Optimize `required_columns` from `BTreeSet<usize>` to `Vec<usize>` in struct PushdownChecker<'schema>

[kosiew]

In row_filter.rs, the required_columns field in both PushdownChecker and PushdownColumns structs uses BTreeSet<usize> to track column indices required for filter evaluation.

Current implementation:

struct PushdownChecker<'schema> {
    required_columns: BTreeSet<usize>,
    // ... other fields
}

struct PushdownColumns {
    required_columns: BTreeSet<usize>,
    nested: NestedColumnSupport,
}

Motivation

For typical filter predicates, the number of columns referenced is usually small (1-5 columns). Using BTreeSet<usize> adds overhead:

• Memory overhead from tree structure
• Insertion cost is O(log n) vs O(1) for append to Vec
• The only operation that benefits from BTreeSet is deduplication, but for small sets, a simple Vec with linear scan would be faster

Proposed Solution

Replace BTreeSet<usize> with Vec<usize> and handle deduplication explicitly if needed

source: #19545 (comment)