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SinceVersion 2.0.4
背景
在数据库中，”Hint” 是一种用于指导查询优化器执行计划的指令。通过在SQL语句中嵌入Hint，可以影响优化器的决策，以选中期望的执行路径。以下是一个使用Hint的背景示例： 假设有一个包含大量数据的表，而你知道在某些特定情况下，在一个查询中，表的连接顺序可能会影响查询性能。Leading Hint允许你指定希望优化器遵循的表连接的顺序。 例如，考虑以下SQL查询：
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] |+-------------------------------------------+7 rows in set (0.06 sec)
在上述例子里面，在执行效率不理想的时候，我们希望调整下join顺序而不改变原始sql以免影响到用户原始场景且能达到调优的目的。我们可以使用leading任意改变tableA和tableB的join顺序。例如可以写成：
mysql> explain shape plan select /*+ leading(t2 t1) */ * from t1 join t2 on c1 = c2;+-----------------------------------------------------------------------------------------------------+| Explain String(Nereids Planner) |+-----------------------------------------------------------------------------------------------------+| PhysicalResultSink || --PhysicalDistribute || ----PhysicalProject || ------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() build RFs:RF0 c1->[c2] || --------PhysicalOlapScan[t2] apply RFs: RF0 || --------PhysicalDistribute || ----------PhysicalOlapScan[t1] || || Hint log: || Used: leading(t2 t1) || UnUsed: || SyntaxError: |+-----------------------------------------------------------------------------------------------------+12 rows in set (0.06 sec)
在这个例子中，使用了 /+ leading(t2 t1) / 这个Hint。这个Hint告诉优化器在执行计划中使用指定表（t2）作为驱动表，并置于(t1)之前。 本文主要阐述如何在Doris里面使用join相关的hint：leading hint、ordered hint 和 distribute hint
Leading hint使用说明
Leading Hint 用于指导优化器确定查询计划的连接顺序。在一个查询中，表的连接顺序可能会影响查询性能。Leading Hint允许你指定希望优化器遵循的表连接的顺序。 在doris里面，其语法为 /+LEADING( tablespec [ tablespec ]… ) /,leading由”/+”和”/“包围并置于select语句里面 select的正后方。注意，leading 后方的 ‘/‘ 和selectlist需要隔开至少一个分割符例如空格。至少需要写两个以上的表才认为这个leadinghint是合理的。且任意的join里面可以用大括号括号起来，来显式地指定joinTree的形状。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: |+------------------------------------------------------------------------------+12 rows in set (0.01 sec)
- 当leadinghint不生效的时候会走正常的流程生成计划，explain会显示使用的hint是否生效，主要分三种来显示：
- Used：leading hint正常生效
- Unused： 这里不支持的情况包含leading指定的join order与原sql不等价或本版本暂不支持特性（详见限制）
- SyntaxError： 指leading hint语法错误，如找不到对应的表等
- leading hint语法默认造出来左深树，例：select /+ leading(t1 t2 t3) / * from t1 join t2 on…默认指定出来
join/ \join t3/ \t1 t2mysql> 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: |+--------------------------------------------------------------------------------+15 rows in set (0.00 sec)
- 同时允许使用大括号指定join树形状。例：/+ leading(t1 {t2 t3}) / join / \ t1 join / \ t2 t3
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: |+----------------------------------------------------------------------------------+15 rows in set (0.02 sec)
- 当有view作为别名参与joinReorder的时候可以指定对应的view作为leading的参数。例：
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: |+--------------------------------------------------------------------------------------+21 rows in set (0.06 sec)
基本用例
（注意这里列命名和表命名相关，例：只有t1中有c1字段，后续例子为了简化会将 t1.c1 直接写成 c1）
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');
举个简单的例子，当我们需要交换t1和t2的join顺序的时候只需要在前面加上leading(t2 t1)即可，explain的时候会 显示是否用上了这个hint。 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] |+-------------------------------------------+7 rows in set (0.06 sec)
Leading plan
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: |+------------------------------------------------------------------------------+12 rows in set (0.00 sec)
hint 效果展示 （Used unused） 若leading hint有语法错误，explain的时候会在syntax error里面显示相应的信息，但是计划能照常生成，只不过没有使用leading而已
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 |+--------------------------------------------------------+11 rows in set (0.01 sec)
扩展场景
左深树
当我们不使用任何括号的情况下leading会默认生成左深树
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: |+--------------------------------------------------------------------------------+15 rows in set (0.10 sec)
右深树
当我们想将计划的形状做成右深树或者bushy树或者zigzag树的时候，只需要加上大括号来限制plan的形状即可，不需要像oracle一样用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: |+-----------------------------------------------+14 rows in set (0.02 sec)
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: |+-----------------------------------------------+17 rows in set (0.02 sec)
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: |+--------------------------------------------------------------------------------------+19 rows in set (0.02 sec)
Non-inner join：
当遇到非inner-join的时候，例如Outer join或者semi/anti join的时候，leading hint会根据原始sql语义自动推导各个join的join方式。若遇到与原始sql语义不同的leading hint或者生成不了的情况则会放到unused里面，但是不影响计划正常流程的生成。 下面是不能交换的例子：
-———- 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: |+--------------------------------------------------------------------------------+15 rows in set (0.01 sec)
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 joinexplain 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 joinexplain 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 joinexplain shape plan select * from t1 where exists (select c2 from t2);
View
遇到别名的情况，可以将别名作为一个完整的子树进行指定，子树里面的joinOrder由文本序生成。
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: |+--------------------------------------------------------------------------------------+21 rows in set (0.02 sec)
与ordered混合使用
当与ordered hint混合使用的时候以ordered hint为主，即ordered hint生效优先级高于leading hint。例：
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: |+--------------------------------------------------------------------------------+18 rows in set (0.02 sec)
使用限制
- 当前版本只支持使用一个leadingHint。若和子查询同时使用leadinghint的话则查询会报错。例（这个例子explain会报错，但是会走正常的路径生成计划）：
mysql> explain shape plan select /*+ leading(alias t1) */ count(*) from t1 join (select /*+ leading(t3 t2) */ 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 || --------------PhysicalOlapScan[t1] || ------------PhysicalDistribute[DistributionSpecHash] || --------------PhysicalProject || ----------------hashJoin[INNER_JOIN] hashCondition=((t2.c2 = t3.c3)) otherCondition=() || ------------------PhysicalProject || --------------------PhysicalOlapScan[t2] || ------------------PhysicalDistribute[DistributionSpecHash] || --------------------PhysicalProject || ----------------------PhysicalOlapScan[t3] || || Hint log: || Used: || UnUsed: leading(alias t1) || SyntaxError: leading(t3 t2) Msg:one query block can only have one leading clause |+----------------------------------------------------------------------------------------+21 rows in set (0.01 sec)
OrderedHint 使用说明
- 使用ordered hint会让join tree的形状固定下来，按照文本序来显示
- 语法为 /+ ORDERED /,leading由”/+”和”/“包围并置于select语句里面 select的正后方，例： explain shape plan select /+ ORDERED / t1.c1 from t2 join t1 on t1.c1 = t2.c2 join t3 on c2 = c3; join / \ join t3 / \ t2 t1
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: |+--------------------------------------------------------------------------------+18 rows in set (0.02 sec)
- 当ordered hint和leading hint同时使用时以ordered hint为准，leading hint会失效
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: |+--------------------------------------------------------------------------------+18 rows in set (0.02 sec)
DistributeHint 使用说明
- 目前只能指定右表的distribute Type 而且只有[shuffle] 和 [broadcast]两种，写在join右表前面且允许中括号和/+ /两种写法
- 目前能使用任意个DistributeHint
- 当遇到无法正确生成计划的 DistributeHint，没有显示，按最大努力生效，最后以explain显示的distribute方式为主
- 当前版本暂不与leading混用，且当distribute指定的表位于join右边才可生效。
- 多与ordered混用，利用文本序把join顺序固定下来，然后再指定相应的join里面我们预期使用什么样的distribute方式。例： 使用前：
mysql> explain shape plan select count(*) from t1 join t2 on t1.c1 = t2.c2;+----------------------------------------------------------------------------------+| Explain String(Nereids Planner) |+----------------------------------------------------------------------------------+| PhysicalResultSink || --hashAgg[GLOBAL] || ----PhysicalDistribute[DistributionSpecGather] || ------hashAgg[LOCAL] || --------PhysicalProject || ----------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() || ------------PhysicalProject || --------------PhysicalOlapScan[t1] || ------------PhysicalDistribute[DistributionSpecHash] || --------------PhysicalProject || ----------------PhysicalOlapScan[t2] |+----------------------------------------------------------------------------------+11 rows in set (0.01 sec)
使用后：
mysql> explain shape plan select /*+ ordered */ count(*) from t2 join[broadcast] t1 on t1.c1 = t2.c2;+----------------------------------------------------------------------------------+| Explain String(Nereids Planner) |+----------------------------------------------------------------------------------+| PhysicalResultSink || --hashAgg[GLOBAL] || ----PhysicalDistribute[DistributionSpecGather] || ------hashAgg[LOCAL] || --------PhysicalProject || ----------hashJoin[INNER_JOIN] hashCondition=((t1.c1 = t2.c2)) otherCondition=() || ------------PhysicalProject || --------------PhysicalOlapScan[t2] || ------------PhysicalDistribute[DistributionSpecReplicated] || --------------PhysicalProject || ----------------PhysicalOlapScan[t1] || || Hint log: || Used: ORDERED || UnUsed: || SyntaxError: |+----------------------------------------------------------------------------------+16 rows in set (0.01 sec)
- Explain shape plan里面会显示distribute算子相关的信息，其中DistributionSpecReplicated表示该算子将对应的数据变成所有be节点复制一份，DistributionSpecGather表示将数据gather到fe节点，DistributionSpecHash表示将数据按照特定的hashKey以及算法打散到不同的be节点。
待支持
- leadingHint待支持子查询解嵌套指定，当前和子查询提升以后不能混用，需要有hint来控制是否可以解嵌套
- 需要新的distributeHint来更好且更全面地控制distribute算子
- 混合使用leadingHint与distributeHint来共同确定join的形状
