Performance improvement for constant integer stats

[tpoterba]

Problem or motivation

vortex/vortex-compressor/src/stats/integer.rs

Lines 490 to 493 in beb7c15

	for &value in values {
	if count_distinct_values {
	*state.distinct_values.entry(NativeValue(value)).or_insert(0) += 1;
	}

I have a dumb table with 5 u32/u8 fields where 4 are almost always const within a chunk but Vortex still hashes them, and this is a serious hotspot.

Proposed solution

Proposed fixes:

1. reorder the control flow for the compressor to check min/max first to identify const before entering this loop
2. replace with a run-aware version, something like this
3. the best solution to my bespoke problem would be to allow passing in a const array and specializing statistics for that.

#[inline(always)]
fn inner_loop_nonnull<T: IntegerPType>(
    values: &[T; 64],
    count_distinct_values: bool,
    state: &mut LoopState<T>,
) where
    NativeValue<T>: Eq + Hash,
{
    let mut pending_count = 0u32;

    for &value in values {
        if value != state.prev {
            if count_distinct_values && pending_count != 0 {
                *state
                    .distinct_values
                    .entry(NativeValue(state.prev))
                    .or_insert(0) += pending_count;
            }

            state.prev = value;
            state.runs += 1;
            pending_count = 0;
        }

        pending_count += 1;
    }

    if count_distinct_values {
        *state
            .distinct_values
            .entry(NativeValue(state.prev))
            .or_insert(0) += pending_count;
    }
}

Additional context

No response