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## Package org.apache.hadoop.mapred.join

Given a set of sorted datasets keyed with the same class and yielding equal partitions, it is possible to effect a join of those datasets prior to the map.

See:
Description

Interface Summary
ComposableInputFormat<K extends WritableComparable,V extends Writable> | Refinement of InputFormat requiring implementors to provide ComposableRecordReader instead of RecordReader.
ComposableRecordReader<K extends WritableComparable,V extends Writable> | Additional operations required of a RecordReader to participate in a join.
ResetableIterator<T extends Writable> | This defines an interface to a stateful Iterator that can replay elements added to it directly.

Class Summary
ArrayListBackedIterator<X extends Writable> | This class provides an implementation of ResetableIterator.
CompositeInputFormat<K extends WritableComparable> | An InputFormat capable of performing joins over a set of data sources sorted and partitioned the same way.
CompositeInputSplit | This InputSplit contains a set of child InputSplits.
CompositeRecordReader<K extends WritableComparable,V extends Writable,X extends Writable> | A RecordReader that can effect joins of RecordReaders sharing a common key type and partitioning.
InnerJoinRecordReader<K extends WritableComparable> | Full inner join.
JoinRecordReader<K extends WritableComparable> | Base class for Composite joins returning Tuples of arbitrary Writables.
MultiFilterRecordReader<K extends WritableComparable,V extends Writable> | Base class for Composite join returning values derived from multiple sources, but generally not tuples.
OuterJoinRecordReader<K extends WritableComparable> | Full outer join.
OverrideRecordReader<K extends WritableComparable,V extends Writable> | Prefer the "rightmost" data source for this key.
Parser | Very simple shift-reduce parser for join expressions.
Parser.Node |
Parser.NodeToken |
Parser.NumToken |
Parser.StrToken |
Parser.Token | Tagged-union type for tokens from the join expression.
ResetableIterator.EMPTY<U extends Writable> |
StreamBackedIterator<X extends Writable> | This class provides an implementation of ResetableIterator.
TupleWritable | Writable type storing multiple `Writable`s.
WrappedRecordReader<K extends WritableComparable,U extends Writable> | Proxy class for a RecordReader participating in the join framework.

Enum Summary
Parser.TType |

## Package org.apache.hadoop.mapred.join Description

Given a set of sorted datasets keyed with the same class and yielding equal partitions, it is possible to effect a join of those datasets prior to the map. This could save costs in re-partitioning, sorting, shuffling, and writing out data required in the general case.

### Interface

The attached code offers the following interface to users of these classes.

property | required | value
--- | --- | ---
mapred.join.expr | yes | Join expression to effect over input data
mapred.join.keycomparator | no | `WritableComparator` class to use for comparing keys
mapred.join.define.<ident> | no | Class mapped to identifier in join expression

The join expression understands the following grammar:

    func ::= <ident>([<func>,]*<func>)
    func ::= tbl(<class>,"<path>");

Operations included in this patch are partitioned into one of two types: join operations emitting tuples and "multi-filter" operations emitting a single value from (but not necessarily included in) a set of input values. For a given key, each operation will consider the cross product of all values for all sources at that node.

Identifiers supported by default:

identifier | type | description
--- | --- | ---
inner | Join | Full inner join
outer | Join | Full outer join
override | MultiFilter | For a given key, prefer values from the rightmost source

A user of this class must set the `InputFormat` for the job to `CompositeInputFormat` and define a join expression accepted by the preceding grammar. For example, both of the following are acceptable:

    inner(tbl(org.apache.hadoop.mapred.SequenceFileInputFormat.class,
              "hdfs://host:8020/foo/bar"),
          tbl(org.apache.hadoop.mapred.SequenceFileInputFormat.class,
              "hdfs://host:8020/foo/baz"))

    outer(override(tbl(org.apache.hadoop.mapred.SequenceFileInputFormat.class,
                       "hdfs://host:8020/foo/bar"),
                   tbl(org.apache.hadoop.mapred.SequenceFileInputFormat.class,
                       "hdfs://host:8020/foo/baz")),
          tbl(org.apache.hadoop.mapred/SequenceFileInputFormat.class,
              "hdfs://host:8020/foo/rab"))

`CompositeInputFormat` includes a handful of convenience methods to aid construction of these verbose statements.

As in the second example, joins may be nested. Users may provide a comparator class in the `mapred.join.keycomparator` property to specify the ordering of their keys, or accept the default comparator as returned by `WritableComparator.get(keyclass)`.

Users can specify their own join operations, typically by overriding `JoinRecordReader` or `MultiFilterRecordReader` and mapping that class to an identifier in the join expression using the `mapred.join.define.ident` property, where ident is the identifier appearing in the join expression. Users may elect to emit- or modify- values passing through their join operation. Consulting the existing operations for guidance is recommended. Adding arguments is considerably more complex (and only partially supported), as one must also add a `Node` type to the parse tree. One is probably better off extending `RecordReader` in most cases.

JIRA

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