Info

Hiding an index can result in cached plans no longer being accessible via the $planCacheStats pipeline stage. db.foo.drop(); db.foo.insert({ a: 1, b: 1 }); db.foo.createIndex({ a: 1 }); db.foo.createIndex({ a: 1, b: 1 }) var q = { a: { $gte: 1 }, b: 1 }; db.foo.count(q); var key = db.foo.explain().count(q).queryPlanner.planCacheKey; //=> E7DCAC4F db.foo.aggregate([{ $planCacheStats: {} }, { $match: { planCacheKey: key } }, {$project: { plan: "$cachedPlan.indexName" } } ]) //=> { "plan" : "a_1_b_1" } db.foo.hideIndex({ a: 1 }) db.foo.count(q); var key2 = db.foo.explain().count(q).queryPlanner.planCacheKey; //=> E7DCAC4F db.foo.aggregate([{ $planCacheStats: {} }, { $match: { planCacheKey: key2 } }, {$project: { plan: "$cachedPlan.indexName" } } ]) //=> no results db.foo.unhideIndex({ a: 1 }) db.foo.count(q); var key3 = db.foo.explain().count(q).queryPlanner.planCacheKey; //=> E7DCAC4F db.foo.aggregate([{ $planCacheStats: {} }, { $match: { planCacheKey: key3 } }, {$project: { plan: "$cachedPlan.indexName" } } ]) //=> { "plan" : "a_1_b_1" }

Top User Comments

alex.bevilacqua commented on Wed, 7 Jul 2021 16:39:25 +0000: This is technically "as designed" as single solution query plans aren't cached. Flagging this as duplicate and leaving it here in case it helps other discover SERVER-13341.

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