A query plan is the set of operations Greenplum Database will perform to produce the answer to a query. Each node or step in the plan represents a database operation such as a table scan, join, aggregation, or sort. Plans are read and executed from bottom to top.
In addition to common database operations such as tables scans, joins, and so on, Greenplum Database has an additional operation type called motion. A motion operation involves moving tuples between the segments during query processing. Note that not every query requires a motion. For example, a targeted query plan does not require data to move across the interconnect.
To achieve maximum parallelism during query execution, Greenplum divides the work of the query plan into slices. A slice is a portion of the plan that segments can work on independently. A query plan is sliced wherever a motion operation occurs in the plan, with one slice on each side of the motion.
For example, consider the following simple query involving a join between two tables:
SELECT customer, amount FROM sales JOIN customer USING (cust_id) WHERE dateCol = '04-30-2016';
Figure 3 shows the query plan. Each segment receives a copy of the query plan and works on it in parallel.
The query plan for this example has a redistribute motion that moves tuples between the segments to complete the join. The redistribute motion is necessary because the customer table is distributed across the segments by cust_id, but the sales table is distributed across the segments by sale_id. To perform the join, the sales tuples must be redistributed by cust_id. The plan is sliced on either side of the redistribute motion, creating slice 1 and slice 2.
This query plan has another type of motion operation called a gather motion. A gather motion is when the segments send results back up to the master for presentation to the client. Because a query plan is always sliced wherever a motion occurs, this plan also has an implicit slice at the very top of the plan (slice 3). Not all query plans involve a gather motion. For example, a CREATE TABLE x AS SELECT... statement would not have a gather motion because tuples are sent to the newly created table, not to the master.
