Yu Xu created CALCITE-7697:
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Summary: Add a rule to remove constant partition keys from Window
Key: CALCITE-7697
URL: https://issues.apache.org/jira/browse/CALCITE-7697
Project: Calcite
Issue Type: Improvement
Components: core
Affects Versions: 1.42.0
Reporter: Yu Xu
Assignee: Yu Xu
When a window function's PARTITION BY clause references a key that is constant,
every row falls into the same partition. The partition key therefore has no
effect on the result but still forces the planner (and downstream engines) to
perform an unnecessary partitioning/shuffle operation. Calcite currently has no
rule to detect and remove such redundant partition keys.
A partition key can be constant in two common ways:
1. *Literal partition key* — the SQL directly partitions by a constant, e.g.
{code:java}
SELECT empno, ROW_NUMBER() OVER (PARTITION BY 1 ORDER BY sal) FROM emp {code}
2. *Predicate-implied constant* — an upstream predicate proves the column is
constant, e.g.
{code:java}
SELECT empno, SUM(sal) OVER (PARTITION BY deptno ORDER BY sal) FROM emp WHERE
deptno = 10
{code}
2. Here deptno is constant for all surviving rows, so PARTITION BY deptno is
redundant.
The second case is the more valuable one in practice: hand-written PARTITION
BY <literal> is rare, but "filter on an equality column, then partition by that
column" arises frequently through views, nested subqueries, and predicate
push-down.
we maybe can add a new rule to implement it.
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