Function writer_props 

fn writer_props(schema: &Schema) -> WriterProperties

Writer properties shared by the one-shot and incremental writers: LZ4, and dictionary encoding disabled for numeric columns (high-entropy vertex data gains nothing from a dictionary while paying significant overhead).

rot0..rot8 are the exception, and the reason is the justification above read backwards. They are the LOWEST-entropy columns in the schema: identity on every row of an identity-plan payload (every streamed batch, and every v6 model with nothing to share), and one of a handful of distinct values on a shared one. Dictionary plus RLE is exactly what that shape is for, where the numeric opt-out would write nine plain f32 per row – 3.6 MB per 100k rows handed to the compressor for a column with one value in it.