作者： jansu-dev
第一章 背景介绍
**好久没发文章了, 发篇曾经研究过的一篇水文.**
通常来说, 永假的 OR 谓词条件是可以被消除的, 并且在通用数据库上可以见到对应逻辑.
如果不消除, 会引入额外的筛选机制, 导致大量计算资源被消耗, 引发 SQL 性能降低的现象.
第二章 理论概括
以下述代码块为例, 演示具体恒假析取表达式怎么被消除:create table jan(id int, name int);
explain select * from jan where id=123 or 1=2;
select version();- PostgreSQL 实验
- 可以看到 Filter: (id = 123) 已经直接将 or 1=2 的永假析取条件消除了.
test=# explain select * from jan where id=123 or 1=2;
QUERY PLAN
-----------------------------------------------------
Seq Scan on jan (cost=0.00..21.25 rows=4 width=62)
Filter: (id = 123)
(2 rows)
test=# select version();
version
-------------------------------------------------------------------------------------------------------------------
PostgreSQL 13.12 on arm-apple-darwin22.4.0, compiled by Apple clang version 14.0.3 (clang-1403.0.22.14.1), 64-bit
(1 row)- Oceanbase 实验
- 可以看到 1 = 2 result is FALSE 并在 Output & filters 中没有出现相关筛选, 说明也消除了永假析取条件.
并且 [OB 官网](https://www.oceanbase.com/docs/common-oceanbase-database-cn-1000000000035699#73f4b831-eadb-45fa-8b3c-79cd86d02e34) 有对该种情况进行描述.
第三章 现状分析
上面介绍了 OB 和 PG 该功能的实现, 下面演示当前 TiDB 的实现情况.
截止至 v7.1.1 TiDB 在单表查询时, or(eq(test.jan.id, 123), 0) 的恒假析取条件直接下推至 TiKV, 而不是直接在执行计划中消除.mysql> explain select * from jan where id=123 or 1=2;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7 | 10.00 | root | | data:Selection_6 |
| └─Selection_6 | 10.00 | cop[tikv] | | or(eq(test.jan.id, 123), 0) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:jan | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)
mysql> select version();
+--------------------+
| version() |
+--------------------+
| 5.7.25-TiDB-v7.1.1 |
+--------------------+
1 row in set (0.00 sec)
第四章 问题分析
其实, 单表查询的使用方法并不一定会造成问题, 但是如果在多表 join 的情况下, 便会加剧该情况. 如 [Issue-45785](https://github.com/pingcap/tidb/issues/45785) 中提到的, 在 TiDB instance 侧仍需要 CARTESIAN inner join 算子过滤 or(and(eq(edmp.t1.a, edmp.t2.a), eq(edmp.t1.b, 1)), 0). 比较理想的情况, 应该是从 CARTESIAN inner join 变为 inner join.CREATE TABLE `t1` (
`a` int(11) DEFAULT NULL,
`b` int(11) DEFAULT NULL,
`c` int(11) DEFAULT NULL,
KEY `idx_a` (`a`)
);
CREATE TABLE `t2` (
`a` int(11) DEFAULT NULL,
`b` int(11) DEFAULT NULL,
`c` int(11) DEFAULT NULL,
KEY `idx_a` (`a`)
);
mysql> explain select /*+ INL_JOIN(t2)*/ * from t1,t2 where t1.a=t2.a and t1.b = 1 or 1=2;
+------------------------------+-----------+-----------+---------------+-----------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------+-----------+-----------+---------------+-----------------------------------------------------------------------------------------+
| HashJoin_9 | 100000.00 | root | | CARTESIAN inner join, other cond:or(and(eq(edmp.t1.a, edmp.t2.a), eq(edmp.t1.b, 1)), 0) |
| ├─TableReader_12(Build) | 10.00 | root | | data:Selection_11 |
| │ └─Selection_11 | 10.00 | cop[tikv] | | or(eq(edmp.t1.b, 1), 0) |
| │ └─TableFullScan_10 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
| └─TableReader_14(Probe) | 10000.00 | root | | data:TableFullScan_13 |
| └─TableFullScan_13 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
+------------------------------+-----------+-----------+---------------+-----------------------------------------------------------------------------------------+
6 rows in set, 1 warning (0.00 sec)
第五章 解决方案
从 PG 的实现方式来看, 会有个 canonicalize\_qual 函数试图将表达式强制转换为规范的 and-of-or 或 or-of-and形式. 在这个过程中, 会调用到 find\_duplicate\_ors 函数(如下所示部分), /\* Get rid of any constant inputs \*/ 注释部分会将永假析取条件直接消除./*
* find_duplicate_ors
* Given a qualification tree with the NOTs pushed down, search for
* OR clauses to which the inverse OR distributive law might apply.
* Only the top-level AND/OR structure is searched.
*
* While at it, we remove any NULL constants within the top-level AND/OR
* structure, eg in a WHERE clause, "x OR NULL::boolean" is reduced to "x".
* In general that would change the result, so eval_const_expressions can't
* do it; but at top level of WHERE, we don't need to distinguish between
* FALSE and NULL results, so it's valid to treat NULL::boolean the same
* as FALSE and then simplify AND/OR accordingly. Conversely, in a top-level
* CHECK constraint, we may treat a NULL the same as TRUE.
*
* Returns the modified qualification. AND/OR flatness is preserved.
*/
static Expr *
find_duplicate_ors(Expr *qual, bool is_check)
{
if (is_orclause(qual))
{
List *orlist = NIL;
ListCell *temp;
/* Recurse */
foreach(temp, ((BoolExpr *) qual)->args)
{
Expr *arg = (Expr *) lfirst(temp);
arg = find_duplicate_ors(arg, is_check);
/* Get rid of any constant inputs */
if (arg && IsA(arg, Const))
{
Const *carg = (Const *) arg;
if (is_check)
{
/* Within OR in CHECK, drop constant FALSE */
if (!carg->constisnull && !DatumGetBool(carg->constvalue))
continue;
/* Constant TRUE or NULL, so OR reduces to TRUE */
return (Expr *) makeBoolConst(true, false);
}
else
{
/* Within OR in WHERE, drop constant FALSE or NULL */
if (carg->constisnull || !DatumGetBool(carg->constvalue))
continue;
/* Constant TRUE, so OR reduces to TRUE */
return arg;
}
}
orlist = lappend(orlist, arg);
}
/* Flatten any ORs pulled up to just below here */
orlist = pull_ors(orlist);
/* Now we can look for duplicate ORs */
return process_duplicate_ors(orlist);
}
else if (is_andclause(qual))
{
List *andlist = NIL;
ListCell *temp;
......因此, TiDB 其实可以仿制类似逻辑, 在 buildSelection 处理 where 条件时, 利用递归的方法实现这个功能.
**期待大佬们解决这个简单的问题了, 望产品逐渐完善!**
第六章 文献参考
- PG 源代码
- PG 源代码解读-PostgreSQL 源码解读（33）- 查询语句#18（查询优化-表达式预处理#3
- TiDB 源代码
