Function update_cell 

pub async fn update_cell(
    client: &Client,
    schema: &str,
    table: &str,
    column: &str,
    column_type: &str,
    new_value: Option<&str>,
    ctid: &str,
) -> Result<UpdateOutcome, PgError>

Issue a single-cell UPDATE. new_value: None means SET NULL. column_type is the column’s pg_type.typname (e.g. int4, timestamptz); used for the server-side text→typed cast.