Shant Hovsepian created IMPALA-10328:
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Summary: Track Keys During Query Planning
Key: IMPALA-10328
URL: https://issues.apache.org/jira/browse/IMPALA-10328
Project: IMPALA
Issue Type: Improvement
Components: Frontend
Reporter: Shant Hovsepian
A key for a tuple is the set of attributes whose values uniquely identify the
tuple. Any key that can uniquely identify a all tuples in a relation is a
_super key_ the minimal set of attributes that make up a _super key_ is usually
called a candidate key or primary key.
Keys can be used to track functional dependencies, and provide a way to
implement many more optimizations with in the impala planner.
Key information can be explicitly defined or inferred. With Kudu we know which
columns make up a primary key of a table or using constraint support from
[IMPALA-3531|https://issues.apache.org/jira/browse/IMPALA-3531]. Keys can be
inferred after certain operations for example: (the list below is not meant to
be exhaustive)
* All slots from a {{SELECT DISTINCT}} query operation
* The grouping expression slots from the result of a {{GROUP BY}}
* Distinct version of {{UNION/EXCEPT/INTERSECT}}
Armstrong's axioms and a variation of the transitive closure used for value
transfers in the Analyzer can be used to calculate any new a derived keys
through the query plan.
Keys would need to be implemented to support slots from a single tuple, slots
across multiple tuples in the case of joins or unions, and a means to verify if
result expressions from a {{QueryStmt}} form a key.
This functionality would help provide the following possible optimizations
* Remove redundant {{DISTINCT}} aggregation
* Minimize the number of slots that need to be hashed for grouping, shuffle,
and certain join operations
* Reduce redundant shuffles in distributed planning
* More reliable cardinality estimate
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