Code Review

Summary: This PR adds logic to prefer bitpacking over RLE when bitpacking produces smaller output. The approach is sound and the test coverage is good.

P1 Issue: Estimation formula may underestimate bitpacking size

In estimate_inline_bitpacking_bytes, the calculation appears to assume all chunks are full 1024-element chunks. The words_per_chunk is hardcoded to 1 (for the bit-width header), but the actual implementation in InlineBitpacking::bitpack_chunked stores the bit-width as a single element of type T (e.g., 8 bytes for u64), not 1 byte.

Looking at compression.rs:241-247:

let words_per_chunk: u128 = 1;
let word_bytes: u128 = (bits / 8) as u128;
// ...
let packed_words = (1024u128 * bit_width) / (bits as u128);
total_words = total_words.saturating_add(words_per_chunk.saturating_add(packed_words));

This correctly accounts for the header as 1 word (element) per chunk plus the packed data words. However, the comparison should be bitpack_bytes < rle_bytes not bitpack_bytes < rle_bytes returning None for RLE. The current logic is:

• If bitpacking is smaller than RLE, skip RLE (return None)
• Then bitpacking will be tried separately

This seems correct, but I'd suggest adding a brief comment in try_rle_for_mini_block explaining that we're checking if bitpacking would be better to avoid selecting RLE when it's not optimal.

Suggestion (not blocking)

Consider adding a comment in the RLE function explaining the fallback to bitpacking check:

// If bitpacking would produce smaller output than RLE, skip RLE
// and let the subsequent bitpacking check handle compression.

Test coverage

The new test test_low_cardinality_prefers_bitpacking_over_rle is well-designed and validates the behavior. The modification to test_rle_encoding_verification to use i32::MIN values ensures RLE is still tested when bitpacking can't help (high bit set means bit-width equals type width).

LGTM with the optional suggestion above.