Function field_column_start 

pub(crate) fn field_column_start(
    metadata: &RegionMetadata,
    num_columns: usize,
) -> usize

Returns the start index of field columns in a flat batch.

num_columns is the total number of columns in the flat batch schema, including tag columns (if present), field columns, and fixed position columns (time index, primary key, sequence, op type).

For Dense encoding (raw PK columns included): field_column_start = primary_key.len() For Sparse encoding (no raw PK columns): field_column_start = 0