In StarRocks, query rewrite refers to the process of automatically transforming an original user query into an alternative formulation that achieves the same result but potentially performs more efficiently. This transformation can take various forms, such as:
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Exploiting materialized views: StarRocks can rewrite queries on base tables to utilize pre-computed data stored in corresponding materialized views. This significantly reduces redundant calculations and accelerates data access.
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Reordering operations: The optimizer might rearrange the order of operations in the query to take advantage of data locality or specific execution engine capabilities.
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Choosing different join algorithms: For joins, StarRocks might select the most efficient join method based on the data size and distribution.
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Eliminating redundant computations: The optimizer might identify and remove unnecessary parts of the query that don’t affect the final result.
The value of query rewrite lies in its ability to:
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Improve query performance: By making better use of available resources and data, rewritten queries can execute significantly faster than their original counterparts.
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Reduce computational cost: Eliminating redundant calculations and optimizing execution plans lowers the overall workload on the system, leading to improved resource utilization.
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Simplify complex queries: Rewritten queries might be easier to understand and maintain, especially if they leverage materialized views or exploit specific execution engine features.
How to use query rewrite in StarRocks:
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Leverage the hybrid optimizer: By default, StarRocks uses a hybrid optimizer that combines rule-based and cost-based approaches to choose the most efficient execution plan. This includes automatic query rewrite when applicable.
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Monitor query plans: You can use tools like
EXPLAINto analyze the execution plan generated for your query and see if it involves any rewrites. -
Tune materialized views: Carefully designing and maintaining materialized views can significantly improve the effectiveness of query rewrite for specific workloads.
Remember, query rewrite is not a deterministic process, and the resulting plan might differ from the original user query. However, StarRocks aims to ensure that the rewritten query produces the same results while achieving better performance.