Number of input and output link

Number of input link	:	One
Number of output link	:	One

Pre-Join Assembly () is used to collect multiple values that share the same key into a single list before a join operation is performed.

For each unique key, the assembly groups all corresponding values and produces one output record in the following structure:

   [key, list(values)] 

Purpose of Pre-Join Assembly

The Pre-Join Assembly is typically used to:

• Prepare data for join operations
• Convert multiple records with the same key into a single composite record
• Improve efficiency in downstream joins by pre-grouping related values

How It Works

• Incoming records are read one by one.
• Records with the same key are grouped together.
• All values associated with that key are collected into a list.
• A new output record is emitted containing:
  • The key
  • A list of all values belonging to that key

Example

Input Records

Key	Value
A	10
A	20
A	30
B	5
B	15

Output Records

Key	Values
A	[10, 20, 30]
B	[5, 15]

Each key appears only once in the output, with all its values aggregated into a list.

Supported Engines

Map Reduce	Spark 1.x RDD	Spark 2.x RDD	Spark 2.x Dataset	Apache Beam 2.29.0 Batch	Apache Beam 2.29.0 Streaming	Apache Flink 1.12.0 Batch	Apache Flink 1.12.0 Streaming	Apache Tez 0.9.1	PySpark