mongo/jstests/core/explain_execution_error.js

// Test that even when the execution of a query fails, explain reports query
// planner information.

var t = db.explain_execution_error;
t.drop();

var result;

/**
 * Asserts that explain reports an error in its execution stats section.
 */
function assertExecError(explain) {
    var errorObj;

    var execStats = explain.executionStats;
    if (execStats.executionStages.stage == "SINGLE_SHARD") {
        errorObj = execStats.executionStages.shards[0];
    }
    else {
        errorObj = execStats;
    }

    assert.eq(false, errorObj.executionSuccess);
    assert("errorMessage" in errorObj);
    assert("errorCode" in errorObj);
}

/**
 * Asserts that explain reports success in its execution stats section.
 */
function assertExecSuccess(explain) {
    var errorObj;

    var execStats = explain.executionStats;
    if (execStats.executionStages.stage == "SINGLE_SHARD") {
        errorObj = execStats.executionStages.shards[0];
    }
    else {
        errorObj = execStats;
    }

    assert.eq(true, errorObj.executionSuccess);
    assert(!("errorMessage" in errorObj));
    assert(!("errorCode" in errorObj));
}

// Make a string that exceeds 1 MB.
var bigStr = "x";
while (bigStr.length < (1024 * 1024)) {
    bigStr += bigStr;
}

// Make a collection that is about 40 MB.
for (var i = 0; i < 40; i++) {
    assert.writeOK(t.insert({a: bigStr, b: 1, c: i}));
}

// A query which sorts the whole collection by "b" should throw an error due to hitting the
// memory limit for sort.
assert.throws(function() {
    t.find({a: {$exists: true}}).sort({b: 1}).itcount();
});

// Explain of this query should succeed at query planner verbosity.
result = db.runCommand({
    explain: {
        find: t.getName(),
        filter: {a: {$exists: true}},
        sort: {b: 1}
    },
    verbosity: "queryPlanner"
});
assert.commandWorked(result);
assert("queryPlanner" in result);

// Explaining the same query at execution stats verbosity should succeed, but indicate that the
// underlying operation failed.
result = db.runCommand({
    explain: {
        find: t.getName(),
        filter: {a: {$exists: true}},
        sort: {b: 1}
    },
    verbosity: "executionStats"
});
assert.commandWorked(result);
assert("queryPlanner" in result);
assert("executionStats" in result);
assertExecError(result);

// The underlying operation should also report a failure at allPlansExecution verbosity.
result = db.runCommand({
    explain: {
        find: t.getName(),
        filter: {a: {$exists: true}},
        sort: {b: 1}
    },
    verbosity: "allPlansExecution"
});
assert.commandWorked(result);
assert("queryPlanner" in result);
assert("executionStats" in result);
assert("allPlansExecution" in result.executionStats);
assertExecError(result);

// Now we introduce two indices. One provides the requested sort order, and
// the other does not.
t.ensureIndex({b: 1});
t.ensureIndex({c: 1});

// The query should no longer fail with a memory limit error because the planner can obtain
// the sort by scanning an index.
assert.eq(40, t.find({c: {$lt: 40}}).sort({b: 1}).itcount());

// The explain should succeed at all verbosity levels because the query itself succeeds.
// First test "queryPlanner" verbosity.
result = db.runCommand({
    explain: {
        find: t.getName(),
        filter: {c: {$lt: 40}},
        sort: {b: 1}
    },
    verbosity: "queryPlanner"
});
assert.commandWorked(result);
assert("queryPlanner" in result);

result = db.runCommand({
    explain: {
        find: t.getName(),
        filter: {c: {$lt: 40}},
        sort: {b: 1}
    },
    verbosity: "executionStats"
});
assert.commandWorked(result);
assert("queryPlanner" in result);
assert("executionStats" in result);
assertExecSuccess(result);

// We expect allPlansExecution verbosity to show execution stats for both candidate plans.
result = db.runCommand({
    explain: {
        find: t.getName(),
        filter: {c: {$lt: 40}},
        sort: {b: 1}
    },
    verbosity: "allPlansExecution"
});
assert.commandWorked(result);
assert("queryPlanner" in result);
assert("executionStats" in result);
assert("allPlansExecution" in result.executionStats);
assertExecSuccess(result);