【发布时间】：2014-07-30 20:14:51
【问题描述】：
我有一个包含以下对象的表：
> db.shapes.find()
{ "_id" : "P1", "amenity" : "restaurant", "shape" : { "type" : "Point",
"coordinates" : [ 2, 2 ] } }
{ "_id" : "P2", "amenity" : "restaurant", "shape" : { "type" : "Point",
"coordinates" : [ 2, 4 ] } }
{ "_id" : "P3", "amenity" : "police", "shape" : { "type" : "Point",
"coordinates" : [ 4, 2 ] } }
{ "_id" : "P4", "amenity" : "police", "shape" : { "type" : "Point",
"coordinates" : [ 4, 4 ] } }
以下查询中的 explain() 给出了一个奇怪的（在我看来）结果：
> db.shapes.find({shape:{$nearSphere:{$geometry:{type: "Point", coordinates:
[0,0]}}}}, {id:1, amenity:1}).limit(2).explain()
{
"cursor" : "S2NearCursor",
"isMultiKey" : false,
"n" : 2,
"nscannedObjects" : 22,
"nscanned" : 22,
"nscannedObjectsAllPlans" : 22,
"nscannedAllPlans" : 22,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 1,
"indexBounds" : {
},
"server" : "DBTest:27017",
"filterSet" : false
}
为什么要扫描这么多物体？我的意思是，桌子只有 4 对象和 mongodb 扫描 22 ?
感谢您的任何解释。
再见，安德烈
> db.shapes.find({shape:{$nearSphere:{$geometry:{type: "Point", coordinates:
... [0,0]}}}}, {id:1, amenity:1}).limit(2).explain(1)
{
"cursor" : "S2NearCursor",
"isMultiKey" : false,
"n" : 2,
"nscannedObjects" : 22,
"nscanned" : 22,
"nscannedObjectsAllPlans" : 22,
"nscannedAllPlans" : 22,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 1,
"indexBounds" : {
},
"allPlans" : [
{
"cursor" : "S2NearCursor",
"isMultiKey" : false,
"n" : 2,
"nscannedObjects" : 22,
"nscanned" : 22,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
}
}
],
"server" : "DBTest:27017",
"filterSet" : false,
"stats" : {
"type" : "LIMIT",
"works" : 22,
"yields" : 0,
"unyields" : 0,
"invalidates" : 0,
"advanced" : 2,
"needTime" : 20,
"needFetch" : 0,
"isEOF" : 1,
"children" : [
{
"type" : "PROJECTION",
"works" : 22,
"yields" : 0,
"unyields" : 0,
"invalidates" : 0,
"advanced" : 2,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 0,
"children" : [
{
"type" : "GEO_NEAR_2DSPHERE",
"works" : 22,
"yields" : 0,
"unyields" : 0,
"invalidates" : 0,
"advanced" : 2,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 0,
"children" : [ ]
}
]
}
]
}
}
【问题讨论】：
-
如果您输入一个 $maxDistance 值会发生什么，作为一个感兴趣的问题。我认为你在 shape 字段上有一个 2dsphere 索引？
-
是的，形状字段上有一个 2dsphere-Index。
-
如果添加到查询中，$maxDistance 会做什么？
-
请显示解释（{详细：真实}）
标签： mongodb spatial sql-execution-plan