问题
I am trying to reproduce the first example of index intersection instruction (http://docs.mongodb.org/manual/core/index-intersection/) but facing a problem: mongo doesn't uses both indexes
My steps:
- Download mongo (3.0.3) and install it
- Run mongod: mongod.exe --dbpath d:\data (folder is empty)
- Run mongo: mongo.exe
Add indexes:
db.orders.ensureIndex({ qty: 1 }) db.orders.ensureIndex({ item: 1 }) db.orders.getIndexes() [{ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "test.orders" }, { "v" : 1, "key" : { "qty" : 1 }, "name" : "qty_1", "ns" : "test.orders" }, { "v" : 1, "key" : { "item" : 1 }, "name" : "item_1", "ns" : "test.orders" }]Check query explain:
db.orders.find( { item: "abc123", qty: { $gt: 15 } } ).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "test.orders", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ { "item" : { "$eq" : "abc123" } }, { "qty" : { "$gt" : 15 } } ] }, "winningPlan" : { "stage" : "KEEP_MUTATIONS", "inputStage" : { "stage" : "FETCH", "filter" : { "qty" : { "$gt" : 15 } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "item" : 1 }, "indexName" : "item_1", "isMultiKey" : false, "direction" : "forward", "indexBounds" : { "item" : [ "[\"abc123\", \"abc123\"]" ] } } } }, "rejectedPlans" : [ { "stage" : "KEEP_MUTATIONS", "inputStage" : { "stage" : "FETCH", "filter" : { "item" : { "$eq" : "abc123" } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "qty" : 1 }, "indexName" : "qty_1", "isMultiKey" : false, "direction" : "forward", "indexBounds" : { "qty" : [ "(15.0, 1.#INF]" ] } } } } ] }, "serverInfo" : { "host" : "localhost", "port" : 27017, "version" : "3.0.3", "gitVersion" : "b40106b36eecd1b4407eb1ad1af6bc60593c6105" }, "ok" : 1 }
As you can see winningPlan contains only item_1 index. There is rejectedPlans which contains qty_1 index. But there is no plans which contains index intersection. I know that there are a lot of conditions to select specific index. But in my case mongo doesn't even plans it!
Could anybody help me?
回答1:
There are some details about the index selection in the SERVER-3071 JIRA issue but I cannot say if all is still relevant for 3.0. Anyway:
MongoDB 3.0.2 seems not consider index interaction for range query. But it will for point intervals:
> db.orders.find( { item: {$eq : "abc123"}, qty: { $eq: 15 } } ).explain()
...
{
"stage" : "FETCH",
"inputStage" : {
"stage" : "KEEP_MUTATIONS",
"inputStage" : {
"stage" : "AND_SORTED",
"inputStages" : [
{
"stage" : "IXSCAN",
"keyPattern" : {
"qty" : 1
},
"indexName" : "qty_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"qty" : [
"[15.0, 15.0]"
]
}
},
{
"stage" : "IXSCAN",
"keyPattern" : {
"item" : 1
},
"indexName" : "item_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"item" : [
"[\"abc123\", \"abc123\"]"
]
}
}
]
}
回答2:
We are hosting a multi-tenant site. So there is a mongo collection holding some info about all customers, for example,
{customerId: 22, category: "category", region: "region", balance: 23434... }
After we learnt about Index Intersection, we tried to create multiple single field indexes to support queries with different query criteria with variable fields, such as
{ customerId: 22, category: "1" }
{ customerId: 22, region: "somewhere" }
{ customerId: 22, balance: {$gt:0} }
{ customerId: 22, region: {$in: ["R1", "R2"]},balance: {$gt:0} }
....
But it simply turned out no intersection happened by learning from "db.collection.explain". As the fields are limited, we finally went for a solution to build a compound index includes all fields.
{customerId: 1, category: 1, region: 1, balance:1...}
And then it turned out that below all queries use the "large" compound index, as far as "customerId" is in the query.
{ customerId: 22, category: "1" }
{ customerId: 22, region: "somewhere" }
{ customerId: 22, balance: {$gt:0} }
{ customerId: 22, region: {$in: ["R1", "R2"]},balance: {$gt:0} }
....
来源：https://stackoverflow.com/questions/30626726/why-mongodb-doesnt-use-index-intersection