Spark2.1和2.2 SQL物理執行策略關鍵原始碼分析
摘要：
先附上一句SQL，使用tpc-ds的表結構，我們圍繞這句SQL講。
SQL：
SQL> selectavg(cs_ext_discount_amt)fromcatalog_sales, date_dimwhered_date between ‘...
先附上一句SQL，使用tpc-ds的表結構，我們圍繞這句SQL講。
- SQL：
SQL> selectavg(cs_ext_discount_amt)fromcatalog_sales, date_dimwhered_date between ‘1999-02-22’andcast(‘1999-05-22’ as date)andd_date_sk = cs_sold_date_skgroup by cs_sold_date_sk;
- 邏輯計劃：
Aggregate [cs_sold_date_sk#24], [cast((avg(UnscaledValue(cs_ext_discount_amt#46)) / 100.0) as decimal(11,6)) AS avg(cs_ext_discount_amt)#149] +- Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] +- Join Inner, (d_date_sk#58 = cs_sold_date_sk#24) :- Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] :+- Filter isnotnull(cs_sold_date_sk#24) :+- Relation[cs_sold_date_sk#24,cs_sold_time_sk#25,cs_ship_date_sk#26,cs_bill_customer_sk#27,cs_bill_cdemo_sk#28,cs_bill_hdemo_sk#29,cs_bill_addr_sk#30,cs_ship_customer_sk#31,cs_ship_cdemo_sk#32,cs_ship_hdemo_sk#33,cs_ship_addr_sk#34,cs_call_center_sk#35,cs_catalog_page_sk#36,cs_ship_mode_sk#37,cs_warehouse_sk#38,cs_item_sk#39,cs_promo_sk#40,cs_order_number#41,cs_quantity#42,cs_wholesale_cost#43,cs_list_price#44,cs_sales_price#45,cs_ext_discount_amt#46,cs_ext_sales_price#47,... 10 more fields] +- Project [d_date_sk#58] +- Filter (((isnotnull(d_date#60) && (cast(d_date#60 as string) >= 1999-02-22)) && (d_date#60 <= 10733)) && isnotnull(d_date_sk#58)) +- Relation[d_date_sk#58,d_date_id#59,d_date#60,d_month_seq#61,d_week_seq#62,d_quarter_seq#63,d_year#64,d_dow#65,d_moy#66,d_dom#67,d_qoy#68,d_fy_year#69,d_fy_quarter_seq#70,d_fy_week_seq#71,d_day_name#72,d_quarter_name#73,d_holiday#74,d_weekend#75,d_following_holiday#76,d_first_dom#77,d_last_dom#78,d_same_day_ly#79,d_same_day_lq#80,d_current_day#81,... 4 more fields]
物理計劃原始碼分析
物理策略
def strategies: Seq[Strategy] = extraStrategies ++ ( FileSourceStrategy :: DataSourceStrategy :: DDLStrategy :: SpecialLimits :: Aggregation :: JoinSelection :: InMemoryScans :: BasicOperators :: Nil)
其中，extraStrategies是提供給外部人員可以自己新增的策略。呼叫這些strategies的程式碼如下：
// Collect physical plan candidates. val candidates = strategies.iterator.flatMap(_(plan))
將strategies逐個去應用在邏輯計劃上，然後做flat操作，返回一個PhysicalPlan
的iterator。那麼每個策略什麼作用？
FileSourceStrategy
一個針對Hadoop檔案系統做的策略，當執行計劃的底層Relation是HadoopFsRelation
時會呼叫到，用來掃描檔案。
DataSourceStrategy
Spark針對DataSource預定義了四種scan介面，TableScan
、PrunedScan
、PrunedFilteredScan
、CatalystScan
(其中CatalystScan
是unstable的，也是不常用的)，如果開發者（使用者）自己實現的DataSource是實現了這四種介面之一的，在scan到執行計劃的底層Relation時，就會呼叫來掃描檔案。
DDLStrategy（2.2中已經消失了，2.1中有）
會在create table的時候呼叫，因為後續版本不會存在，所以不做解釋。
SpecialLimits
在Spark SQL中加limit n時候回撥用到（如果不指定，Spark 預設也會limit 20），在原始碼中，會給每種case的limit節點的子節點使用PlanLater
，這是個很神奇的東西下文會講到。
Aggregation
顧名思義，執行聚合函式的策略。
JoinSelection
執行join的策略。Join的執行策略也同樣分BroadcastJoin（也就是MapSideJoin），和ShuffledJoin，這個之後的文章會展開講。
InMemoryScans
當資料在記憶體中被快取過，就會用到該策略。
BasicOperators
一些基本操作的執行策略，如flatMap，sort，project等，但是實際上大都是給這些節點的子節點套上一個PlanLater
。
PlanLater
Spark SQL物理計劃裡一個非常重要的概念。字面意思很好理解，就是之後再計劃。那麼經過以上策略逐個去執行以後，原來的邏輯計劃會變成什麼樣呢？
ReturnAnswer +- GlobalLimit 21 +- LocalLimit 21 +- PlanLater Aggregate [cs_sold_date_sk#24], [cast((avg(UnscaledValue(cs_ext_discount_amt#46)) / 100.0) as decimal(11,6)) AS avg(cs_ext_discount_amt)#149] , Aggregate [cs_sold_date_sk#24], [cast((avg(UnscaledValue(cs_ext_discount_amt#46)) / 100.0) as decimal(11,6)) AS avg(cs_ext_discount_amt)#149] +- PlanLater Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] , Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] +- PlanLater Join Inner, (d_date_sk#58 = cs_sold_date_sk#24) :- PlanLater Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] , Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] :+- Filter isnotnull(cs_sold_date_sk#24) :+- Relation[cs_sold_date_sk#24,cs_sold_time_sk#25,cs_ship_date_sk#26,cs_bill_customer_sk#27,cs_bill_cdemo_sk#28,cs_bill_hdemo_sk#29,cs_bill_addr_sk#30,cs_ship_customer_sk#31,cs_ship_cdemo_sk#32,cs_ship_hdemo_sk#33,cs_ship_addr_sk#34,cs_call_center_sk#35,cs_catalog_page_sk#36,cs_ship_mode_sk#37,cs_warehouse_sk#38,cs_item_sk#39,cs_promo_sk#40,cs_order_number#41,cs_quantity#42,cs_wholesale_cost#43,cs_list_price#44,cs_sales_price#45,cs_ext_discount_amt#46,cs_ext_sales_price#47,... 10 more fields] +- PlanLater Project [d_date_sk#58] , Project [d_date_sk#58] +- Filter (((isnotnull(d_date#60) && (cast(d_date#60 as string) >= 1999-02-22)) && (d_date#60 <= 10733)) && isnotnull(d_date_sk#58)) +- Relation[d_date_sk#58,d_date_id#59,d_date#60,d_month_seq#61,d_week_seq#62,d_quarter_seq#63,d_year#64,d_dow#65,d_moy#66,d_dom#67,d_qoy#68,d_fy_year#69,d_fy_quarter_seq#70,d_fy_week_seq#71,d_day_name#72,d_quarter_name#73,d_holiday#74,d_weekend#75,d_following_holiday#76,d_first_dom#77,d_last_dom#78,d_same_day_ly#79,d_same_day_lq#80,d_current_day#81,... 4 more fields]
有什麼差別呢？主要有二：
-
-
頂層多了個
ReturnAnswer和Limit節點
-
頂層多了個
-
-
Aggregate、Project、Join節點都用了PlanLater
-
（其實Filter
節點也是可以用PlanLater
的，但是由於邏輯計劃已經將Filter
下推至底部，所以最底部的Project->Filter->Relation的三層節點是可以直接呼叫一個策略去執行的，因此只需要三層節點的最上層也就是Project節點使用PlanLater
即可。）
言歸正傳，語法樹頂部多了ReturnAnswer
和Limit
節點，很容易理解，Limit
是Spark SQL預設限制行數，ReturnAnswer
是將結果返回。那麼加的PlanLater有什麼作用？我的理解是，將物理計劃分割成一段段，每一段物理計劃會有其對應策略來執行。具體原始碼如下：
def plan(plan: LogicalPlan): Iterator[PhysicalPlan] = {
// Obviously a lot to do here still...
// Collect physical plan candidates.
val candidates = strategies.iterator.flatMap(_(plan))
// The candidates may contain placeholders marked as [[planLater]],
// so try to replace them by their child plans.
val plans = candidates.flatMap { candidate =>
val placeholders = collectPlaceholders(candidate)
if (placeholders.isEmpty) {
// Take the candidate as is because it does not contain placeholders.
Iterator(candidate)
} else {
// Plan the logical plan marked as [[planLater]] and replace the placeholders.
placeholders.iterator.foldLeft(Iterator(candidate)) {
case (candidatesWithPlaceholders, (placeholder, logicalPlan)) =>
// Plan the logical plan for the placeholder.
val childPlans = this.plan(logicalPlan)
candidatesWithPlaceholders.flatMap { candidateWithPlaceholders =>
childPlans.map { childPlan =>
// Replace the placeholder by the child plan
candidateWithPlaceholders.transformUp {
case p if p == placeholder => childPlan
}
}
}
}
}
}
val pruned = prunePlans(plans)
assert(pruned.hasNext, s"No plan for $plan")
pruned
}
可以看到，經過策略迭代器和flat過後的candidates候選計劃們（一般來說只有一個，是最頂層的planLater），然後收集placeholder（其實就是planlater），這個時候對placeholders進行迭代，並對每個placeholder的child plan遞迴呼叫plan方法。舉例文章這句SQL，遞迴呼叫plan方法，得到每個placeholder及其child plan節點（也就是 case (candidatesWithPlaceholders, (placeholder, logicalPlan))這句話的placeholder和logicalPlan兩個變數）如下：
placeholder： PlanLater Aggregate [cs_sold_date_sk#24], [cast((avg(UnscaledValue(cs_ext_discount_amt#46)) / 100.0) as decimal(11,6)) AS avg(cs_ext_discount_amt)#149] logicalPlan： Aggregate [cs_sold_date_sk#24], [cast((avg(UnscaledValue(cs_ext_discount_amt#46)) / 100.0) as decimal(11,6)) AS avg(cs_ext_discount_amt)#149] +- Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] +- Join Inner, (d_date_sk#58 = cs_sold_date_sk#24) :- Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] :+- Filter isnotnull(cs_sold_date_sk#24) :+- Relation[cs_sold_date_sk#24,cs_sold_time_sk#25,cs_ship_date_sk#26,cs_bill_customer_sk#27,cs_bill_cdemo_sk#28,cs_bill_hdemo_sk#29,cs_bill_addr_sk#30,cs_ship_customer_sk#31,cs_ship_cdemo_sk#32,cs_ship_hdemo_sk#33,cs_ship_addr_sk#34,cs_call_center_sk#35,cs_catalog_page_sk#36,cs_ship_mode_sk#37,cs_warehouse_sk#38,cs_item_sk#39,cs_promo_sk#40,cs_order_number#41,cs_quantity#42,cs_wholesale_cost#43,cs_list_price#44,cs_sales_price#45,cs_ext_discount_amt#46,cs_ext_sales_price#47,... 10 more fields] +- Project [d_date_sk#58] +- Filter (((isnotnull(d_date#60) && (cast(d_date#60 as string) >= 1999-02-22)) && (d_date#60 <= 10733)) && isnotnull(d_date_sk#58)) +- Relation[d_date_sk#58,d_date_id#59,d_date#60,d_month_seq#61,d_week_seq#62,d_quarter_seq#63,d_year#64,d_dow#65,d_moy#66,d_dom#67,d_qoy#68,d_fy_year#69,d_fy_quarter_seq#70,d_fy_week_seq#71,d_day_name#72,d_quarter_name#73,d_holiday#74,d_weekend#75,d_following_holiday#76,d_first_dom#77,d_last_dom#78,d_same_day_ly#79,d_same_day_lq#80,d_current_day#81,... 4 more fields]
placeholder: PlanLater Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] logicalPlan： Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] +- Join Inner, (d_date_sk#58 = cs_sold_date_sk#24) :- Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] :+- Filter isnotnull(cs_sold_date_sk#24) :+- Relation[cs_sold_date_sk#24,cs_sold_time_sk#25,cs_ship_date_sk#26,cs_bill_customer_sk#27,cs_bill_cdemo_sk#28,cs_bill_hdemo_sk#29,cs_bill_addr_sk#30,cs_ship_customer_sk#31,cs_ship_cdemo_sk#32,cs_ship_hdemo_sk#33,cs_ship_addr_sk#34,cs_call_center_sk#35,cs_catalog_page_sk#36,cs_ship_mode_sk#37,cs_warehouse_sk#38,cs_item_sk#39,cs_promo_sk#40,cs_order_number#41,cs_quantity#42,cs_wholesale_cost#43,cs_list_price#44,cs_sales_price#45,cs_ext_discount_amt#46,cs_ext_sales_price#47,... 10 more fields] +- Project [d_date_sk#58] +- Filter (((isnotnull(d_date#60) && (cast(d_date#60 as string) >= 1999-02-22)) && (d_date#60 <= 10733)) && isnotnull(d_date_sk#58)) +- Relation[d_date_sk#58,d_date_id#59,d_date#60,d_month_seq#61,d_week_seq#62,d_quarter_seq#63,d_year#64,d_dow#65,d_moy#66,d_dom#67,d_qoy#68,d_fy_year#69,d_fy_quarter_seq#70,d_fy_week_seq#71,d_day_name#72,d_quarter_name#73,d_holiday#74,d_weekend#75,d_following_holiday#76,d_first_dom#77,d_last_dom#78,d_same_day_ly#79,d_same_day_lq#80,d_current_day#81,... 4 more fields]
placeholder: PlanLater Join Inner, (d_date_sk#58 = cs_sold_date_sk#24) logicalPlan： Join Inner, (d_date_sk#58 = cs_sold_date_sk#24) :- Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] :+- Filter isnotnull(cs_sold_date_sk#24) :+- Relation[cs_sold_date_sk#24,cs_sold_time_sk#25,cs_ship_date_sk#26,cs_bill_customer_sk#27,cs_bill_cdemo_sk#28,cs_bill_hdemo_sk#29,cs_bill_addr_sk#30,cs_ship_customer_sk#31,cs_ship_cdemo_sk#32,cs_ship_hdemo_sk#33,cs_ship_addr_sk#34,cs_call_center_sk#35,cs_catalog_page_sk#36,cs_ship_mode_sk#37,cs_warehouse_sk#38,cs_item_sk#39,cs_promo_sk#40,cs_order_number#41,cs_quantity#42,cs_wholesale_cost#43,cs_list_price#44,cs_sales_price#45,cs_ext_discount_amt#46,cs_ext_sales_price#47,... 10 more fields] +- Project [d_date_sk#58] +- Filter (((isnotnull(d_date#60) && (cast(d_date#60 as string) >= 1999-02-22)) && (d_date#60 <= 10733)) && isnotnull(d_date_sk#58)) +- Relation[d_date_sk#58,d_date_id#59,d_date#60,d_month_seq#61,d_week_seq#62,d_quarter_seq#63,d_year#64,d_dow#65,d_moy#66,d_dom#67,d_qoy#68,d_fy_year#69,d_fy_quarter_seq#70,d_fy_week_seq#71,d_day_name#72,d_quarter_name#73,d_holiday#74,d_weekend#75,d_following_holiday#76,d_first_dom#77,d_last_dom#78,d_same_day_ly#79,d_same_day_lq#80,d_current_day#81,... 4 more fields]
placeholder: PlanLater Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] logicalPlan： Project [cs_sold_date_sk#24, cs_ext_discount_amt#46] +- Filter isnotnull(cs_sold_date_sk#24) +- Relation[cs_sold_date_sk#24,cs_sold_time_sk#25,cs_ship_date_sk#26,cs_bill_customer_sk#27,cs_bill_cdemo_sk#28,cs_bill_hdemo_sk#29,cs_bill_addr_sk#30,cs_ship_customer_sk#31,cs_ship_cdemo_sk#32,cs_ship_hdemo_sk#33,cs_ship_addr_sk#34,cs_call_center_sk#35,cs_catalog_page_sk#36,cs_ship_mode_sk#37,cs_warehouse_sk#38,cs_item_sk#39,cs_promo_sk#40,cs_order_number#41,cs_quantity#42,cs_wholesale_cost#43,cs_list_price#44,cs_sales_price#45,cs_ext_discount_amt#46,cs_ext_sales_price#47,... 10 more fields]
placeholder: PlanLater Project [d_date_sk#58] logicalPlan: Project [d_date_sk#58] +- Filter (((isnotnull(d_date#60) && (cast(d_date#60 as string) >= 1999-02-22)) && (d_date#60 <= 10733)) && isnotnull(d_date_sk#58)) +- Relation[d_date_sk#58,d_date_id#59,d_date#60,d_month_seq#61,d_week_seq#62,d_quarter_seq#63,d_year#64,d_dow#65,d_moy#66,d_dom#67,d_qoy#68,d_fy_year#69,d_fy_quarter_seq#70,d_fy_week_seq#71,d_day_name#72,d_quarter_name#73,d_holiday#74,d_weekend#75,d_following_holiday#76,d_first_dom#77,d_last_dom#78,d_same_day_ly#79,d_same_day_lq#80,d_current_day#81,... 4 more fields]
那麼可以看到，遞迴到最底處，就是project->filter->relation的三層節點組合，由於我實際是重寫過了DataSource，這個時候會呼叫DataSourceStrategy
，去讀取獲取資料，然後遞迴逐個返回根據每個planLater分割點會有對應的策略去對資料進行相應的操作。