explain 运算符用于显示关系的逻辑，物理和MapReduce执行计划。

语法

下面给出了 explain 运算符的语法。

grunt> explain Relation_name;

例

假设在HDFS中有一个包含以下内容的文件 student_data.txt 。

001,Rajiv,Reddy,9848022337,Hyderabad
002,siddarth,Battacharya,9848022338,Kolkata
003,Rajesh,Khanna,9848022339,Delhi
004,Preethi,Agarwal,9848022330,Pune
005,Trupthi,Mohanthy,9848022336,Bhuwaneshwar
006,Archana,Mishra,9848022335,Chennai.

使用LOAD运算符将它读入关系 student ，如下所示。

grunt> student = LOAD 'hdfs://localhost:9000/pig_data/student_data.txt' USING PigStorage(',')
   as ( id:int, firstname:chararray, lastname:chararray, phone:chararray, city:chararray );

现在，让我们使用 explain 运算符解释名为student的关系，如下所示。

grunt> explain student;

输出

它将产生以下输出。

$ explain student;

2015-10-05 11:32:43,660 [main]
2015-10-05 11:32:43,660 [main] INFO  org.apache.pig.newplan.logical.optimizer
.LogicalPlanOptimizer -
{RULES_ENABLED=[AddForEach, ColumnMapKeyPrune, ConstantCalculator,
GroupByConstParallelSetter, LimitOptimizer, LoadTypeCastInserter, MergeFilter, 
MergeForEach, PartitionFilterOptimizer, PredicatePushdownOptimizer,
PushDownForEachFlatten, PushUpFilter, SplitFilter, StreamTypeCastInserter]}  
#-----------------------------------------------
# New Logical Plan: 
#-----------------------------------------------
student: (Name: LOStore Schema:
id#31:int,firstname#32:chararray,lastname#33:chararray,phone#34:chararray,city#
35:chararray)
| 
|---student: (Name: LOForEach Schema:
id#31:int,firstname#32:chararray,lastname#33:chararray,phone#34:chararray,city#
35:chararray)
    |   |
    |   (Name: LOGenerate[false,false,false,false,false] Schema:
id#31:int,firstname#32:chararray,lastname#33:chararray,phone#34:chararray,city#
35:chararray)ColumnPrune:InputUids=[34, 35, 32, 33,
31]ColumnPrune:OutputUids=[34, 35, 32, 33, 31]
    |   |   | 
    |   |   (Name: Cast Type: int Uid: 31) 
    |   |   |     |   |   |---id:(Name: Project Type: bytearray Uid: 31 Input: 0 Column: (*))
    |   |   |     
    |   |   (Name: Cast Type: chararray Uid: 32)
    |   |   | 
    |   |   |---firstname:(Name: Project Type: bytearray Uid: 32 Input: 1
Column: (*))
    |   |   |
    |   |   (Name: Cast Type: chararray Uid: 33)
    |   |   |
    |   |   |---lastname:(Name: Project Type: bytearray Uid: 33 Input: 2
	 Column: (*))
    |   |   | 
    |   |   (Name: Cast Type: chararray Uid: 34)
    |   |   |  
    |   |   |---phone:(Name: Project Type: bytearray Uid: 34 Input: 3 Column:
(*))
    |   |   | 
    |   |   (Name: Cast Type: chararray Uid: 35)
    |   |   |  
    |   |   |---city:(Name: Project Type: bytearray Uid: 35 Input: 4 Column:
(*))
    |   | 
    |   |---(Name: LOInnerLoad[0] Schema: id#31:bytearray)
    |   |  
    |   |---(Name: LOInnerLoad[1] Schema: firstname#32:bytearray)
    |   |
    |   |---(Name: LOInnerLoad[2] Schema: lastname#33:bytearray)
    |   |
    |   |---(Name: LOInnerLoad[3] Schema: phone#34:bytearray)
    |   | 
    |   |---(Name: LOInnerLoad[4] Schema: city#35:bytearray)
    |
    |---student: (Name: LOLoad Schema: 
id#31:bytearray,firstname#32:bytearray,lastname#33:bytearray,phone#34:bytearray
,city#35:bytearray)RequiredFields:null 
#-----------------------------------------------
# Physical Plan: #-----------------------------------------------
student: Store(fakefile:org.apache.pig.builtin.PigStorage) - scope-36
| 
|---student: New For Each(false,false,false,false,false)[bag] - scope-35
    |   |
    |   Cast[int] - scope-21
    |   |
    |   |---Project[bytearray][0] - scope-20
    |   |  
    |   Cast[chararray] - scope-24
    |   |
    |   |---Project[bytearray][1] - scope-23
    |   | 
    |   Cast[chararray] - scope-27
    |   |  
    |   |---Project[bytearray][2] - scope-26 
    |   |  
    |   Cast[chararray] - scope-30 
    |   |  
    |   |---Project[bytearray][3] - scope-29
    |   |
    |   Cast[chararray] - scope-33
    |   | 
    |   |---Project[bytearray][4] - scope-32
    | 
    |---student: Load(hdfs://localhost:9000/pig_data/student_data.txt:PigStorage(',')) - scope19
2015-10-05 11:32:43,682 [main]
INFO  org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MRCompiler - 
File concatenation threshold: 100 optimistic? false
2015-10-05 11:32:43,684 [main]
INFO  org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MultiQueryOp timizer - 
MR plan size before optimization: 1 2015-10-05 11:32:43,685 [main]
INFO  org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.
MultiQueryOp timizer - MR plan size after optimization: 1 
#--------------------------------------------------
# Map Reduce Plan                                   
#--------------------------------------------------
MapReduce node scope-37
Map Plan
student: Store(fakefile:org.apache.pig.builtin.PigStorage) - scope-36
|
|---student: New For Each(false,false,false,false,false)[bag] - scope-35
    |   |
    |   Cast[int] - scope-21 
    |   |
    |   |---Project[bytearray][0] - scope-20
    |   |
    |   Cast[chararray] - scope-24
    |   |
    |   |---Project[bytearray][1] - scope-23
    |   |
    |   Cast[chararray] - scope-27
    |   | 
    |   |---Project[bytearray][2] - scope-26 
    |   | 
    |   Cast[chararray] - scope-30 
    |   |  
    |   |---Project[bytearray][3] - scope-29 
    |   | 
    |   Cast[chararray] - scope-33
    |   | 
    |   |---Project[bytearray][4] - scope-32 
    |  
    |---student:
Load(hdfs://localhost:9000/pig_data/student_data.txt:PigStorage(',')) - scope
19-------- Global sort: false
 ---------------- 

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