Constant SELECT_CHECKPOINT_SQL 

pub const SELECT_CHECKPOINT_SQL: &str = r#"
SELECT
    thread_id,
    checkpoint_ns,
    checkpoint_id,
    parent_checkpoint_id,
    type,
    checkpoint,
    metadata
FROM checkpoints
"#;

Select base columns for a checkpoint row. Channel values and pending writes are fetched separately (SQLite lacks array_agg).