Sampling somehow compresses into 0 bytes

[connortsui20]

We have this code in our sampling compressor (which has existed for a long time, even before the most recent changes):

let after = scheme
    .compress(compressor, &mut sample_data, sample_ctx)?
    .nbytes();
let before = sample_data.array().nbytes();

let ratio = before as f64 / after as f64;

tracing::debug!("estimate_compression_ratio_with_sampling(compressor={scheme:#?}) = {ratio}",);

Ok(ratio)

If I add some checks:

let mut sample_data = ArrayAndStats::new(sample_array, scheme.stats_options());
let cascade_history = ctx.cascade_history().to_vec();
let sample_ctx = ctx.with_sampling();

let before = sample_data.array().nbytes();
let after = scheme
    .compress(compressor, &mut sample_data, sample_ctx)?
    .nbytes();

if after == 0 {
    tracing::warn!(
        scheme = %scheme.id(),
        ?cascade_history,
        "sample compressed to 0 bytes, which should only happen for constant arrays",
    );
}

we get hundreds of warnings saying that bitpacking compresses to 0 bytes. This also means that the ratio ends up being infinity, which we interpret as invalid.

Here are some examples. These lists are in order of descent, so the first in the list is the parent scheme.

WARN vortex_compressor::estimate: vortex-compressor/src/estimate.rs:129: sample compressed to 0 bytes, which should only happen for constant arrays scheme=vortex.int.bitpacking
cascade_history=[
(SchemeId { name: "vortex.string.dict" }, 0),
(SchemeId { name: "vortex.string.fsst" }, 0),
(SchemeId { name: "vortex.int.dict" }, 1)]

utf8 encoded as dict with values as fsst, fsst lengths as dict, and the codes as bitpacked

WARN vortex_compressor::estimate: vortex-compressor/src/estimate.rs:129: sample compressed to 0 bytes, which should only happen for constant arrays scheme=vortex.int.bitpacking
cascade_history=[
(SchemeId { name: "vortex.string.fsst" }, 0),
(SchemeId { name: "vortex.int.rle" }, 1)]

utf8 encoded as fsst with lengths as rle, and then rle indices as bitpacking

[connortsui20]

output.zip

This is all of the warnings I get when generating TPC-H SF 10 partition files

[robert3005]

I wonder if you get an all null samples? Sequence and bitpacking can also be 0 bytes but a lot harder for bitpacking

[connortsui20]

That shouldn't happen since we check for all null and empty earlier in the compressor.

But maybe it is possible when we sample and we happen to choose a bunch of slices that are all null?

Will have to look into this more

[connortsui20]

I (am merging) this into develop commented out via #7230:

    // TODO(connor): Issue https://github.com/vortex-data/vortex/issues/7268.
    // if after == 0 {
    //     tracing::warn!(
    //         scheme = %scheme.id(),
    //         "sample compressed to 0 bytes, which should only happen for constant arrays",
    //     );
    // }