How is the execution plan in Impala generated?

The generation process of Impala execution plans is as follows:

1. Query Parsing: The SQL query submitted by the user is first parsed by the query parser, which converts it into an Abstract Syntax Tree (AST) representation.
2. Query optimization: After parsing, the query will go through a series of optimization steps, such as syntax validation, semantic analysis, query rewriting, and predicate pushdown, to improve the query’s execution efficiency.
3. Execution Plan Generation: After optimization, Impala will generate an execution plan based on the optimized query plan. The execution plan is a tree structure that contains specific execution steps and the order of execution for the query, guiding its actual execution.
4. Plan optimization: The generated execution plan undergoes a series of optimization steps, including load balancing, query rearrangement, data localization, and more, in order to further enhance the efficiency of query execution.
5. The execution plan will be executed by the Impala execution engine, following the specified steps and order in the plan, to perform the query operations and return the results to the user.

Overall, the process of generating Impala execution plans involves steps like query parsing, query optimization, execution plan generation, plan optimization, and plan execution to ensure that queries run in the most efficient manner.