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  1. Core Server
  2. SERVER-56105

[SBE] $split expression succeeds with SBE off but fails with SBE on

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    • Type: Icon: Bug Bug
    • Resolution: Fixed
    • Priority: Icon: Major - P3 Major - P3
    • 5.0.0-rc0
    • Affects Version/s: None
    • Component/s: Query Execution
    • Labels:
      None
    • Fully Compatible
    • ALL
    • Hide

      I've created a short repro script to demonstrate the problem:

      (function() {

function createCollectionAndRunProblemQuery(conn) {
    const testDb = conn.getDB("testdb");
    const coll = testDb.test_collection;
    coll.drop();
    assert.commandWorked(coll.insert({}));

    printjson(coll.explain("queryPlanner").aggregate([
        {$project: {_id: 0, out: {$split: ["$missing", {$toLower: "$missing"}]}}}
    ]));
    return coll
        .aggregate([{$project: {_id: 0, out: {$split: ["$missing", {$toLower: "$missing"}]}}}])
        .toArray()[0];
};

let conn = MongoRunner.runMongod();
assert.neq(null, conn, "failed to start mongod");
const withoutSbe = createCollectionAndRunProblemQuery(conn);
MongoRunner.stopMongod(conn);

conn = MongoRunner.runMongod({setParameter: "featureFlagSBE=true"});
assert.neq(null, conn, "failed to start mongod");
const withSbe = createCollectionAndRunProblemQuery(conn);
MongoRunner.stopMongod(conn);

assert.eq(withoutSbe, withSbe);
}());
      Show
      I've created a short repro script to demonstrate the problem: (function() { function createCollectionAndRunProblemQuery(conn) { const testDb = conn.getDB( "testdb" ); const coll = testDb.test_collection; coll.drop(); assert .commandWorked(coll.insert({})); printjson(coll.explain( "queryPlanner" ).aggregate([ {$project: {_id: 0, out: {$split: [ "$missing" , {$toLower: "$missing" }]}}} ])); return coll .aggregate([{$project: {_id: 0, out: {$split: [ "$missing" , {$toLower: "$missing" }]}}}]) .toArray()[0]; }; let conn = MongoRunner.runMongod(); assert .neq( null , conn, "failed to start mongod" ); const withoutSbe = createCollectionAndRunProblemQuery(conn); MongoRunner.stopMongod(conn); conn = MongoRunner.runMongod({setParameter: "featureFlagSBE= true " }); assert .neq( null , conn, "failed to start mongod" ); const withSbe = createCollectionAndRunProblemQuery(conn); MongoRunner.stopMongod(conn); assert .eq(withoutSbe, withSbe); }());
    • Query Execution 2021-05-03

      When the collection contains a single document (which has no field named "missing"), the following query succeeds with SBE off but fails when SBE is on:

      coll.aggregate([{$project: {_id: 0, out: {$split: ["$missing", {$toLower: "$missing"}]}}}])

      I believe the problem relates to different evaluation orders. The classic engine first checks whether each of $split's arguments are nullish, and if so returns null. This means that the $split expression returns null due to to its first argument evaluating to null, prior to doing any validation checks for the second argument. SBE, on the other hand, uses a plan which appears to validate the second argument before the first. The $toLower expression returns an empty string, which fails since this is an illegal value as the second parameter to $split. Here is the SBE plan, with a few edits I made to the spacing to make it more legible:

      [2] traverse s10 s9 s2 {} {}
from
    [1] scan s2 s3 [] @\"09a5b5d4-ba95-401e-bbd8-076034ee9c93\" true
in
    [2] mkbson s9 s2 [] keep [out = s8] true false
    [2] project [s8 = let [l2.0 = s5, l2.1 = let [l1.0 = s7]
        if (! exists (l1.0) || typeMatch (l1.0, 0x00000440),
            \"\",
            if (typeMatch (l1.0, 0x000F4206),
                toLower (coerceToString (l1.0)),
                fail ( 5066300 ,$toLower input type is not supported)))]
        if (! exists (l2.1) || typeMatch (l2.1, 0x00000440),
            null,
            if (! isString (l2.1),
                fail ( 5155400 ,$split delimiter must be a string),
                if (l2.1 == \"\",
                    fail ( 5155401 ,$split delimiter must not be an empty string),
                    if (! exists (l2.0) || typeMatch (l2.0, 0x00000440),
                        null,
                        if (! isString (l2.0),
                            fail ( 5155402 ,$split string expression must be a string),
                            if (l2.0 == \"\",
                                [\"\"],
                                split (l2.0, l2.1)))))))]
    [2] traverse s7 s7 s6 [s4, s5] {} {}
    from
        [2] project [s6 = getField (s2, \"missing\")]
        [2] traverse s5 s5 s4 {} {}
        from
            [2] project [s4 = getField (s2, \"missing\")]
            [2] limit 1
            [2] coscan
        in
            [2] project [s5 = s4]
            [2] limit 1
            [2] coscan

    in
        [2] project [s7 = s6]
        [2] limit 1
        [2] coscan

            Assignee:
            andrii.dobroshynski@mongodb.com Andrii Dobroshynski (Inactive)
            Reporter:
            david.storch@mongodb.com David Storch
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            Watchers:
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              Created:
              Updated:
              Resolved: