COUNT_SCAN 和 IXSCAN 有什么区别? [英] What is the difference between COUNT_SCAN and IXSCAN?

问题描述

每当我使用解释在 MongoDB 上运行计数查询时，我都会看到两个不同的阶段 COUNT_SCAN 和 IXSCAN.我想根据性能了解它们之间的区别以及如何改进查询.字段已编入索引.

Whenever I run a count query on MongoDB with explain I can see two different stages COUNT_SCAN and IXSCAN. I want to know the difference between them according to performance and how can I improve the query. field is indexed.

query db.collection.explain(true).count({field:1}}) 使用 COUNT_SCAN 和查询像

query db.collection.explain(true).count({field:1}}) uses COUNT_SCAN and query like

db.collection.explain(true).count({field:"$in":[1,2]}) 使用 IXSCAN.

db.collection.explain(true).count({field:"$in":[1,2]}) uses IXSCAN.

推荐答案

The short: COUNT_SCAN 是通过从索引中读取值来获取计数的最有效方法，但只能执行在某些情况下.否则，IXSCAN 会在对文档进行一些过滤和内存中的计数之后执行.

The short: COUNT_SCAN is the most efficient way to get a count by reading the value from an index, but can only be performed in certain situations. Otherwise, IXSCAN is performed following by some filtering of documents and a count in memory.

当从二级读取时，使用读取关注 available.此关注级别不考虑分片集群中的孤立文档，因此不会执行 SHARDING_FILTER 阶段.这是您看到 COUNT_SCAN 的时候.

When reading from secondary the read concern available is used. This concern level doesn't consider orphan documents in sharded clusters, and so no SHARDING_FILTER stage will be performed. This is when you see COUNT_SCAN.

但是，如果我们使用读取关注local，我们需要获取文档以执行 SHARDING_FILTER 过滤阶段.在这种情况下，有多个阶段来完成查询:IXSCAN，然后是 FETCH，然后是 SHAARDING_FILTER.

However, if we use read concern local, we need to fetch the documents in order to perform the SHARDING_FILTER filter stage. In this case, there are multiple stages to fulfill the query: IXSCAN, then FETCH then SHARDING_FILTER.

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