fix: Preserve quoted mixed-case identifiers in the pivot_unpivot example

[niebayes]

Summary

This PR fixes a bug in the datafusion-examples/examples/relation_planner/pivot_unpivot.rs
example implementation.

The example planner rewrites PIVOT into a GROUP BY plus CASE expressions. During that
rewrite, it rebuilt column references using unquoted col("...") expressions. That loses the
original identifier quoting and case-sensitivity, which breaks queries that pivot on quoted
mixed-case columns such as "pointNumber".

This PR preserves the original parsed column expression instead of reconstructing it from a plain
string.

Problem

Consider a query like:

SELECT *
FROM point_stats
PIVOT (
  MAX(max_value)
  FOR "pointNumber" IN ('16951' AS p16951, '16952' AS p16952)
)
ORDER BY ts

Before this change, the example planner:

1. Parsed "pointNumber" correctly as a quoted, case-sensitive identifier.
2. Extracted its name as pointNumber.
3. Reconstructed new expressions with col(&pivot_col_name) and col(...) for GROUP BY.

That reconstruction treated the identifier as an unquoted column reference, which could be
normalized differently from the original schema field. In practice, this means the planner could
end up looking for pointnumber while the schema still contained "pointNumber".

Root Cause

Two places in the example rewrite logic rebuilt column references from bare strings:

1. The generated CASE expression for the pivot column
2. The inferred GROUP BY expressions

That is fine for simple lowercase identifiers, but it is not correct for quoted identifiers or
qualified fields because the reconstructed expression no longer carries the original identifier
semantics.

Fix

The fix is minimal:

1. Reuse the already planned pivot_col expression when building the CASE expression.
2. Build GROUP BY expressions directly from the input schema via Expr::from(...) rather than
   re-creating them with col(field_name).

This preserves:

• quoted mixed-case identifiers
• qualifiers
• original field resolution semantics

Code Changes

File changed:

• datafusion-examples/examples/relation_planner/pivot_unpivot.rs

Main changes:

• Replace:

case(col(&pivot_col_name))

with:

case(pivot_col.clone())

• Replace string-based GROUP BY reconstruction:

schema
    .fields()
    .iter()
    .map(|f| f.name().as_str())
    ...
    .map(col)

with schema-derived expressions:

schema
    .iter()
    .filter(...)
    .map(Expr::from)

Example Coverage

This PR also adds an additional example scenario to the same file:

• a point_stats input table
• a PIVOT query using quoted mixed-case column "pointNumber"
• a snapshot asserting the expected output

This makes the bug and the fix visible directly in the example itself.

[alamb]

Thanks @niebayes -- I had just one question about one of the changes, but I don't think it blocks this PR

[alamb]

is this change necessary? It seems like it is more verbose but does the same thing

[alamb]

this is the real fix, right?

[alamb]

Thanks again @niebayes