Some notes on Mongo indexes

This article mainly introduces some instructions about Mongo index, which has certain reference value. Now I share it with you. Friends in need can refer to it

Index usage scenarios

Advantages: Speed ​​up the query

Disadvantages: Additions, deletions and modifications will cause additional overhead and occupy space

tips: Return more than half of the data in the collection, and the full table scan is more efficient

Index basics

Basic operations

View index: db.test.getIndexes()

Create index: db.test.ensureIndex({" username":1},{"background":true,"name":"index_test_name"}) //When there is a large amount of data, it can be executed in the background without blocking

Delete the index: db.test.dropIndex({ "username":1})

View the index size: db.test.totalIndexSize()

Attribute

Index order:

1 is positive order, -1 is reverse order

In the composite index, you need to pay attention to the order (id:1, age:-1)

Index attributes:

• Uniqueness

db.test.ensureIndex({x:1,y:1},{unique:true})

• Sparseness

db.test.ensureIndexx({},{sparse:true/false})
不稀疏（默认）：
1. 可插入不存在索引字段的数据，null; 
2. 可筛选不存在字段: db.test.find({m:{$exist:ture}})
稀疏：

Optimization analysis method

• explain

  Learn how the system handles requests

cursor  返回游标类型（BasicCursor或BtreeCursor）
nscanned  被扫描的文档数量
n 返回的文档数
millis  耗时（毫秒）
indexBounds  所使用的索引

• hint

  Force the use of an index

db.test.find({"age":20}).hint({"name":1,"age":1}) // .hint(name_1_age_1)

• profile

  Set the log level and record slow queries

##Tips

1. Automatically adjust the order of query conditions

2. Can hit the index for the prefix regular expression (/^z /)

3. Create indexes for keys that require a large amount of sorting to avoid loading all data into memory

4. $ne, $nin will not use indexes

Index type

• _id index

• Single key index

• Multi-key index

• Compound index

• Expired index

• Full-text index

• Geographical Location Index

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