Mihai Budiu created CALCITE-7608:
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Summary: Introduce a LogicalSelectMany operator
Key: CALCITE-7608
URL: https://issues.apache.org/jira/browse/CALCITE-7608
Project: Calcite
Issue Type: Improvement
Components: core
Affects Versions: 1.42.0
Reporter: Mihai Budiu
Assignee: Mihai Budiu
Today UNNEST is implemented using the Uncollect operator. We propose adding an
alternative LogicalSelectMany operator, which generalizes Uncollect. (Notice
that Enumerable API already has a SelectMany.) The main difference between
Uncollect and SelectMany is that Uncollect unnests all the fields of its input
relation, whereas LogicalSelectMany would only unnest SOME of the fields of the
input collection, preserving the other ones in each output row.
This distinction is very important, because:
* LogicalSelectMany can be directly and efficiently implemented using the
Enumerable SelectMany
* UNNEST used in a cross-join is implemented using an Uncollect and a
LogicalCorrelate. However, the same UNNEST can be represented using just one
LogicalSelectMany node
* Neither the old nor the new decorrelator can actually eliminate
LogicalCorrelate nodes that are paired with Uncollect. Using LogicalSelectMany
we can decorrelate many more plans.
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