How Distributed SQL Works
When an SQL statement is submitted for execution, the SQL service parses and optimizes it using Apache Calcite. The result is an executable plan that is cached and reused by subsequent executions of the same statement.
The plan contains a tree of query fragments. A query fragment is a tree of operators that could be executed on a single member independently. Child fragments supply data to parent fragments, possibly over a network, until the root fragment is reached. The root fragment returns the query results.