Leading Hint 控制 Join 顺序
一句话定义
Leading Hint 是 Doris 中用于手动指定多表 Join 顺序的查询提示（Hint），通过在 SELECT 关键字后添加 /*+ LEADING(...) */ 注释，引导优化器按照指定顺序生成 Join 计划，从而提升复杂查询性能。
适用场景速览
在以下场景中可考虑使用 Leading Hint 或 Ordered Hint：
- 优化器自动选择的 Join 顺序不理想，需要人工干预。
- 复杂多表 Join 中希望显式指定左深树、右深树、Bushy 树或 zig-zag 树的形状。
- View 或子查询作为别名参与 Join Reorder，需要把整个子树作为整体定位。
- 希望强制按 SQL 文本中的表书写顺序进行 Join（使用 Ordered Hint）。
使用前 Checklist
在编写 Hint 前，请确认：
- SQL 中至少包含两个参与 Join 的表。
- Hint 中使用的表名/别名与
FROM子句中的一致。 - 已通过
EXPLAIN SHAPE PLAN查看原始 Plan 与目标 Plan 的差异。 - 评估目标 Join Order 与原 SQL 在语义上等价（尤其是 Outer Join / Semi / Anti Join 场景）。
快速导航
- 常规 Leading Hint：基础语法、生效状态、典型用法。
- 典型场景示例：基础用法 + 左深树 / 右深树 / Bushy 树 / zig-zag 树 / Non-inner Join / View。
- Ordered Hint：固定 Join 顺序为文本序的特例。
- 常见问题（FAQ）：Hint 不生效、与其他 Hint 优先级等。
常规 Leading Hint
语法
Leading Hint 用于指定希望优化器遵循的表连接顺序。在 Doris 中，基本语法如下：
SELECT /*+ LEADING(tablespec [tablespec]...) */ ...
语法要点：
- Leading Hint 由
/*+和*/包围，并置于 SQL 语句中SELECT关键字之后。 tablespec是表名或表别名，至少需要指定两个表。- 多个表之间用空格或
,分隔。 - 可以使用大括号
{}显式地指定 Join Tree 的形状。
最小示例：
mysql> explain shape plan select /*+ leading(t2 t1) */ * from t1 join t2 on c1 = c2;
+------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| --------PhysicalOlapScan[t2] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalOlapScan[t1] |
| |
| Hint log: |
| Used: leading(t2 t1) |
| UnUsed: |
| SyntaxError: |
+------------------------------------------------------------------------------+
Hint 生效状态
当 Leading Hint 不生效时，会走正常流程生成计划。EXPLAIN 会显示该 Hint 是否生效，主要分为三种状态：
| 状态 | 描述 |
|---|---|
Used | Leading Hint 正常生效。 |
UnUsed | 这里不支持的情况包含 Leading Hint 指定的 Join Order 与原 SQL 不等价，或本版本暂不支持的特性（详见限制）。 |
SyntaxError | Leading Hint 语法错误，例如找不到对应的表等。 |
核心规则
-
默认构造左深树：Leading Hint 在不使用任何括号的情况下，默认构造出左深树。
mysql> explain shape plan select /*+ leading(t1 t2 t3) */ * from t1 join t2 on c1 = c2 join t3 on c2=c3;
+--------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| --------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| ----------PhysicalOlapScan[t1] |
| ----------PhysicalDistribute[DistributionSpecHash] |
| ------------PhysicalOlapScan[t2] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalOlapScan[t3] |
| |
| Hint log: |
| Used: leading(t1 t2 t3) |
| UnUsed: |
| SyntaxError: |
+--------------------------------------------------------------------------------+ -
使用大括号指定 Join 树形状：通过
{}可以显式控制 Join Tree 的形状。mysql> explain shape plan select /*+ leading(t1 {t2 t3}) */ * from t1 join t2 on c1 = c2 join t3 on c2=c3;
+----------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+----------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| --------PhysicalOlapScan[t1] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| ------------PhysicalOlapScan[t2] |
| ------------PhysicalDistribute[DistributionSpecHash] |
| --------------PhysicalOlapScan[t3] |
| |
| Hint log: |
| Used: leading(t1 { t2 t3 }) |
| UnUsed: |
| SyntaxError: |
+----------------------------------------------------------------------------------+ -
支持 View / 别名作为参数：当 View 作为别名参与 Join Reorder 时，可以指定对应的 View 作为 Leading Hint 的参数。
mysql> explain shape plan select /*+ leading(alias t1) */ count(*) from t1 join (select c2 from t2 join t3 on t2.c2 = t3.c3) as alias on t1.c1 = alias.c2;
+--------------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------------+
| PhysicalResultSink |
| --hashAgg[GLOBAL] |
| ----PhysicalDistribute[DistributionSpecGather] |
| ------hashAgg[LOCAL] |
| --------PhysicalProject |
| ----------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = alias.c2)) otherCondition=() |
| ------------PhysicalProject |
| --------------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| ----------------PhysicalProject |
| ------------------PhysicalOlapScan[t2] |
| ----------------PhysicalDistribute[DistributionSpecHash] |
| ------------------PhysicalProject |
| --------------------PhysicalOlapScan[t3] |
| ------------PhysicalDistribute[DistributionSpecHash] |
| --------------PhysicalProject |
| ----------------PhysicalOlapScan[t1] |
| |
| Hint log: |
| Used: leading(alias t1) |
| UnUsed: |
| SyntaxError: |
+--------------------------------------------------------------------------------------+
典型场景示例
准备：建表语句
以下示例均基于这一组测试表。
CREATE DATABASE testleading;
USE testleading;
create table t1 (c1 int, c11 int) distributed by hash(c1) buckets 3 properties('replication_num' = '1');
create table t2 (c2 int, c22 int) distributed by hash(c2) buckets 3 properties('replication_num' = '1');
create table t3 (c3 int, c33 int) distributed by hash(c3) buckets 3 properties('replication_num' = '1');
create table t4 (c4 int, c44 int) distributed by hash(c4) buckets 3 properties('replication_num' = '1');
基础场景：交换两个表的 Join 顺序
-
原始 Plan：
mysql> explain shape plan select * from t1 join t2 on t1.c1 = c2;
+-------------------------------------------+
| Explain String |
+-------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN](t1.c1 = t2.c2) |
| --------PhysicalOlapScan[t2] |
| --------PhysicalDistribute |
| ----------PhysicalOlapScan[t1] |
+-------------------------------------------+ -
当需要交换 t1 和 t2 的 Join 顺序时，只需在前面加上
leading(t2 t1)即可。在执行EXPLAIN时，Used表示该 Hint 正常生效。mysql> explain shape plan select /*+ leading(t2 t1) */ * from t1 join t2 on c1 = c2;
+------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| --------PhysicalOlapScan[t2] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalOlapScan[t1] |
| |
| Hint log: |
| Used: leading(t2 t1) |
| UnUsed: |
| SyntaxError: |
+------------------------------------------------------------------------------+ -
如果 Leading Hint 存在语法错误，
EXPLAIN时会在SyntaxError中显示相应信息，但计划仍能照常生成，只是不会使用 Leading Hint。mysql> explain shape plan select /*+ leading(t2 t3) */ * from t1 join t2 on t1.c1 = c2;
+--------------------------------------------------------+
| Explain String |
+--------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN](t1.c1 = t2.c2) |
| --------PhysicalOlapScan[t1] |
| --------PhysicalDistribute |
| ----------PhysicalOlapScan[t2] |
| |
| Used: |
| UnUsed: |
| SyntaxError: leading(t2 t3) Msg:can not find table: t3 |
+--------------------------------------------------------+
扩展场景：构造不同形状的 Join 树
下表汇总了四种常见 Join 树形状的写法：
| 形状 | Leading 写法 | 说明 |
|---|---|---|
| 左深树 | leading(t1 t2 t3) | 默认行为，无需大括号。 |
| 右深树 | leading(t1 {t2 t3}) | 用 {} 把右子树包起来。 |
| Bushy 树 | leading({t1 t2} {t3 t4}) | 左右子树都用 {} 包起来。 |
| zig-zag 树 | leading(t1 {t2 t3} t4) | 中间嵌入子树形成 zig-zag。 |
左深树
如前所述，在不使用任何括号的情况下，Leading Hint 默认生成左深树。
mysql> explain shape plan select /*+ leading(t1 t2 t3) */ * from t1 join t2 on t1.c1 = c2 join t3 on c2 = c3;
+--------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| --------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| ----------PhysicalOlapScan[t1] |
| ----------PhysicalDistribute[DistributionSpecHash] |
| ------------PhysicalOlapScan[t2] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalOlapScan[t3] |
| |
| Hint log: |
| Used: leading(t1 t2 t3) |
| UnUsed: |
| SyntaxError: |
+--------------------------------------------------------------------------------+
右深树
当需要将计划的形状做成右深树、Bushy 树或 zig-zag 树时，只需加上大括号来限制 Plan 的形状即可，无需像 Oracle 那样使用 swap 从左深树一步步调整。
mysql> explain shape plan select /*+ leading(t1 {t2 t3}) */ * from t1 join t2 on t1.c1 = c2 join t3 on c2 = c3;
+-----------------------------------------------+
| Explain String |
+-----------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN](t1.c1 = t2.c2) |
| --------PhysicalOlapScan[t1] |
| --------PhysicalDistribute |
| ----------hashJoin[INNER_JOIN](t2.c2 = t3.c3) |
| ------------PhysicalOlapScan[t2] |
| ------------PhysicalDistribute |
| --------------PhysicalOlapScan[t3] |
| |
| Used: leading(t1 { t2 t3 }) |
| UnUsed: |
| SyntaxError: |
+-----------------------------------------------+
Bushy 树
mysql> explain shape plan select /*+ leading({t1 t2} {t3 t4}) */ * from t1 join t2 on t1.c1 = c2 join t3 on c2 = c3 join t4 on c3 = c4;
+-----------------------------------------------+
| Explain String |
+-----------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN](t2.c2 = t3.c3) |
| --------hashJoin[INNER_JOIN](t1.c1 = t2.c2) |
| ----------PhysicalOlapScan[t1] |
| ----------PhysicalDistribute |
| ------------PhysicalOlapScan[t2] |
| --------PhysicalDistribute |
| ----------hashJoin[INNER_JOIN](t3.c3 = t4.c4) |
| ------------PhysicalOlapScan[t3] |
| ------------PhysicalDistribute |
| --------------PhysicalOlapScan[t4] |
| |
| Used: leading({ t1 t2 } { t3 t4 }) |
| UnUsed: |
| SyntaxError: |
+-----------------------------------------------+
zig-zag 树
mysql> explain shape plan select /*+ leading(t1 {t2 t3} t4) */ * from t1 join t2 on t1.c1 = c2 join t3 on c2 = c3 join t4 on c3 = c4;
+--------------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t3.c3 = t4.c4)) otherCondition=() |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| ------------PhysicalOlapScan[t1] |
| ------------PhysicalDistribute[DistributionSpecHash] |
| --------------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| ----------------PhysicalOlapScan[t2] |
| ----------------PhysicalDistribute[DistributionSpecHash] |
| ------------------PhysicalOlapScan[t3] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalOlapScan[t4] |
| |
| Hint log: |
| Used: leading(t1 { t2 t3 } t4) |
| UnUsed: |
| SyntaxError: |
+--------------------------------------------------------------------------------------+
Non-inner Join 场景
当遇到非 Inner Join（如 Outer Join 或 Semi/Anti Join）时，Leading Hint 会根据原始 SQL 语义自动推导各个 Join 的方式。若 Leading Hint 与原始 SQL 语义不同或无法生成，则会将其放入 UnUsed 中，但这并不影响计划正常流程的生成。
以下是一个不能交换的例子：
-------- test outer join which can not swap
-- t1 leftjoin (t2 join t3 on (P23)) on (P12) != (t1 leftjoin t2 on (P12)) join t3 on (P23)
mysql> explain shape plan select /*+ leading(t1 {t2 t3}) */ * from t1 left join t2 on c1 = c2 join t3 on c2 = c3;
+--------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| --------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| ----------PhysicalOlapScan[t1] |
| ----------PhysicalDistribute[DistributionSpecHash] |
| ------------PhysicalOlapScan[t2] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalOlapScan[t3] |
| |
| Hint log: |
| Used: |
| UnUsed: leading(t1 { t2 t3 }) |
| SyntaxError: |
+--------------------------------------------------------------------------------+
下面是一些可以交换的例子和不能交换的例子，读者可自行验证。
-------- test outer join which can swap
-- (t1 leftjoin t2 on (P12)) innerjoin t3 on (P13) = (t1 innerjoin t3 on (P13)) leftjoin t2 on (P12)
explain shape plan select * from t1 left join t2 on c1 = c2 join t3 on c1 = c3;
explain shape plan select /*+ leading(t1 t3 t2) */ * from t1 left join t2 on c1 = c2 join t3 on c1 = c3;
-- (t1 leftjoin t2 on (P12)) leftjoin t3 on (P13) = (t1 leftjoin t3 on (P13)) leftjoin t2 on (P12)
explain shape plan select * from t1 left join t2 on c1 = c2 left join t3 on c1 = c3;
explain shape plan select /*+ leading(t1 t3 t2) */ * from t1 left join t2 on c1 = c2 left join t3 on c1 = c3;
-- (t1 leftjoin t2 on (P12)) leftjoin t3 on (P23) = t1 leftjoin (t2 leftjoin t3 on (P23)) on (P12)
select /*+ leading(t2 t3 t1) SWAP_INPUT(t1) */ * from t1 left join t2 on c1 = c2 left join t3 on c2 = c3;
explain shape plan select /*+ leading(t1 {t2 t3}) */ * from t1 left join t2 on c1 = c2 left join t3 on c2 = c3;
explain shape plan select /*+ leading(t1 {t2 t3}) */ * from t1 left join t2 on c1 = c2 left join t3 on c2 = c3;
-------- test outer join which can not swap
-- t1 leftjoin (t2 join t3 on (P23)) on (P12) != (t1 leftjoin t2 on (P12)) join t3 on (P23)
-- eliminated to inner join
explain shape plan select /*+ leading(t1 {t2 t3}) */ * from t1 left join t2 on c1 = c2 join t3 on c2 = c3;
explain graph select /*+ leading(t1 t2 t3) */ * from t1 left join (select * from t2 join t3 on c2 = c3) on c1 = c2;
-- test semi join
explain shape plan select * from t1 where c1 in (select c2 from t2);
explain shape plan select /*+ leading(t2 t1) */ * from t1 where c1 in (select c2 from t2);
-- test anti join
explain shape plan select * from t1 where exists (select c2 from t2);
View / 别名场景
在涉及别名（Alias）的情况下，可以将别名作为一个完整独立的子树进行指定，并在这些子树内部根据文本序生成 Join 顺序。
mysql> explain shape plan select /*+ leading(alias t1) */ count(*) from t1 join (select c2 from t2 join t3 on t2.c2 = t3.c3) as alias on t1.c1 = alias.c2;
+--------------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------------+
| PhysicalResultSink |
| --hashAgg[GLOBAL] |
| ----PhysicalDistribute[DistributionSpecGather] |
| ------hashAgg[LOCAL] |
| --------PhysicalProject |
| ----------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = alias.c2)) otherCondition=() |
| ------------PhysicalProject |
| --------------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| ----------------PhysicalProject |
| ------------------PhysicalOlapScan[t2] |
| ----------------PhysicalDistribute[DistributionSpecHash] |
| ------------------PhysicalProject |
| --------------------PhysicalOlapScan[t3] |
| ------------PhysicalDistribute[DistributionSpecHash] |
| --------------PhysicalProject |
| ----------------PhysicalOlapScan[t1] |
| |
| Hint log: |
| Used: leading(alias t1) |
| UnUsed: |
| SyntaxError: |
+--------------------------------------------------------------------------------------+
Ordered Hint
Ordered Hint 可以看作 Leading Hint 的一种特例，用于控制 Join Order 为 SQL 文本序。
语法
Ordered Hint 的语法为 /*+ ORDERED */，放置在 SELECT 语句中的 SELECT 关键字之后，紧接着查询的其余部分。
示例
以下是一个使用 Ordered Hint 的示例：
mysql> explain shape plan select /*+ ORDERED */ t1.c1 from t2 join t1 on t1.c1 = t2.c2 join t3 on c2 = c3;
+--------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| --------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| ----------PhysicalProject |
| ------------PhysicalOlapScan[t2] |
| ----------PhysicalDistribute[DistributionSpecHash] |
| ------------PhysicalProject |
| --------------PhysicalOlapScan[t1] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalProject |
| ------------PhysicalOlapScan[t3] |
| |
| Hint log: |
| Used: ORDERED |
| UnUsed: |
| SyntaxError: |
+--------------------------------------------------------------------------------+
与 Leading Hint 的优先级关系
当 Ordered Hint 和 Leading Hint 同时使用时，Ordered Hint 优先于 Leading Hint。这意味着即使指定了 Leading Hint，如果同时存在 Ordered Hint，查询计划将按照 Ordered Hint 的规则执行，而 Leading Hint 将被忽略。
以下示例展示了两者同时使用时的情况：
mysql> explain shape plan select /*+ ORDERED LEADING(t1 t2 t3) */ t1.c1 from t2 join t1 on t1.c1 = t2.c2 join t3 on c2 = c3;
+--------------------------------------------------------------------------------+
| Explain String(Nereids Planner) |
+--------------------------------------------------------------------------------+
| PhysicalResultSink |
| --PhysicalDistribute[DistributionSpecGather] |
| ----PhysicalProject |
| ------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() |
| --------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() |
| ----------PhysicalProject |
| ------------PhysicalOlapScan[t2] |
| ----------PhysicalDistribute[DistributionSpecHash] |
| ------------PhysicalProject |
| --------------PhysicalOlapScan[t1] |
| --------PhysicalDistribute[DistributionSpecHash] |
| ----------PhysicalProject |
| ------------PhysicalOlapScan[t3] |
| |
| Hint log: |
| Used: ORDERED |
| UnUsed: leading(t1 t2 t3) |
| SyntaxError: |
+--------------------------------------------------------------------------------+
Leading Hint vs Ordered Hint 对比
| 维度 | Leading Hint | Ordered Hint |
|---|---|---|
| 语法 | /*+ LEADING(t1 t2 ...) */ | /*+ ORDERED */ |
| Join 顺序 | 按 Hint 中表的列举顺序 | 按 SQL 文本中 FROM 子句中的表书写顺序 |
| 控制 Join 树形状 | 支持，通过 {} 显式指定 | 不支持，固定为左深树 |
| 灵活性 | 高，可指定任意形状的 Join 树 | 低，仅按文本序 |
| 同时使用时 | 被放入 UnUsed | 优先生效 |
常见问题（FAQ）
Q1：为什么 EXPLAIN 中的 Leading Hint 出现在 UnUsed 而不是 Used？
通常有两类原因：
- Leading Hint 指定的 Join Order 与原 SQL 语义不等价（例如 Outer Join 不可交换的场景）。
- 当前版本暂不支持该特性。
此时 Doris 会回退到正常计划生成流程，查询仍可正常执行。
Q2：SyntaxError 提示找不到表怎么办？
SyntaxError 表示 Hint 自身存在语法错误，例如 leading(t2 t3) 中的 t3 未出现在 SQL 的 FROM 子句中。请检查 Hint 中的表名或别名是否与 SQL 中实际使用的一致。
Q3：Ordered Hint 与 Leading Hint 同时使用时哪个生效？
Ordered Hint 优先级更高。当两者同时出现时，按 Ordered Hint 执行（Join 顺序为 SQL 文本序），Leading Hint 会被放入 UnUsed。
Q4：默认生成什么形状的 Join 树？如何调整？
不使用大括号时默认生成左深树。需要右深树、Bushy 树或 zig-zag 树时，使用大括号 {} 显式指定形状即可，无需像 Oracle 那样从左深树逐步 swap。
Q5：Hint 中的表名需要写全限定名吗？
不需要。Hint 中的表名/别名只需与当前查询 FROM 子句中使用的名称保持一致即可，不要求带库名前缀。
总结
Leading Hint 是一个强大的手工控制 Join Order 的特性，在生产业务调优中应用广泛。合理使用 Leading Hint 能够满足现场针对 Join Order 的调优需求，提升系统控制的灵活性。Ordered Hint 是一种特殊的 Leading Hint，用于将当前业务的 Join Order 固定为文本序，使用时需要注意它与其他 Hint 之间的优先级关系。
