Function decode_value 

pub fn decode_value(
    oid: u32,
    data: &[u8],
    ext: &ExtensionOids,
) -> Result<DecodedValue>

Decodes one field’s raw buffer (already length-stripped, matching what postgres_types::FromSql::from_sql receives) into a DecodedValue, given its Postgres type OID. NULL is handled by the caller (a -1 field length never reaches this function) — see decode_record/ decode_array for where that’s checked.