diff --git a/explain-subqueries.md b/explain-subqueries.md index 74b1a97f9257..a395f2c9863e 100644 --- a/explain-subqueries.md +++ b/explain-subqueries.md @@ -256,6 +256,85 @@ tidb> EXPLAIN SELECT * FROM t WHERE (a, b) NOT IN (SELECT * FROM s); > > `Not Exists` 操作符也会被转成 Anti Semi Join，但是 `Not Exists` 符号本身不具有集合运算 Null-Aware 的性质。 +## `... < ALL (SELECT ... FROM ...)` 或者 `... > ANY (SELECT ... FROM ...)` + +对于这种情况，可以将 `ALL` 或者 `ANY` 用 `MAX` 以及 `MIN` 来代替。不过由于在表为空时，`MAX(EXPR)` 以及 `MIN(EXPR)` 的结果会为 `NULL`，其表现形式和 `EXPR` 有 `NULL` 值的结果一样。同时，外部表达式结果为 `NULL` 时也会影响表达式的最终结果，因此完整的改写为如下形式： + +- `t.id < all(select s.id from s)` 会被改写为 `t.id < min(s.id) and if(sum(s.id is null) != 0, null, true)`。 +- `t.id > any (select s.id from s)` 会被改写为 `t.id > max(s.id) or if(sum(s.id is null) != 0, null, false)`。 + +示例如下： + +``` +CREATE TABLE products ( + product_id INT PRIMARY KEY, + product_name VARCHAR(100), + unit_price DECIMAL(10, 2) +); + +CREATE TABLE order_details ( + order_id INT, + product_id INT, + quantity INT, + discount_price DECIMAL(10, 2) +); +``` + +``` +tidb> EXPLAIN SELECT product_id, product_name, unit_price FROM products WHERE unit_price < ALL (SELECT DISTINCT discount_price FROM order_details ); ++------------------------------+----------+-----------+---------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ +| id | estRows | task | access object | operator info | ++------------------------------+----------+-----------+---------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ +| HashJoin_27 | 10000.00 | root | | CARTESIAN inner join, other cond:or(and(lt(test.products.unit_price, Column#9), if(ne(Column#10, 0), NULL, 1)), or(eq(Column#11, 0), if(isnull(test.products.unit_price), NULL, 0))) | +| ├─HashAgg_55(Build) | 1.00 | root | | funcs:min(Column#16)->Column#9, funcs:sum(Column#17)->Column#10, funcs:count(1)->Column#11 | +| │ └─Projection_82 | 8000.00 | root | | test.order_details.discount_price->Column#16, cast(isnull(test.order_details.discount_price), decimal(20,0) BINARY)->Column#17 | +| │ └─HashAgg_66 | 8000.00 | root | | group by:test.order_details.discount_price, funcs:firstrow(test.order_details.discount_price)->test.order_details.discount_price | +| │ └─TableReader_67 | 8000.00 | root | | data:HashAgg_59 | +| │ └─HashAgg_59 | 8000.00 | cop[tikv] | | group by:test.order_details.discount_price, | +| │ └─TableFullScan_65 | 10000.00 | cop[tikv] | table:order_details | keep order:false, stats:pseudo | +| └─TableReader_30(Probe) | 10000.00 | root | | data:TableFullScan_29 | +| └─TableFullScan_29 | 10000.00 | cop[tikv] | table:products | keep order:false, stats:pseudo | ++------------------------------+----------+-----------+---------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ + +tidb> EXPLAIN SELECT product_id, product_name, unit_price FROM products WHERE unit_price > ALL (SELECT DISTINCT discount_price FROM order_details ); ++------------------------------+----------+-----------+---------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ +| id | estRows | task | access object | operator info | ++------------------------------+----------+-----------+---------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ +| HashJoin_27 | 10000.00 | root | | CARTESIAN inner join, other cond:or(and(gt(test.products.unit_price, Column#9), if(ne(Column#10, 0), NULL, 1)), or(eq(Column#11, 0), if(isnull(test.products.unit_price), NULL, 0))) | +| ├─HashAgg_55(Build) | 1.00 | root | | funcs:max(Column#16)->Column#9, funcs:sum(Column#17)->Column#10, funcs:count(1)->Column#11 | +| │ └─Projection_82 | 8000.00 | root | | test.order_details.discount_price->Column#16, cast(isnull(test.order_details.discount_price), decimal(20,0) BINARY)->Column#17 | +| │ └─HashAgg_66 | 8000.00 | root | | group by:test.order_details.discount_price, funcs:firstrow(test.order_details.discount_price)->test.order_details.discount_price | +| │ └─TableReader_67 | 8000.00 | root | | data:HashAgg_59 | +| │ └─HashAgg_59 | 8000.00 | cop[tikv] | | group by:test.order_details.discount_price, | +| │ └─TableFullScan_65 | 10000.00 | cop[tikv] | table:order_details | keep order:false, stats:pseudo | +| └─TableReader_30(Probe) | 10000.00 | root | | data:TableFullScan_29 | +| └─TableFullScan_29 | 10000.00 | cop[tikv] | table:products | keep order:false, stats:pseudo | ++------------------------------+----------+-----------+---------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ +``` + +## `... IN (SELECT ... FROM ...)` + +对于这种情况，TiDB 会将其 `IN` 的子查询改写为 `SELECT ... FROM ... GROUP ...` 的形式，然后将 `IN` 改写为普通的 `JOIN` 的形式。如 `SELECT * FROM t1 WHERE t1.a IN (SELECT t2.a FROM t2)` 会被改写为 `SELECT t1.* FROM t1, (SELECT DISTINCT a AS a FROM t2) t2 WHERE t1.a = t2.a` 的形式。同时这里的 `DISTINCT` 可以在 `t2.a` 具有 `UNIQUE` 属性时被自动消去。 + +```sql +EXPLAIN SELECT * FROM t1 WHERE t1.a IN (SELECT t2.a FROM t2); +``` + +```sql ++------------------------------+---------+-----------+------------------------+----------------------------------------------------------------------------+ +| id | estRows | task | access object | operator info | ++------------------------------+---------+-----------+------------------------+----------------------------------------------------------------------------+ +| IndexJoin_12 | 9990.00 | root | | inner join, inner:TableReader_11, outer key:test.t2.a, inner key:test.t1.a | +| ├─HashAgg_21(Build) | 7992.00 | root | | group by:test.t2.a, funcs:firstrow(test.t2.a)->test.t2.a | +| │ └─IndexReader_28 | 9990.00 | root | | index:IndexFullScan_27 | +| │ └─IndexFullScan_27 | 9990.00 | cop[tikv] | table:t2, index:idx(a) | keep order:false, stats:pseudo | +| └─TableReader_11(Probe) | 7992.00 | root | | data:TableRangeScan_10 | +| └─TableRangeScan_10 | 7992.00 | cop[tikv] | table:t1 | range: decided by [test.t2.a], keep order:false, stats:pseudo | ++------------------------------+---------+-----------+------------------------+----------------------------------------------------------------------------+ +``` + +这个改写会在 `IN` 子查询相对较小、而外部查询相对较大时产生更好的执行性能。因为不经过改写的情况下，TiDB 无法使用以 `t2` 为驱动表的 `index join`。需要注意，当改写生成的聚合无法被自动消去且 `t2` 表比较大时，反而会影响查询的性能。你可以使用 [`tidb_opt_insubq_to_join_and_agg`](/system-variables.md#tidb_opt_insubq_to_join_and_agg) 变量来控制该优化。当遇到适用的情况，可以手动关闭。 + ## 其他类型查询的执行计划 + [MPP 模式查询的执行计划](/explain-mpp.md)