perf: don't re-inline CSE'd expensive expressions in projection pushdown

[fordN]

Which issue does this PR close?

• ClosesPreserve CSE for expensive expressions when pushing projections into file scans #23425.

Follow-up to #23395 (the volatile correctness fix, now merged), extending the same projection-pushdown guard to the performance case.

Rationale for this change

The logical CSE pass extracts a repeated scalar-function call (e.g. power(a, 2)) into a single intermediate projection referenced by column, so it is evaluated once per row. For file sources that can absorb computed projections (e.g. Parquet), the physical projection-pushdown rule then merges that projection into the scan's DataSourceExec, re-inlining the expression at every reference site and re-evaluating it N times per row — undoing the deduplication.

This is the performance sibling of #23220 (which fixed the correctness case for volatile expressions). It reproduces on file scans (Parquet/CSV), not in-memory tables.

EXPLAIN, before — power(a, 2) evaluated 3× per row:

DataSourceExec: projection=[power(a,2)+b as x, power(a,2)-b as y, power(a,2)*c as z]

EXPLAIN, after — power(a, 2) evaluated once, kept in a ProjectionExec:

ProjectionExec: [__common_expr_1+b as x, __common_expr_1-b as y, __common_expr_1*c as z]
DataSourceExec: projection=[power(a,2) as __common_expr_1, b, c]

What changes are included in this PR?

• Extends the projection-pushdown guard in FileScanConfig::try_swapping_with_projection so it also declines the merge when it would duplicate a non-trivial expression. "Non-trivial" is decided by the existing expression-placement signal (KeepInPlace) — the same signal try_collapse_projection_chain uses — covering arithmetic, casts, and most scalar functions. Cheap leaf-pushable expressions (columns, get_field, input_file_name) still merge, so struct-field pushdown is unaffected. It reuses the volatile guard's multiplicity-aware reference counting, so an expression is blocked only when referenced more than once.
• Adds the cse_projection_pushdown benchmark used to measure the change.

Are these changes tested?

Yes:

• Unit tests in file_scan_config.rs: a non-trivial expr (arithmetic / scalar function) referenced ≥2 → blocked, once → allowed; a leaf-pushable scalar function (get_field) → allowed; volatile referenced ≥2 → blocked, once → allowed.
• datafusion-sqllogictest suite passes. One existing plan in window.slt improves: a repeated c2 >= 2 comparison over a CSV scan is now computed once instead of being inlined twice into the DataSourceExec.
• Benchmark A/B on cse_projection_pushdown (sample-size 50, 5s), with the no_repeated_exprs control confirming no change on unaffected queries:

Benchmark	Change
repeated_power (power(a,2) ×3)	−40%
repeated_nested_fn (ln(abs(a)) ×3)	−38%
repeated_sqrt (sqrt(a) ×3)	−37%
mixed_repeated_unique	−35%
repeated_cheap_abs (abs(a) ×3)	within noise
no_repeated_exprs (control)	no change

The gain scales with expression cost. For a single-instruction function like abs, caching the value in a ProjectionExec versus recomputing it is a wash (run-to-run noise), since the extra plan node roughly offsets the recomputation it saves.

Reproduce with:

cargo bench -p datafusion --bench cse_projection_pushdown --features parquet

Are there any user-facing changes?

No API changes. Queries that repeat an expensive expression over a file scan run faster; their physical plans retain a ProjectionExec above the scan (the expression evaluated once) instead of inlining it into the DataSourceExec projection.

[fordN]

@xudong963 here's a follow up to #23395 that expands the set of expressions to not be "re-inline'd" to include performance considerations along with the correctness considerations.

In practice, this change has shown to have a significant effect on the performance of queries where outer statement references expensive computation results from inner.

[xudong963]

How about adding a small projection_pushdown.slt EXPLAIN test asserting:

ProjectionExec: ... __common_expr ...
DataSourceExec: projection=[power(a, 2) as __common_expr, ...]

[codecov-commenter]

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[xudong963]

Thanks again. I'll merge tomorrow this if no new comments