Function decode_primary_key 

fn decode_primary_key(
    primary_key: &[u8],
    min_ts: i64,
    max_ts: i64,
    row_count: u64,
    tag_columns: &[(ColumnId, String)],
    offsets: &mut SparseOffsetsCache,
    buf: &mut Vec<u8>,
) -> Result<SeriesIndexRow>

Decodes one sparse primary key into a series-index row by extracting only the reserved ids and the tag columns the index needs, instead of decoding every label into owned values.