graphitesql 0.0.5

A pure, safe, no_std Rust re-implementation of SQLite, compatible with the SQLite 3 file format.
Documentation 
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//! `SELECT count(*)` answered via a covering secondary index (roadmap B2b).
//!
//! When a single rowid table has exactly one full secondary index, sqlite (and
//! graphitesql) counts that index's entries — `EXPLAIN QUERY PLAN` reports
//! `SCAN t USING COVERING INDEX <name>`. With zero or multiple such indexes,
//! graphitesql conservatively keeps the plain `SCAN t` plan (no guessing).

#![cfg(feature = "std")]

use graphitesql::{Connection, Value};

fn detail(conn: &Connection, sql: &str) -> Vec<String> {
    let r = conn.query(sql).unwrap();
    assert_eq!(r.columns.len(), 4);
    r.rows
        .iter()
        .map(|row| match &row[3] {
            Value::Text(s) => s.clone(),
            other => panic!("detail not text: {other:?}"),
        })
        .collect()
}

fn count(conn: &Connection, sql: &str) -> i64 {
    let r = conn.query(sql).unwrap();
    assert_eq!(r.rows.len(), 1);
    assert_eq!(r.rows[0].len(), 1);
    match &r.rows[0][0] {
        Value::Integer(n) => *n,
        other => panic!("count not integer: {other:?}"),
    }
}

#[test]
fn one_index_uses_covering_index_in_eqp() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    c.execute("INSERT INTO t VALUES(1,10),(2,20)").unwrap();
    assert_eq!(
        detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t"),
        ["SCAN t USING COVERING INDEX ib"]
    );
}

#[test]
fn one_index_count_is_correct() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    c.execute("INSERT INTO t VALUES(1,10),(2,20),(3,30)")
        .unwrap();
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 3);
}

#[test]
fn no_index_keeps_plain_scan() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("INSERT INTO t VALUES(1,10),(2,20)").unwrap();
    assert_eq!(
        detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t"),
        ["SCAN t"]
    );
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 2);
}

#[test]
fn multiple_indexes_fall_back_to_plain_scan() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b, c)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    c.execute("CREATE INDEX ic ON t(c)").unwrap();
    c.execute("INSERT INTO t VALUES(1,10,100),(2,20,200)")
        .unwrap();
    // Ambiguous index choice => keep the plain SCAN (no guessing).
    assert_eq!(
        detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t"),
        ["SCAN t"]
    );
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 2);
}

#[test]
fn count_correct_after_delete() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    c.execute("INSERT INTO t VALUES(1,10),(2,20),(3,30),(4,40)")
        .unwrap();
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 4);
    c.execute("DELETE FROM t WHERE a IN (2,3)").unwrap();
    // Still uses the covering index, and the count reflects the deletes.
    assert_eq!(
        detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t"),
        ["SCAN t USING COVERING INDEX ib"]
    );
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 2);
}

#[test]
fn empty_table_counts_zero() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 0);
}

#[test]
fn partial_index_does_not_cover_count() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b) WHERE b > 5").unwrap();
    c.execute("INSERT INTO t VALUES(1,1),(2,10)").unwrap();
    // A partial index does not have one entry per row => plain SCAN, full count.
    assert_eq!(
        detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t"),
        ["SCAN t"]
    );
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 2);
}

#[test]
fn without_rowid_table_uses_plain_scan() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a TEXT PRIMARY KEY, b) WITHOUT ROWID")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    c.execute("INSERT INTO t VALUES('x',1),('y',2)").unwrap();
    assert_eq!(
        detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t"),
        ["SCAN t"]
    );
    assert_eq!(count(&c, "SELECT count(*) FROM t"), 2);
}

#[test]
fn count_with_where_is_not_index_covered() {
    let mut c = Connection::open_memory().unwrap();
    c.execute("CREATE TABLE t(a INTEGER PRIMARY KEY, b)")
        .unwrap();
    c.execute("CREATE INDEX ib ON t(b)").unwrap();
    c.execute("INSERT INTO t VALUES(1,10),(2,20),(3,10)")
        .unwrap();
    // A WHERE clause excludes the count-covering fast path.
    let plan = detail(&c, "EXPLAIN QUERY PLAN SELECT count(*) FROM t WHERE b = 10");
    assert_ne!(plan, ["SCAN t USING COVERING INDEX ib"]);
    assert_eq!(count(&c, "SELECT count(*) FROM t WHERE b = 10"), 2);
}