【发布时间】：2016-06-30 00:26:24
【问题描述】：
我问Is cursor.skip() on indexed keys always faster?
现在我正在尝试利用 multi-key indexing （第 94 页）来执行更快的查询。
使用此脚本创建了 2 个数据库：
var a = 0;
while(a++ < 4096*2){
var randomnumber=Math.ceil(Math.random()*1000000)
// create string in length of 3
var name = Math.random().toString(36).substring(2,5);
db.fast.insert({name:name, email:name + ".email@example.com", age:randomnumber});
db.slow.insert({name:name, email:name + ".email@example.com", age:randomnumber});
}
数据库索引如下：
> db.fast.getIndexes()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.fast"
},
{
"v" : 1,
"key" : {
"age" : 1
},
"name" : "age_1",
"ns" : "test.fast"
},
{
"v" : 1,
"key" : {
"age" : 1,
"name" : 1,
"email" : 1
},
"name" : "age_1_name_1_email_1",
"ns" : "test.fast"
},
{
"v" : 1,
"key" : {
"age" : 0,
"name" : 0,
"email" : 0
},
"name" : "age_0_name_0_email_0",
"ns" : "test.fast"
}
]
>
> db.slow.getIndexes()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.slow"
}
]
我执行了这两个查询：
>pageNumber=18;nPerPage=20; db.slow.find({name:{$gt:"1234", $lt:"z"}, age:{$gt:200, $lt:777217}, email:{$gt:"bdnsa28831283d", $lt:"z"} }).skip(pageNumber > 0 ? ((pageNumber-1)*nPerPage) : 0).limit(nPerPage).explain("executionStats")
"executionSuccess" : true,
"nReturned" : 20,
"executionTimeMillis" : 93,
"totalKeysExamined" : 0,
"totalDocsExamined" : 688,
>pageNumber=18;nPerPage=20; db.fast.find({name:{$gt:"1234", $lt:"z"}, age:{$gt:200, $lt:777217}, email:{$gt:"bdnsa28831283d", $lt:"z"} }).skip(pageNumber > 0 ? ((pageNumber-1)*nPerPage) : 0).limit(nPerPage).explain("executionStats")
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 20,
"executionTimeMillis" : 70,
"totalKeysExamined" : 559,
"totalDocsExamined" : 360,
如您所见，fast db 的执行时间比预期的要好。
但在那之后我执行了以下两个，并且慢了很多：
>pageNumber=58;nPerPage=300; db.slow.find({name:{$gt:"1Adsa34", $lt:"zZ123z"}, age:{$gt:4555, $lt:988888}, email:{$gt:"abdnsa28831283d", $lt:"z2"} }).skip(pageNumber > 0 ? ((pageNumber-1)*nPerPage) : 0).limit(nPerPage).explain("executionStats")
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 30,
"totalKeysExamined" : 0,
"totalDocsExamined" : 8192,
>pageNumber=58;nPerPage=300; db.fast.find({name:{$gt:"1Adsa34", $lt:"zZ123z"}, age:{$gt:4555, $lt:988888}, email:{$gt:"abdnsa28831283d", $lt:"z2"} }).skip(pageNumber > 0 ? ((pageNumber-1)*nPerPage) : 0).limit(nPerPage).explain("executionStats")
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 101,
"totalKeysExamined" : 8058,
"totalDocsExamined" : 5484,
可能是某种服务的缓存问题，尽管我重新启动了 mongod。
慢速执行计划：
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.slow",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"age" : {
"$lt" : 988888
}
},
{
"email" : {
"$lt" : "z2"
}
},
{
"name" : {
"$lt" : "zZ123z"
}
},
{
"age" : {
"$gt" : 4555
}
},
{
"email" : {
"$gt" : "abdnsa28831283d"
}
},
{
"name" : {
"$gt" : "1Adsa34"
}
}
]
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 300,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 11616,
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"$and" : [
{
"age" : {
"$lt" : 988888
}
},
{
"email" : {
"$lt" : "z2"
}
},
{
"name" : {
"$lt" : "zZ123z"
}
},
{
"age" : {
"$gt" : 4555
}
},
{
"email" : {
"$gt" : "abdnsa28831283d"
}
},
{
"name" : {
"$gt" : "1Adsa34"
}
}
]
},
"direction" : "forward"
}
}
},
"rejectedPlans" : [ ]
},
快速数据库执行计划：
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.fast",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"age" : {
"$lt" : 988888
}
},
{
"email" : {
"$lt" : "z2"
}
},
{
"name" : {
"$lt" : "zZ123z"
}
},
{
"age" : {
"$gt" : 4555
}
},
{
"email" : {
"$gt" : "abdnsa28831283d"
}
},
{
"name" : {
"$gt" : "1Adsa34"
}
}
]
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 300,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 17100,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 0,
"name" : 0,
"email" : 0
},
"indexName" : "age_0_name_0_email_0",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"(4555.0, 988888.0)"
],
"name" : [
"(\"1Adsa34\", \"zZ123z\")"
],
"email" : [
"(\"abdnsa28831283d\", \"z2\")"
]
}
}
}
}
},
"rejectedPlans" : [
{
"stage" : "LIMIT",
"limitAmount" : 300,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 17100,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1,
"name" : 1,
"email" : 1
},
"indexName" : "age_1_name_1_email_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"(4555.0, 988888.0)"
],
"name" : [
"(\"1Adsa34\", \"zZ123z\")"
],
"email" : [
"(\"abdnsa28831283d\", \"z2\")"
]
}
}
}
}
},
{
"stage" : "LIMIT",
"limitAmount" : 300,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 17100,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"email" : {
"$lt" : "z2"
}
},
{
"name" : {
"$lt" : "zZ123z"
}
},
{
"email" : {
"$gt" : "abdnsa28831283d"
}
},
{
"name" : {
"$gt" : "1Adsa34"
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1
},
"indexName" : "age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"(4555.0, 988888.0)"
]
}
}
}
}
}
]
},
【问题讨论】：
-
您正在查询非索引键，它被理解为较慢。很抱歉，但我不明白这一点。
-
@Saleem，非索引键是什么意思。查看
db.fast.getIndexes()的输出。显然所有的键都是字符串并且是可排序的并且可以存储在 B-Tree 数据结构中。 -
好吧，你知道在所有键上都有索引并不意味着索引实际上被用于查询。如果可能，请向我们展示完整的执行计划
-
@Saleem，附上。
标签： javascript mongodb indexing query-optimization