t2, t3
                WHERE t2.c1 = t3.c1) v
                WHERE t1.c2 = v.c2;
<==>
Q2:
obclient>SELECT t1.c1, t2.c1
                FROM t1, t2, t3
                WHERE t2.c1 = t3.c1 AND t1.c2 = t3.c2;

如果 Q1 不进行改写，则其联接顺序有以下几种：

t1, v(t2,t3)
t1, v(t3,t2)
v(t2,t3), t1
v(t3,t2), t1

进行视图合并改写后，可选择的联接顺序有：

t1, t2, t3
t1, t3, t2
t2, t1, t3
t2, t3, t1
t3, t1, t2
t3, t2, t1

可以看出，进行视图合并后，联接顺序可选择空间增加。对于复杂查询，视图合并后，对路径的选择和可改写的空间均会增大，从而使得优化器可生成更优的计划。

子查询展开

子查询展开是指将 WHERE 条件中子查询提升到父查询中，并作为联接条件与父查询并列进行展开。转换后子查询将不存在，外层父查询中会变成多表联接。

这样改写的好处是优化器在进行路径选择、联接方法和联接排序时都会考虑到子查询中的表，从而可以获得更优的执行计划。涉及的子查询表达式一般有 NOT IN、IN、NOT EXIST、EXIST、ANY、ALL。

子查询展开的方式如下：

改写条件使生成的联接语句能够返回与原始语句相同的行。

展开为半联接（SEMI JOIN / ANTI JOIN）

如下例所示，t2.c2 不具有唯一性，改为 SEMI JOIN，该语句改写后执行计划为：

obclient>CREATE TABLE t1 (c1 INT, c2 INT);
Query OK, 0 rows affected (0.17 sec)
obclient>CREATE TABLE t2 (c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.01 sec)
obclient>EXPLAIN SELECT * FROM t1 WHERE t1.c1 IN (SELECT t2.c2 FROM t2)\G;
*************************** 1. row ***************************
Query Plan:
=======================================
|ID|OPERATOR      |NAME|EST. ROWS|COST|
---------------------------------------
|0 |HASH SEMI JOIN|    |495      |3931|
|1 | TABLE SCAN   |t1  |1000     |499 |
|2 | TABLE SCAN   |t2  |1000     |433 |
=======================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2]), filter(nil),
      equal_conds([t1.c1 = t2.c2]), other_conds(nil)
  1 - output([t1.c1], [t1.c2]), filter(nil),
      access([t1.c1], [t1.c2]), partitions(p0)
  2 - output([t2.c2]), filter(nil),
      access([t2.c2]), partitions(p0)

将查询前面操作符改为 NOT IN 后，可改写为 ANTI JOIN，具体计划如下例所示：

obclient>EXPLAIN SELECT * FROM t1 WHERE t1.c1 NOT IN (SELECT t2.c2 FROM t2)\G;
*************************** 1. row ***************************
Query Plan:
================================================
|ID|OPERATOR             |NAME|EST. ROWS|COST  |
------------------------------------------------
|0 |NESTED-LOOP ANTI JOIN|    |0        |520245|
|1 | TABLE SCAN          |t1  |1000     |499   |
|2 | TABLE SCAN          |t2  |22       |517   |
================================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2]), filter(nil),
      conds(nil), nl_params_([t1.c1], [(T_OP_IS, t1.c1, NULL, 0)])
  1 - output([t1.c1], [t1.c2], [(T_OP_IS, t1.c1, NULL, 0)]), filter(nil),
      access([t1.c1], [t1.c2]), partitions(p0)
  2 - output([t2.c2]), filter([(T_OP_OR, ? = t2.c2, ?, (T_OP_IS, t2.c2, NULL, 0))]),
      access([t2.c2]), partitions(p0)

子查询展开为内联接

上面示例的 Q1 中如果将 t2.c2 改为 t2.c1，由于 t2.c1 为主键，子查询输出具有唯一性，此时可以直接转换为内联接，如下例所示：

Q1:
obclient>SELECT * FROM t1 WHERE t1.c1 IN  (SELECT t2.c1 FROM t2)\G;
<==>
Q2:
obclient>SELECT t1.* FROM t1, t2 WHERE t1.c1 = t2.c1;

Q1 改写后的计划如下例所示：

obclient>EXPLAIN SELECT * FROM t1 WHERE t1.c1 IN (SELECT t2.c1 FROM t2)\G;
*************************** 1. row ***************************
Query Plan:
 ====================================
|ID|OPERATOR   |NAME|EST. ROWS|COST|
------------------------------------
|0 |HASH JOIN  |    |1980     |3725|
|1 | TABLE SCAN|t2  |1000     |411 |
|2 | TABLE SCAN|t1  |1000     |499 |
====================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2]), filter(nil),
      equal_conds([t1.c1 = t2.c1]), other_conds(nil)
  1 - output([t2.c1]), filter(nil),
      access([t2.c1]), partitions(p0)
  2 - output([t1.c1], [t1.c2]), filter(nil),
      access([t1.c1], [t1.c2]), partitions(p0)

对于 NOT IN、IN、NOT EXIST、EXIST、ANY、ALL 子查询表达式都可以对应做类似的改写操作。

ANY/ALL 使用 MAX/MIN 改写

对于 ANY/ALL 的子查询，如果子查询中没有 GROUP BY 子句、聚集函数以及 HAVING 时，以下表达式可以使用聚集函数 MIN/MAX 进行等价转换，其中 col_item为单独列且有非 NULL 属性：

val > ALL(SELECT col_item ...)  <==> val > ALL(SELECT MAX(col_item) ...);
val >= ALL(SELECT col_item ...) <==> val >= ALL(SELECT MAX(col_item) ...);
val < ALL(SELECT col_item ...)  <==> val < ALL(SELECT MIN(col_item) ...);
val <= ALL(SELECT col_item ...) <==> val <= ALL(SELECT MIN(col_item) ...);
val > ANY(SELECT col_item ...)  <==> val > ANY(SELECT MIN(col_item) ...);
val >= ANY(SELECT col_item ...) <==> val >= ANY(SELECT MIN(col_item) ...);
val < ANY(SELECT col_item ...)  <==> val < ANY(SELECT MAX(col_item) ...);
val <= ANY(SELECT col_item ...) <==> val <= ANY(SELECT MAX(col_item) ...);

将子查询更改为含有 MAX/MIN 的子查询后，再结合使用 MAX/MIN 的改写，可减少改写前对内表的多次扫描，如下例所示：

obclient>SELECT c1 FROM t1 WHERE c1 > ANY(SELECT c1 FROM t2);
<==>
obclient>SELECT c1 FROM t1 WHERE c1 > ANY(SELECT MIN(c1) FROM t2);

结合 MAX/MIN 的改写后，可利用 t2.c1 的主键序将 LIMIT 1 直接下压到 TABLE SCAN，将 MIN 值输出，执行计划如下：

obclient>EXPLAIN SELECT c1 FROM t1 WHERE c1 > ANY(SELECT c1 FROM t2)\G;
*************************** 1. row ***************************
Query Plan:
 ===================================================
|ID|OPERATOR        |NAME          |EST. ROWS|COST|
---------------------------------------------------
|0 |SUBPLAN FILTER  |              |1        |73  |
|1 | TABLE SCAN     |t1            |1        |37  |
|2 | SCALAR GROUP BY|              |1        |37  |
|3 |  SUBPLAN SCAN  |subquery_table|1        |37  |
|4 |   TABLE SCAN   |t2            |1        |36  |
===================================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1]), filter([t1.c1 > ANY(subquery(1))]),
      exec_params_(nil), onetime_exprs_(nil), init_plan_idxs_([1])
  1 - output([t1.c1]), filter(nil),
      access([t1.c1]), partitions(p0)
  2 - output([T_FUN_MIN(subquery_table.c1)]), filter(nil),
      group(nil), agg_func([T_FUN_MIN(subquery_table.c1)])
  3 - output([subquery_table.c1]), filter(nil),
      access([subquery_table.c1])
  4 - output([t2.c1]), filter(nil),
      access([t2.c1]), partitions(p0),
      limit(1), offset(nil)

外联接消除

外联接操作可分为左外联接、右外联接和全外联接。在联接过程中，由于外联接左右顺序不能变换，优化器对联接顺序的选择会受到限制。外联接消除是指将外联接转换成内联接，从而可以提供更多可选择的联接路径，供优化器考虑。

如果进行外联接消除，需要存在“空值拒绝条件”，即在 WHERE 条件中存在，当内表生成的值为 NULL 时，输出为 FALSE 的条件。

如下例所示：

obclient>SELECT t1.c1, t2.c2 FROM t1 LEFT JOIN t2 ON t1.c2 = t2.c2;

这是一个外联接，在其输出行中 t2.c2 可能为 NULL。如果加上一个条件 t2.c2 > 5，则通过该条件过滤后，t2.c1 输出不可能为 NULL，从而可以将外联接转换为内联接。

obclient>SELECT t1.c1, t2.c2 FROM t1 LEFT JOIN t2 ON t1.c2 = t2.c2 WHERE t2.c2 > 5;
<==>
obclient>SELECT t1.c1, t2.c2 FROM t1 LEFT INNER JOIN t2 ON t1.c2 = t2.c2
            WHERE t2.c2 > 5;

简化条件改写

HAVING 条件消除

如果查询中没有聚集操作及 GROUP BY，则 HAVING 可以合并到 WHERE 条件中，并将 HAVING 条件删除，从而可以将 HAVING 条件在 WHERE 条件中统一管理，并进行进一步相关优化。

obclient>SELECT * FROM t1, t2 WHERE t1.c1 = t2.c1 HAVING t1.c2 > 1;
<==>
obclient>SELECT * FROM t1, t2 WHERE t1.c1 = t2.c1 AND t1.c2 > 1;

改写后计划如下例所示, t1.c2 > 1条件被下压到了 TABLE SCAN 层。

obclient>EXPLAIN SELECT * FROM t1, t2 WHERE t1.c1 = t2.c1 HAVING t1.c2 > 1\G;
*************************** 1. row ***************************
Query Plan:
=========================================
|ID|OPERATOR        |NAME|EST. ROWS|COST|
-----------------------------------------
|0 |NESTED-LOOP JOIN|    |1        |59  |
|1 | TABLE SCAN     |t1  |1        |37  |
|2 | TABLE GET      |t2  |1        |36  |
=========================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil),
      conds(nil), nl_params_([t1.c1])
  1 - output([t1.c1], [t1.c2]), filter([t1.c2 > 1]),
      access([t1.c1], [t1.c2]), partitions(p0)
  2 - output([t2.c1], [t2.c2]), filter(nil),
      access([t2.c1], [t2.c2]), partitions(p0)

等价关系推导

等价关系推导是指利用比较操作符的传递性，推倒出新的条件表达式，从而减少需要处理的行数或者选择到更有效的索引。

OceanBase 数据库可对等值联接进行推导，比如 a ＝ b AND a > 1可以推导出 a ＝ b AND a > 1 AND b > 1，如果 b 上有索引，且 b > 1在该索引选择率很低，则可以大大提升访问 b 列所在表的性能。

如下例所示，条件 t1.c1 = t2.c2 AND t1.c1 > 2，等价推导后为 t1.c1 = t2.c2 AND t1.c1 > 2 AND t2.c2 > 2，从计划中可以看到 t2.c2 已下压到 TABLE SCAN，并且使用 t2.c2 对应的索引。

obclient>CREATE TABLE t1(c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.15 sec)
obclient>CREATE TABLE t2(c1 INT PRIMARY KEY, c2 INT, c3 INT, KEY IDX_c2(c2));
Query OK, 0 rows affected (0.10 sec)
/*此命令需运行于 MySQL 模式下*/
obclient>EXPLAIN EXTENDED_NOADDR SELECT t1.c1, t2.c2 FROM t1, t2
              WHERE t1.c1 = t2.c2 AND t1.c1 > 2\G;
*************************** 1. row ***************************
Query Plan:
==========================================
|ID|OPERATOR   |NAME      |EST. ROWS|COST|
------------------------------------------
|0 |MERGE JOIN |          |5        |78  |
|1 | TABLE SCAN|t2(IDX_c2)|5        |37  |
|2 | TABLE SCAN|t1        |3        |37  |
==========================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t2.c2]), filter(nil),
      equal_conds([t1.c1 = t2.c2]), other_conds(nil)
  1 - output([t2.c2]), filter(nil),
      access([t2.c2]), partitions(p0),
      is_index_back=false,
      range_key([t2.c2], [t2.c1]), range(2,MAX ; MAX,MAX),
      range_cond([t2.c2 > 2])
  2 - output([t1.c1]), filter(nil),
      access([t1.c1]), partitions(p0),
      is_index_back=false,
      range_key([t1.c1]), range(2 ; MAX),
      range_cond([t1.c1 > 2])

恒真/假消除

对于如下恒真恒假条件可以进行消除：

false and expr ＝恒 false
true or expr = 恒 true

如下例所示，对于 WHERE 0 > 1 AND c1 ＝ 3，由于 0 > 1使得 AND 恒假，所以该 SQL 不用执行，可直接返回，从而加快查询的执行。

obclient>EXPLAIN EXTENDED_NOADDR SELECT * FROM t1 WHERE 0 > 1 AND c1 = 3\G;
*************************** 1. row ***************************
Query Plan:
===================================
|ID|OPERATOR  |NAME|EST. ROWS|COST|
-----------------------------------
|0 |TABLE SCAN|t1  |0        |38  |
===================================
Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2]), filter([0], [t1.c1 = 3]), startup_filter([0]),
      access([t1.c1], [t1.c2]), partitions(p0),
      is_index_back=false, filter_before_indexback[false,false],
      range_key([t1.__pk_increment], [t1.__pk_cluster_id], [t1.__pk_partition_id]),
      range(MAX,MAX,MAX ; MIN,MIN,MIN)always false

非 SPJ 的改写

冗余排序消除

冗余排序消除是指删除 order item 中不需要的项，减少排序开销。以下三种情况可进行排序消除：

ORDER BY 表达式列表中有重复列，可进行去重后排序。

obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c1, c2, c3 ;
<==>
obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c2, c3;

ORDER BY 列中存在 where 中有单值条件的列，该列排序可删除。

obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c2, c3;
<==>
obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c3;

如果本层查询有 ORDER BY 但是没有 LIMIT，且本层查询位于父查询的集合操作中，则 ORDER BY 可消除。因为对两个有序的集合做 UNION 操作，其结果是乱序的。但是如果 ORDER BY 中有 LIMIT，则语义是取最大/最小的 N 个，此时不能消除 ORDER BY，否则有语义错误。
```
obclient>(SELECT c1,c2 FROM t1 ORDER BY c1) UNION (SELECT c3,c4 FROM t2 ORDER BY c3)；
<==>
obclient>(SELECT c1,c2 FROM t1) UNION (SELECT c3,c4 FROM t2)；
```

LIMIT 下压

LIMIT 下压改写是指将 LIMIT 下降到子查询中，OceanBase 数据库现在支持在不改变语义的情况下，将 LIMIT 下压到视图（示例 1）及 UNION 对应子查询（示例 2）中。

示例 1：

obclient>SELECT * FROM (SELECT * FROM t1 ORDER BY c1) a LIMIT 1;
<==>
obclient>SELECT * FROM (SELECT * FROM t1 ORDER BY c1 LIMIT 1) a LIMIT 1;

示例 2：

obclient>(SELECT c1,c2 FROM t1) UNION ALL (SELECT c3,c4 FROM t2) LIMIT 5;
<==>
obclient>(SELECT c1,c2 FROM t1 LIMIT 5) UNION ALL (SELECT c3,c4 FROM t2 limit 5) LIMIT 5;

DISTINCT 消除

如果 select item 中只包含常量，则可以消除 DISTINCT，并加上 LIMIT 1。

obclient>SELECT DISTINCT 1,2 FROM t1 ;
<==>
obclient>SELECT DISTINCT 1,2 FROM t1 LIMIT 1;
obclient>CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.17 sec)
obclient>EXPLAIN EXTENDED_NOADDR SELECT DISTINCT 1,2 FROM t1\G;
*************************** 1. row ***************************
Query Plan:
===================================
|ID|OPERATOR  |NAME|EST. ROWS|COST|
-----------------------------------
|0 |TABLE SCAN|t1  |1        |36  |
===================================
Outputs & filters:
-------------------------------------
  0 - output([1], [2]), filter(nil),
      access([t1.c1]), partitions(p0),
      limit(1), offset(nil),
      is_index_back=false,
      range_key([t1.c1]), range(MIN ; MAX)always true

如果 select item 中包含确保唯一性约束的列，则 DISTINCT 能够消除，如下示例中 (c1, c2)为主键，可确保 c1、c2 和 c3 唯一性，从而 DISTINCT 可消除。

obclient>CREATE TABLE t2(c1 INT, c2 INT, c3 INT, PRIMARY KEY(c1, c2));
Query OK, 0 rows affected (0.17 sec)
obclient>SELECT DISTINCT c1, c2, c3 FROM t2;
<==>
obclient>SELECT c1, c2 c3 FROM t2;
obclient>EXPLAIN SELECT DISTINCT c1, c2, c3 FROM t2\G;
*************************** 1. row ***************************
Query Plan:
===================================
|ID|OPERATOR  |NAME|EST. ROWS|COST|
-----------------------------------
|0 |TABLE SCAN|t2  |1000     |455 |
===================================
Outputs & filters:
-------------------------------------
  0 - output([t2.c1], [t2.c2], [t2.c3]), filter(nil),
      access([t2.c1], [t2.c2], [t2.c3]), partitions(p0)

MIN/MAX 改写

当 MIN/MAX 函数中参数为索引前缀列，且不含 GROUP BY 时，可将该 scalar aggregate 转换为走索引扫描 1 行的情况，如下例所示：

obclient>CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 INT, c3 INT, KEY IDX_c2_c3(c2,c3));
Query OK, 0 rows affected (0.17 sec)
obclient>SELECT MIN(c2) FROM t1;
<==>
obclient>SELECT MIN(c2) FROM (SELECT c2 FROM t2 ORDER BY c2 LIMIT 1) AS t;
obclient>EXPLAIN SELECT MIN(c2) FROM t1\G;
*************************** 1. row ***************************
Query Plan:
==================================================
|ID|OPERATOR       |NAME          |EST. ROWS|COST|
--------------------------------------------------
|0 |SCALAR GROUP BY|              |1        |37  |
|1 | SUBPLAN SCAN  |subquery_table|1        |37  |
|2 |  TABLE SCAN   |t1(idx_c2_c3) |1        |36  |
==================================================
Outputs & filters:
-------------------------------------
  0 - output([T_FUN_MIN(subquery_table.c2)]), filter(nil),
      group(nil), agg_func([T_FUN_MIN(subquery_table.c2)])
  1 - output([subquery_table.c2]), filter(nil),
      access([subquery_table.c2])
  2 - output([t1.c2]), filter([(T_OP_IS_NOT, t1.c2, NULL, 0)]),
      access([t1.c2]), partitions(p0),
      limit(1), offset(nil)

如果 SELECT MIN/MAX的参数为常量，而且包含 GROUP BY，可以将 MIN/MAX 改为常量，从而减少 MIN/MAX 的计算开销。

obclient>SELECT MAX(1) FROM t1 GROUP BY c1;
<==>
obclient>SELECT 1 FROM t1 GROUP BY c1;
obclient>EXPLAIN EXTENDED_NOADDR SELECT MAX(1) FROM t1 GROUP BY c1\G;
*************************** 1. row ***************************
Query Plan:
===================================
|ID|OPERATOR  |NAME|EST. ROWS|COST|
-----------------------------------
|0 |TABLE SCAN|t1  |1000     |411 |
===================================
Outputs & filters:
-------------------------------------
  0 - output([1]), filter(nil),
      access([t1.c1]), partitions(p0),
      is_index_back=false,
      range_key([t1.c1]), range(MIN ; MAX)always true

如果 SELECT MIN/MAX的参数为常量，而且不含 GROUP BY，可以按照如下示例进行改写，从而走索引只需扫描 1 行。

obclient>SELECT MAX(1) FROM t1;
<==>
obclient>SELECT MAX(t.a) FROM (SELECT 1 AS a FROM t1 LIMIT 1) t;
obclient>EXPLAIN EXTENDED_NOADDR SELECT MAX(1) FROM t1\G;
*************************** 1. row ***************************
Query Plan:
==================================================
|ID|OPERATOR       |NAME          |EST. ROWS|COST|
--------------------------------------------------
|0 |SCALAR GROUP BY|              |1        |37  |
|1 | SUBPLAN SCAN  |subquery_table|1        |37  |
|2 |  TABLE SCAN   |t1            |1        |36  |
==================================================
Outputs & filters:
-------------------------------------
  0 - output([T_FUN_MAX(subquery_table.subquery_col_alias)]), filter(nil),
      group(nil), agg_func([T_FUN_MAX(subquery_table.subquery_col_alias)])
  1 - output([subquery_table.subquery_col_alias]), filter(nil),
      access([subquery_table.subquery_col_alias])
  2 - output([1]), filter(nil),
      access([t1.c1]), partitions(p0),
      limit(1), offset(nil),
      is_index_back=false,
      range_key([t1.c1]), range(MIN ; MAX)always true

上一节： OceanBase 查询改写概述

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"云数据库OceanBase入门教程"的目录