LakshSingla opened a new pull request, #15559:
URL: https://github.com/apache/druid/pull/15559
### Description
The code in the groupBy engine and the topN engine assume that the
dimensions are comparable and can call `dimA.compareTo(dimB)` to sort the
dimensions and group them together.
This works well for the primitive dimensions, because they are Comparable,
however falls apart when the dimensions can be arrays (or in future scenarios
complex columns). In cases when the dimensions are not comparable, Druid
resorts to having a wrapper type `ComparableStringArray` and `ComparableList`,
which is a Comparable, based on the list comparator.
A cleaner approach would be to assume that each type is associated with a
comparator - for primitives, it would be the natural comparator, and for arrays
it would be the list comparator, and use the comparator to compare the
dimensions for sorting. This code achieves the same, each dimension is
comparable if:
1. We know its type -> We can use the ComparisonUtil to fetch the comparator
associated with the type
2. We know the dimension is an Object in the desired type -> We resort to
coercion using the DimensionHandlerUtils to coerce the dimension (from the
column value selectors) into the necessary type (interchange numerics to their
desired type, convert List to Object[] for arrays, parse strings if permitted
etc). This coercion is necessary to allow rogue selectors (perhaps from
extensions or expressions) to work well with the engine. Druid has been lenient
in what selector's #getObject() can return, because it casts such cases in
multiple places. Till we have strictly typed selectors, this coercion will be
needed. The good thing about refactoring is that it is separate from 1, so once
we feel coercion is not required, we can get rid of it easily.
Since this is a refactor, there's no user-facing impact of the same.
<hr>
##### Key changed/added classes in this PR
<hr>
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This PR has:
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- [ ] using the [concurrency
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- [ ] added documentation for new or modified features or behaviors.
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- [ ] added or updated version, license, or notice information in
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is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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