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In Greenplum, the notation "Redistribute Motion 4:4" in a query plan indicates the following:

• 4:4: The first number (4) represents the number of segments sending data, and the second number (4) represents the number of segments receiving data. In this case, it means that data is being redistributed across all 4 segments to all 4 segments.

Detailed Explanation

1. Redistribute Motion:

   • This operator is used when data needs to be redistributed across segments based on a specific key. In your example, the sales table is being redistributed by cust_id to ensure that rows with the same cust_id are on the same segment for efficient join operations.

2. 4:4 Notation:

   • The notation "4:4" indicates that there are 4 segments involved in both sending and receiving data. Each segment sends data to each of the other segments based on the hash value of the cust_id column.

3. Execution Steps:

   • Hash Calculation: Each segment calculates the hash value of the cust_id for each row in the sales table.

   • Data Transfer: Rows are sent to the appropriate segments based on the calculated hash values. This ensures that related rows are on the same segment for the join operation.

4. Local Join:

   • After redistribution, each segment performs a local join between the redistributed sales data and its local customer data. This ensures that the join operation is performed efficiently without the need for further data movement.

Example Query Plan

Here’s an example of what the query plan might look like for the given query:

Gather Motion 4:1  (slice1; segments: 4)->  Hash JoinHash Cond: (s.cust_id = c.cust_id)->  Redistribute Motion 4:4  (slice2; segments: 4)Hash Key: s.cust_id->  Seq Scan on sales s->  Seq Scan on customer c

Conclusion

The "Redistribute Motion 4:4" notation in the query plan indicates that data is being redistributed across all 4 segments to all 4 segments based on the cust_id column. This ensures that related rows are on the same segment for efficient join operations.