Mongodb index -index

1. summary

• Index support in MongoDB Efficiently execute queries in . If there is no index ,MongoDB A full set scan must be performed , That is, scanning every document in the collection , To select and query statements
  Matching documents . The query efficiency of scanning the whole collection is very low , Especially when dealing with large amounts of data , Queries can take tens of seconds or even minutes , This is right and wrong for the performance of the website Often fatal .
• If the query has an appropriate index ,MongoDB You can use this index to limit the number of documents that must be checked .
• An index is a special data structure , It stores a small portion of the collection dataset in an easy to traverse form . An index stores the values of a particular field or group of fields , Sort by field value . Row of index entries
  Order supports effective equality matching and range based query operations . Besides ,MongoDB You can also use the sort in the index to return the sort result .
  Official document ：https://docs.mongodb.com/manual/indexes/

2. Type of index

1. Single field index
   MongoDB Supports the creation of user-defined ascending order on a single field of a document / descending index , It's called a single field index （Single Field Index）.
   For single field indexing and sorting operations , Sort order of index keys （ In ascending or descending order ） Not important , because MongoDB The index can be traversed in any direction .
2. Composite index
   MongoDB It also supports user-defined indexes for multiple fields , Composite index （Compound Index）. The order of the fields listed in a composite index is significant . for example , If the composite index is from { userid: 1, score: -1 } form , Then the index first presses userid Positive sequence sort , then
   At every userid Within the value of , Press... Again score Reverse sort .
3. Other indexes
   Geospatial index （Geospatial Index）, Text index （Text Indexes）, Hash index （Hashed Indexes）

3. Index management operations

View index
explain ：
Returns an array of all indexes in a collection .
grammar ：

db.collection.getIndexes()

Tips ： The syntax command is required to run MongoDB 3.0+

Index creation
explain ：
Create an index on a collection .
grammar ：

db.collection.createIndex(keys, options)

createIndex() Receive optional parameters , The list of optional parameters is as follows ：

Parameter	Type	Description
background	Boolean	The indexing process blocks other database operations ,background You can specify backchannel mode to create indexes , That is, increase “background” Optional parameters . “background” The default value is false.
unique	Boolean	Is the index created unique . Designated as true Create unique index . The default value is false.
name	string	Name of index . If not specified ,MongoDB Generates an index name by concatenating the field name of the index and the sort order .
dropDups	Boolean	3.0+ Version obsolete . Whether to delete duplicate records when establishing a unique index , Appoint true Create unique index . The default value is false.
sparse	Boolean	Indexes are not enabled for field data that does not exist in the document ; This parameter requires special attention , If set to true Words , Documents that do not contain corresponding fields are not queried in index fields .. The default value is false.
expireAfterSeconds	integer	Specifies a value in seconds , complete TTL Set up , Sets the lifetime of the collection .
v	index version	The version number of the index . The default index version depends on mongod The version that runs when the index is created .
weights	document	Index weight , Values in 1 To 99,999 Between , Represents the score weight of the index relative to other index fields .
default_language	string	For text index , This parameter determines the list of stop words and the rules for the word stem and lexicon . Default to English
language_override	string	For text index , This parameter specifies the name of the field to be included in the document , The language overrides the default language, The default value is language.

Index removal
explain ： You can remove the specified index , Or remove all indexes
One 、 Specifies the removal of the index
grammar ：

db.collection.dropIndex(index)

Parameters ：

Parameter	Type	Description
index	string or document	Specify the index to delete . The index can be specified by index name or index specification document . To delete the text index , Please specify the index name .

Two 、 Removal of all indexes
grammar ：

db.collection.dropIndexes()