jstests/core/query_hash_stability.js


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/**
 * Test that 'queryHash' and 'planCacheKey' from explain() output have sensible values
 * across catalog changes.
 */
(function() {
    "use strict";
    load('jstests/libs/fixture_helpers.js');  // For and isMongos().

    const collName = "query_hash_stability";
    const coll = db[collName];
    coll.drop();
    // Be sure the collection exists.
    assert.commandWorked(coll.insert({x: 5}));

    function getPlanCacheKeyFromExplain(explainRes) {
        const hash = FixtureHelpers.isMongos(db)
            ? explainRes.queryPlanner.winningPlan.shards[0].planCacheKey
            : explainRes.queryPlanner.planCacheKey;
        assert.eq(typeof(hash), "string");
        return hash;
    }

    function getQueryHashFromExplain(explainRes) {
        const hash = FixtureHelpers.isMongos(db)
            ? explainRes.queryPlanner.winningPlan.shards[0].queryHash
            : explainRes.queryPlanner.queryHash;
        assert.eq(typeof(hash), "string");
        return hash;
    }

    const query = {x: 3};

    const initialExplain = coll.find(query).explain();

    // Add a sparse index.
    assert.commandWorked(coll.createIndex({x: 1}, {sparse: true}));

    const withIndexExplain = coll.find(query).explain();

    // 'queryHash' shouldn't change across catalog changes.
    assert.eq(getQueryHashFromExplain(initialExplain), getQueryHashFromExplain(withIndexExplain));
    // We added an index so the plan cache key changed.
    assert.neq(getPlanCacheKeyFromExplain(initialExplain),
               getPlanCacheKeyFromExplain(withIndexExplain));

    // Drop the index.
    assert.commandWorked(coll.dropIndex({x: 1}));
    const postDropExplain = coll.find(query).explain();

    // 'queryHash' shouldn't change across catalog changes.
    assert.eq(getQueryHashFromExplain(initialExplain), getQueryHashFromExplain(postDropExplain));

    // The 'planCacheKey' should be the same as what it was before we dropped the index.
    assert.eq(getPlanCacheKeyFromExplain(initialExplain),
              getPlanCacheKeyFromExplain(postDropExplain));
})();