// IXSCAN filter in the explain with the pushdown on: { "w" : { "$_internalExprLt" : null } }
/**
 * A {$expr: {$lt: [<field>, null]}} filter pushed into a full index scan that exists only to
 * provide a sort drops documents where the field is missing: the index key for a missing field is
 * null and null < null is false, while in aggregation expressions missing sorts below null.
 * The result is the same wrong answer in the classic and SBE engines.
 */
const conn = MongoRunner.runMongod({setParameter: {internalQueryDisablePlanCache: true}});
const db = conn.getDB(jsTestName());
assert.commandWorked(db.c.insertMany([
    {_id: 1, c: 1},
    {_id: 2, c: 2, w: null},
    {_id: 3, c: 3, w: 1},
    {_id: 4, c: 4, w: "x"},
    {_id: 5, c: 5},
]));
assert.commandWorked(db.c.createIndex({c: 1, w: 1}));

const pipeline = [{$match: {$expr: {$lt: ["$w", null]}}}, {$sort: {c: 1}}, {$project: {_id: 1}}];
const ids = (docs) => docs.map((d) => d._id);
function ixscanFilters(explain) {
    const found = [];
    (function walk(x) {
        if (Array.isArray(x)) return x.forEach(walk);
        if (x === null || typeof x !== "object") return;
        if (x.stage === "IXSCAN") found.push(x.filter);
        Object.values(x).forEach(walk);
    })(explain);
    return found;
}
function run(label, engine, pushdown, hint) {
    assert.commandWorked(db.adminCommand({
        setParameter: 1,
        internalQueryFrameworkControl: engine,
        internalQueryPlannerPushdownFilterToIxscanForSort: pushdown,
    }));
    const opts = hint ? {hint} : {};
    const filters = ixscanFilters(db.c.explain().aggregate(pipeline, opts));
    const got = ids(db.c.aggregate(pipeline, opts).toArray());
    jsTest.log.info("run", {label, engine, pushdown, hint, ixscanFilters: filters, got});
    return {got, filters};
}

// Controls: missing < null is true, so documents 1 and 5 match, when the filter is not pushed
// into the index scan or when the query scans the collection.
assert.eq(run("pushdown off", "forceClassicEngine", false).got, [1, 5], "pushdown off, classic");
assert.eq(run("pushdown off", "trySbeEngine", false).got, [1, 5], "pushdown off, SBE");
assert.eq(run("collection scan", "forceClassicEngine", true, {$natural: 1}).got, [1, 5], "collscan");
// Mechanism: with the pushdown on, the filter is in the index scan.
assert(run("pushdown on", "forceClassicEngine", true).filters.some((f) => f !== undefined),
       "the filter was not pushed into the index scan; the repro does not test the bug");
// Fails at the local build (2026-09-17): returns [] in both engines.
assert.eq(run("pushdown on", "forceClassicEngine", true).got, [1, 5], "pushdown on, classic");
assert.eq(run("pushdown on", "trySbeEngine", true).got, [1, 5], "pushdown on, SBE");
MongoRunner.stopMongod(conn);