**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.

   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();

   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.

   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)

   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)

   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!**