我有一个拥有五百万个文档的集合，文档的结构如下：

{     "_id" : ObjectId("5ed23bd292845eaf1e488965"),     "categoryId" : 18790,     "targetType" : 43,     "targetId" : 433,     "priority" : 0 } 

集合拥有索引categoryId_1_priority_-1。

我期望如下的 Aggregation Pipeline 使用到索引，但是事实上并没有。

db.collection.aggregate([{$facet: {"category_1_order_by_priority_limit_2": [{$match: {categoryId: 1}}, {$sort: {priority: -1}}, {$limit: 2}],"category_2_order_by_priority_limit_3": [{$match: {categoryId: 2}}, {$sort: {priority: -1}}, {$limit: 3}]}}]) 

explain 的结果如下：

{     "stages" : [         {             "$cursor" : {                 "query" : {                  },                 "queryPlanner" : {                     "plannerVersion" : 1,                     "namespace" : "test.collection",                     "indexFilterSet" : false,                     "parsedQuery" : {                      },                     "queryHash" : "8B3D4AB8",                     "planCacheKey" : "8B3D4AB8",                     "winningPlan" : {                         "stage" : "COLLSCAN",                         "direction" : "forward"                     },                     "rejectedPlans" : [ ]                 }             }         },         {             "$facet" : {                 ...             }         }     ],     "serverInfo" : {         ...     },     "ok" : 1 } 

我应该怎么做才能使用到索引呢？

indexing – MongoDB Aggregation Pipeline $match $sort doesn’t use index – Stack Overflow