MongoDB Learning Notes (ii)

Other ways to MongoDB:

Displays the specified number of data:

1, in MongoDB query a specified number of records, you can use:

Db.collectionname.find (). limit (number);

2, skip the specified number of records in MongoDB, you can use

Db.collectionname.find (). Skip (Number1). Limit (number2); Corresponds to the NUMBER2 data record that shows the beginning of the data in section (NUMBER1+1).

Sort:

Sort the displayed data records in ascending or descending order:

Db.collectionname.find (). Sort ({key:flag}); Key for the field to be sorted based on, Flag 1 is ascending, flag is-1 is descending

When querying records, only the fields of interest are displayed:

Db.collectionname.find ({},{_id:0,key1:1,key2:1})//_id:0 indicates that id,key1:1&key2:1 is not displayed, indicating that the corresponding data for the Key1&key2 is displayed. (0 not shown, 1 shown)

Index:

Index concept: The index is a special data structure, the index is stored in the form of key-value pairs in a data collection that is easy to traverse, key is the value of the field to create the index, and value is the pointer corresponding to the data record block;

Indexes can use the values of one or more columns in a database table, and all values of the field are sorted after the index is created. Indexes are created to improve the efficiency of full-table scans in specific scenarios.

The index in MongoDB:

Use the Ensureindex () method to create an index in MongoDB

DB. Collection_name. Ensureindex({KEY:1})//key value is the index field you want to create, 1 is ascending, 1 is descending

Ensureindex () receives optional parameters, the optional parameter list is as follows:

Parameter	Type	Description
Background	Boolean	The indexing process blocks other database operations, and background can specify a later way to create an index, which is to add "background" optional parameters. The default value for "Background" is false.
Unique	Boolean	Whether the established index is unique. Specifies true to create a unique index. The default value is false.
Name	String	The name of the index. If not specified, MongoDB generates an index name by the field name and sort order of the connection index.
Dropdups	Boolean	Whether to delete duplicate records when establishing a unique index, specifying true to create a unique index. The default value is false.
Sparse	Boolean	Do not enable indexing for field data that does not exist in the document; This parameter requires special attention, and if set to true, documents that do not contain corresponding fields are not queried in the index field ... The default value is false.
Expireafterseconds	Integer	Specify a value in seconds, complete the TTL setting, and set the lifetime of the collection.
V	Index version	The version number of the index. The default index version depends on the version that is run when the index is mongod created.
Weights	Document	The index weight value, which is between 1 and 99,999, that represents the score weight of the index relative to the other indexed fields.
Default_language	String	For a text index, this parameter determines the list of rules for the stop word and the stemming and the word changer. Default to English
Language_override	String	For a text index, this parameter specifies the name of the field contained in the document, the language overrides the default language, and the default value is language.

MongoDB Learning Notes (ii)