mongodb AND查詢遇到多個index時候可能會做交集——和復合索引不同

關於MongoDB中索引文檔的一個問題？

 -

To illustrate index intersection, consider a collection orders that has the following indexes:

{ qty: 1 }
{ item: 1 }

MongoDB can use the intersection of the two indexes to support the following query:

db.orders.find( { item: "abc123", qty: { $gt: 15 } } )

上面是MongoDB的索引文檔，說下面這個查詢能交叉利用上面兩個索引進行優化，可是根據我的理解，需要建立一個多重索引才行，如下：

{ qty: 1, item: 1 }

建立兩個索引是建立兩個獨立的B+樹，多重索引則是建立一顆B+樹，如果兩顆B+樹是獨立的，怎么才能交叉利用呢？

還有下面這個例子，也不知道如何才能混合利用起來？

Consider a collection orders with the following indexes:

{ qty: 1 }
{ status: 1, ord_date: -1 }

To fulfill the following query which specifies a condition on both the qty field and the status field, MongoDB can use the intersection of the two indexes:

db.orders.find( { qty: { $gt: 10 } , status: "A" } )

 

看問題似乎很神奇。。不過實際看過去也沒啥變化。。


determine if MongoDB used index intersection, run explain(); the results of explain()will include either an AND_SORTED stage or an AND_HASH stage.

歸並排序或hash 組合。 類似join的做法。

Stages are descriptive of the operation; e.g.

• COLLSCAN for a collection scan
• IXSCAN for scanning index keys
• FETCH for retrieving documents
• SHARD_MERGE for merging results from shards

Index Intersection

For an index intersection plan, the result will include either an AND_SORTED stage or an AND_HASH stage with an inputStages array that details the indexes; e.g.:

{
   "stage" : "AND_SORTED", "inputStages" : [ { "stage" : "IXSCAN", ... }, { "stage" : "IXSCAN", ... } ] } 

In previous versions of MongoDB, cursor.explain() returned the cursor field with the value of Complex Plan for index intersections.

 

轉自：http://www.ihowandwhy.com/z/%E5%85%B3%E4%BA%8EMongoDB%E4%B8%AD%E7%B4%A2%E5%BC%95%E6%96%87%E6%A1%A3%E7%9A%84%E4%B8%80%E4%B8%AA%E9%97%AE%E9%A2%98%EF%BC%9F