Aggregation and index optimization of mongodb basic operations

mongodb Aggregation of basic operations 、 Index optimization

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Catalog

• Aggregation operation

  • Polymerization pipeline operation

  • Map-Reduce operation

• Optimize queries using indexes

  • Index operation

    • Look at the index

    • View index size

    • Create index

    • Delete index

Aggregation operation

Polymerization pipeline operation

$group Group documents in the collection , Facilitate subsequent statistical results

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$limit Used to limit the MongoDB The number of documents returned by the aggregation pipeline

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$match For filtering data , Output only documents that meet the criteria

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$sort Sort the input documents first , Then the output

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$project Used to modify the structure of the input document （ increase , Delete fields, etc ） And name

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$skip Skip the specified number of documents in the aggregation pipeline , And return the remaining documents

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Specific syntax ：

# $group
db.COLLECTION_NAME.aggregate([{$group:{<key1>:"$<key2>"}}]).pretty()

# $limit
db.COLLECTION_NAME.aggregate({$limit: integer }).pretty()

# $match
db.COLLECTION_NAME.aggregate([{$match:{<key>:<value>}}]).pretty()

# $sort, -1 Representation of descending order , 1 Expressing ascending order 
db.COLLECTION_NAME.aggregate([{$sort:{<key>:-1 or 1}}]).pretty()

# $project
db.COLLECTION_NAME.aggregate([{$project:{<key>:<value>}}]).pretty()

# $skip
db.COLLECTION_NAME.aggregate({$skip: integer }).pretty()

group： Press... For the documents in the collection userid Grouping

db.firstCollection.aggregate([{$group:{"_id": "$userid"}}]).pretty()

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limit： Specify that the collection only shows two documents

db.firstCollection.aggregate({$limit:2}).pretty()

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match: Put... In the set nickname For Edward's documents

db.firstCollection.aggregate([{$match:{"nickname":" Edward "}}]).pretty()

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sort： Set the documents in the collection according to age Sort in descending order

db.firstCollection.aggregate([{$sort:{"age":-1}}]).pretty()

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project: Show the documents in the collection , And none of the documents contain fields _id,articleid,content

 db.firstCollection.aggregate([{$project:{ "_id":0, "articleid":0, "content":0}}]).pretty()

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skip： Skip in collection _id by 5 Previous documents （ It doesn't contain 5）, Show only _id by 5 Subsequent documents

db.firstCollection.aggregate({$skip:5}).pretty()

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Common pipe expressions and related descriptions

$sum Calculate the sum

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$avg Calculate average

db.COLLECTION_NAME

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$min Get the minimum value of all documents in the collection

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$max Get the maximum value of all documents in the collection

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$push Insert values into an array in the result document

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$first Get the first document in the grouped document

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$last Get the last document in the grouped document

Map-Reduce operation

MongoDB Provide Map-Reduce To perform aggregation operations .

db.firstCollection.mapReduce( 
  function() {emit(this.nickname, 1);}, 
  function(key,values) {return Array.sum(values)}, 
  { 
    query:{state:"1"}, out:"comment_total" 
  } 
)

Output results ：

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result: Save the result set

timeMillis: perform Map-Reduce Time spent in operation

input: If the filter conditions are met, it is input to map Number of documents for the function

emit: stay map Function emit（） The number of times the method was called , That is, the number of documents in the collection that meet the conditions

output: Number of documents in the result set

ok: Judgment execution Map-Reduce Successful operation , If the execution is successful, it displays 1, Instead, use err Express

Optimize queries using indexes

MongoDB The index of can be divided into 6 Kind of ： Single field index 、 Composite index 、 Multi key index 、 Geospatial index 、 Text index and Hashemite index

By default ,MongoDB All sets in _id There is an index on each field . because MongoDB You can traverse the index from any direction , So for single field indexing and sorting operations , The sort order of the index entries is not important .

If field of the document is array type , Then each field is an element in the array ,MongoDB An index will be created for each element in the array , So it is called multi key index .

Index operation

Look at the index

db.COLLECTION_NAME.getIndexes()

Query the collection firstCollection Index in

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Among the results returned are 4 A field , Namely v、key、name as well as ns, Which field v Indicates the version number of the indexing engine , Field key Indicates the field to which the index is added , The default is _id, Its value is 1 Indicates that the index is sorted in ascending order ; Field namne Represents the name of the index , That is, the field plus _, namely _id_; Field ns Represents the namespace of the index store , That is database test Set firstCollection in .

View index size

db.COLLECTION_NAME.totalIndexSize()

Query the collection firstCollection Index size in

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Create index

db.COLLECTION_NAME.createIndex(keys,options)

• keys: The data type of this parameter is document type , Is a document that contains fields and values , Where the field is the index key , The value is the index type that describes the field . If the specified field is an ascending index , The specified value is 1, conversely , The specified value is -1

• options: The data type of this parameter is document type , It's optional , A document containing a set of options that control index creation . Common options are unique and name, One option unique Describe whether the index is unique , If the value is true, Create a unique index , The default value is false; Options name Describes the name of the index created , If no name is specified ,MongoDB An index name will be generated by connecting the field name and sort order of the index

In collection firstCollection Create a single field index in , That is to say userid Create an index on the field of

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Look at the index

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Create composite index , That is to say userid and nickname At the same time, create an index on the field of

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Delete index

# Delete a single index

db.COLLECTION_NAME.dropIndex(index)

# Delete all indexes

db.COLLECTION_NAME.dropIndexes()

Delete userid Indexes

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Delete all indexes

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Indexes _id_ By default , Cannot delete , Only non fields can be deleted _id The index of

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