adriangb opened a new pull request, #25675:
URL: https://github.com/apache/datafusion/pull/25675
## Which issue does this PR close?
- Closes #25669.
## Rationale for this change
`PREPARE p AS SELECT $1 AS a UNION ALL SELECT $2 AS a` fails with `Schema
error: No field named a.` The same happens when a caller optimizes such a plan
before binding its parameters.
`Expr::get_type` had a special case for an alias of an untyped placeholder:
it looked up the alias name as a column in the schema it receives. The `UNION`
coercion code calls `get_type` on each projection expression with the
projection's input schema, which does not contain the alias, so the lookup
fails. `Expr::to_field` has no such special case and returns a `Null` field for
the same expression.
The special case came from #4701. At that time `get_type` returned an error
for a bare untyped placeholder. Today a bare untyped placeholder has type
`Null`, so the special case is no longer needed.
## What changes are included in this PR?
- Remove the `Alias(Placeholder { field: None })` special case from
`Expr::get_type`. An alias now has the type of the expression it wraps, the
same as in `to_field`.
## What is the testing strategy for this PR?
Tests added:
- sqllogictest cases in `prepare.slt` for `PREPARE` and `EXECUTE` of a
`UNION ALL` of aliased placeholders, including a CTE joined to a table.
- A test in `core/tests/sql/select.rs` that optimizes the plan with unbound
placeholders, then binds and executes it.
- A unit test in `expr_schema.rs` that `get_type` and `to_field` agree for
an aliased untyped placeholder.
The existing prepared statement and placeholder type inference tests pass
without changes.
## Are there any user-facing changes?
The queries above now plan and run. No API changes.
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