MongoDB in Action Second Edition笔记之基于Index查询优化

查看查询过程的方法

查看所有的使用index的查询方案

强制使用某种index的方法

index cache

查看查询过程的方法：

> db.inventory.find({"quantity": 500,
"type":"toys"}).limit(1).explain("executionStats")

查看所有的使用index的查询方案：

mongoDBv3.0 interprets true as allPlansExecution and false as queryPlanner

db.values.find({stock_symbol: "GOOG", close: {$gt: 200}}).explain(true)

强制使用某种index的方法

It’s understandable why the optimizer rejects the collection scan, but it might be less clear why the index on {close: 1} doesn’t satisfy. You can use hint() to find out.

hint() forces the query optimizer to use a particular index:

query = {stock_symbol: "GOOG", close: {$gt: 200}}
db.values.find(query).hint({close: 1}).explain()

index cache

当查询执行成功后，该查询模式会被记录下来，如下：

{

pattern: {
stock_symbol: 'equality',
close: 'bound',
index: {
stock_symbol: 1
},
nscanned: 894
}
}

这个模式记录了当你请求

stock_symbol

的精确匹配(equality)和

close

的区间匹配(bound)时，使用index{stock_symbol:1}

index cache失效的三种情景：

1. 100 writes are made to the collection.

2. Indexes are added or removed from the collection.

3. A query using a cached query plan does a lot more work than expected. Here,what qualifies as “a lot more work” is a value for nscanned exceeding the cached nscanned value by at least a factor of 10.