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What is explain operator?

  • The explain operator is used to display the logical, physical, and MapReduce execution plans of a relation.
  • One of Pig’s goals is to allow you to think in terms of data flow instead of MapReduce. But sometimes you need to peek into the barn and see how Pig is compiling your script into MapReduce jobs.
  • Pig provides explain for this. explain is particularly helpful when you are trying to optimize your scripts or debug errors.
  • It was written so that Pig developers could examine how Pig handled various scripts, thus its output is not the most user-friendly.
  • But with some effort, explain can help you write better Pig Latin.

Syntax:

  • The syntax of the explain operator is
 grunt> explain Relation_name;

Usage:

  • Use the EXPLAIN operator to review the logical, physical, and map reduce execution plans that are used to compute the specified relationship.
  • If no script is given:
  • The logical plan shows a pipeline of operators to be executed to build the relation. Type checking and backend-independent optimizations (such as applying filters early on) also apply.
  • The physical plan shows how the logical operators are translated to backend-specific physical operators. Some backend optimizations also apply.
  • The mapreduce plan shows how the physical operators are grouped into map reduce jobs.
  • If a script without an alias is specified, it will output the entire execution graph (logical, physical, or map reduce).
  • If a script with a alias is specified, it will output the plan for the given alias.

Example:

  • Assume we have a file student_data.txt in HDFS with the following content.
001, Aadhira,Arushi,9848022337, Delhi
002, Mahi,Champa,9848022338, Chennai
003, Avantika,charu,9848022339, Pune
004, Samaira,Hansa,9848022330, Kolkata
005, Abhinav,Akaash,9848022336,Bhuwaneshwar
006, Amarjeet,Aksat,9848022335, Hyderabad
  • And we have read it into a relation student using the LOAD operator as shown below.
  • Now, explain the relation named student using the explain operator as shown below.
grunt> explain student;

Output:

  • It will produce the following output.
$ 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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