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Hive Outer Join Behavior

This document is based on a writeup of DB2 Outer Join Behavior. The original HTML document is attached to the Hive Design Docs and can be downloaded here.

Definitions

Preserved Row table

The table in an Outer Join that must return all rows.
For left outer joins this is the Left table, for right outer joins it is the Right table, and for full outer joins both tables are Preserved Row tables.

Null Supplying table

This is the table that has nulls filled in for its columns in unmatched rows.
In the non-full outer join case, this is the other table in the Join. For full outer joins both tables are also Null Supplying tables.

During Join predicate

A predicate that is in the JOIN ON clause.
For example, in 'R1 join R2 on R1.x = 5' the predicate 'R1.x = 5' is a During Join predicate.

After Join predicate

A predicate that is in the WHERE clause.

Predicate Pushdown Rules

The logic can be summarized by these two rules:

  1. During Join predicates cannot be pushed past Preserved Row tables.
  2. After Join predicates cannot be pushed past Null Supplying tables.

This captured in the following table:

 

Preserved Row Table

Null Supplying Table

Join
Predicate

Case J1:
Not Pushed

Case J2:
Pushed

Where
Predicate

Case W1:
Pushed

Case W2:
Not Pushed

See Examples below for illustrations of cases J1, J2, W1, and W2.

Hive Implementation

Hive enforces the predicate pushdown rules by:

Rule 1: During QBJoinTree construction in Plan Gen, the parse Join Condition logic applies this rule.
Rule 2: During JoinPPD (Join Predicate Pushdown) the get Qualified Alias logic applies this rule.

Examples

Given Src(Key String, Value String) the following Left Outer Join examples show that Hive has the correct behavior.

Case J1: Join Predicate on Preserved Row Table

explain 
select s1.key, s2.key 
from src s1 left join src s2 on s1.key > '2';

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-0 is a root stage

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Alias -> Map Operator Tree:
	s1
	  TableScan
	    alias: s1
	    Reduce Output Operator
	      sort order:
	      tag: 0
	      value expressions:
		    expr: key
		    type: string
	s2
	  TableScan
	    alias: s2
	    Reduce Output Operator
	      sort order:
	      tag: 1
	      value expressions:
		    expr: key
		    type: string
      Reduce Operator Tree:
	Join Operator
	  condition map:
	       Left Outer Join0 to 1
	  condition expressions:
	    0 {VALUE._col0}
	    1 {VALUE._col0}
	  filter predicates:
	    0 {(VALUE._col0 > '2')}
	    1
	  handleSkewJoin: false
	  outputColumnNames: _col0, _col4
	  Select Operator
	    expressions:
		  expr: _col0
		  type: string
		  expr: _col4
		  type: string
	    outputColumnNames: _col0, _col1
	    File Output Operator
	      compressed: false
	      GlobalTableId: 0
	      table:
		  input format: org.apache.hadoop.mapred.TextInputFormat
		  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
		  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe

  Stage: Stage-0
    Fetch Operator
      limit: -1

Case J2: Join Predicate on Null Supplying Table

explain 
select s1.key, s2.key 
from src s1 left join src s2 on s2.key > '2';

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Alias -> Map Operator Tree:
	s1
	  TableScan
	    alias: s1
	    Reduce Output Operator
	      sort order:
	      tag: 0
	      value expressions:
		    expr: key
		    type: string
	s2
	  TableScan
	    alias: s2
	    Filter Operator
	      predicate:
		  expr: (key > '2')
		  type: boolean
	      Reduce Output Operator
		sort order:
		tag: 1
		value expressions:
		      expr: key
		      type: string
      Reduce Operator Tree:
	Join Operator
	  condition map:
	       Left Outer Join0 to 1
	  condition expressions:
	    0 {VALUE._col0}
	    1 {VALUE._col0}
	  handleSkewJoin: false
	  outputColumnNames: _col0, _col4
	  Select Operator
	    expressions:
		  expr: _col0
		  type: string
		  expr: _col4
		  type: string
	    outputColumnNames: _col0, _col1
	    File Output Operator
	      compressed: false
	      GlobalTableId: 0
	      table:
		  input format: org.apache.hadoop.mapred.TextInputFormat
		  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
		  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe

  Stage: Stage-0
    Fetch Operator
      limit: -1

Case W1: Where Predicate on Preserved Row Table

explain 
select s1.key, s2.key 
from src s1 left join src s2 
where s1.key > '2';

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Alias -> Map Operator Tree:
	s1
	  TableScan
	    alias: s1
	    Filter Operator
	      predicate:
		  expr: (key > '2')
		  type: boolean
	      Reduce Output Operator
		sort order:
		tag: 0
		value expressions:
		      expr: key
		      type: string
	s2
	  TableScan
	    alias: s2
	    Reduce Output Operator
	      sort order:
	      tag: 1
	      value expressions:
		    expr: key
		    type: string
      Reduce Operator Tree:
	Join Operator
	  condition map:
	       Left Outer Join0 to 1
	  condition expressions:
	    0 {VALUE._col0}
	    1 {VALUE._col0}
	  handleSkewJoin: false
	  outputColumnNames: _col0, _col4
	  Select Operator
	    expressions:
		  expr: _col0
		  type: string
		  expr: _col4
		  type: string
	    outputColumnNames: _col0, _col1
	    File Output Operator
	      compressed: false
	      GlobalTableId: 0
	      table:
		  input format: org.apache.hadoop.mapred.TextInputFormat
		  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
		  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe

  Stage: Stage-0
    Fetch Operator
      limit: -1

Case W2: Where Predicate on Null Supplying Table

explain
select s1.key, s2.key 
from src s1 left join src s2 
where s2.key > '2';

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Alias -> Map Operator Tree:
	s1
	  TableScan
	    alias: s1
	    Reduce Output Operator
	      sort order:
	      tag: 0
	      value expressions:
		    expr: key
		    type: string
	s2
	  TableScan
	    alias: s2
	    Reduce Output Operator
	      sort order:
	      tag: 1
	      value expressions:
		    expr: key
		    type: string
      Reduce Operator Tree:
	Join Operator
	  condition map:
	       Left Outer Join0 to 1
	  condition expressions:
	    0 {VALUE._col0}
	    1 {VALUE._col0}
	  handleSkewJoin: false
	  outputColumnNames: _col0, _col4
	  Filter Operator
	    predicate:
		expr: (_col4 > '2')
		type: boolean
	    Select Operator
	      expressions:
		    expr: _col0
		    type: string
		    expr: _col4
		    type: string
	      outputColumnNames: _col0, _col1
	      File Output Operator
		compressed: false
		GlobalTableId: 0
		table:
		    input format: org.apache.hadoop.mapred.TextInputFormat
		    output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
		    serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe

  Stage: Stage-0
    Fetch Operator
      limit: -1
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