refactor: parquet pruning simplifications

[crepererum]

Which issue does this PR close?

Prep work for #4695.

Rationale for this change

Makes the actual change in #4695 easier.

What changes are included in this PR?

Internal improvements.

Are these changes tested?

Existing tests pass, no function changes.

Are there any user-facing changes?

-

[crepererum]

I am pretty sure that this test breakage is unrelated, seems flaky to me:

[window.slt] Running query: "select 1 - lag(amount, 1) over (order by idx) as column1 from (values ('a', 1, 100), ('a', 2, 150)) as t (col1, idx, amount)
---"
Error: query result mismatch:
[SQL] select 1 - lag(amount, 1) over (order by idx) as column1 from (values ('a', 1, 100), ('a', 2, 150)) as t (col1, idx, amount)
---
[Diff] (-expected|+actual)
-   NULL
-   -99
+   -99
+   NULL
at tests/sqllogictests/test_files/window.slt:414

error: test failed, to rerun pass `-p datafusion --test sqllogictests`

[waynexia]

Nice simplification, LGTM 👍

[avantgardnerio]

More descriptive variable names would help readability here.

[avantgardnerio]

Oh, so this was always returning Ok?

[crepererum]

Yes

[avantgardnerio]

This is a behavior change for n_columns() == 0. Based on:

    pub fn allways_true(&self) -> bool {
        self.predicate_expr
            .as_any()
            .downcast_ref::<Literal>()
            .map(|l| matches!(l.value(), ScalarValue::Boolean(Some(true))))
            .unwrap_or_default()
    }

I ran the test suite, panicing if n_columns() == 0 and I can't get it to happen, so I guess it LGTM.
I assume that would default to false, in which case I think we'd want to return a None here?

[crepererum]

Yeah, we skip the predicate if we don't refer to any column. However you might be right (at least this is how I read your comment) that we need additional test coverage for a "constant" predicate (i.e. one that doesn't reference any column). I'll check next week if such a test exists and if not, add one.

[ursabot]

Benchmark runs are scheduled for baseline = 7224901 and contender = 1841736. 1841736 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java