zhuqi-lucas opened a new pull request, #24063:
URL: https://github.com/apache/datafusion/pull/24063
## Rationale for this change
`EquivalenceProperties::project` is a central, widely-called method, but its
doc comment was a single line ("Projects the equivalences within according to
`mapping` and `output_schema`"). That does not convey that it carries
orderings, the equivalence group, and constraints through the mapping, nor the
key subtlety that an ordering is preserved only through an order-preserving
expression (`SortProperties`). This tripped me up while reasoning about why an
ordering survived a projection containing computed expressions, so I expanded
the docs.
## What changes are included in this PR?
Documentation only. Expands the rustdoc on `EquivalenceProperties::project`
to describe:
- it carries orderings, the equivalence group, and constraints through the
mapping (not just a column-index remap);
- orderings survive only through order-preserving expressions, determined
from each expression's `SortProperties` (e.g. preserved through `c + 1`,
dropped through `abs(c)`; compound orderings like `a + b` map to `a_new +
b_new`);
- expressions/orderings not representable in `output_schema` are dropped.
## Are these changes tested?
No code change. `cargo doc -p datafusion-physical-expr --no-deps` builds
cleanly with `RUSTDOCFLAGS="-D warnings"`.
## Are there any user-facing changes?
Documentation only.
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