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Mongodb unique index

2022-06-10 19:49:00 【51CTO】

A unique index ensures that the index field does not store duplicate values ; That is, to enforce the uniqueness of index fields . By default ,MongoDB When you create a collection, you create it in id Create a unique index on the field .
from Mongodb4.2 Version start , The functional compatibility version is 4.2( Or higher ),MongoDB A new internal format is used for unique indexes , It's different from the early MongoDB Version incompatible , The new format is applicable to existing unique indexes and newly created indexes / Unique index rebuilt .

1、 Updating mongodb There is a prompt that the index is null when the document field

db.version()
4.2.18

> db.test2.update({"cr_user":"0d69c51de9ea464da4b3aff397fa3d0a"}, {"us_amount": NumberLong(104857600)});
WriteResult({
	"nMatched" : 0,
	"nUpserted" : 0,
	"nModified" : 0,
	"writeError" : {
		"code" : 11000,
		"errmsg" : "E11000 duplicate key error collection: jia_test.test2 index: cr_user_1 dup key: { cre_user: null }"
	}
})

     
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1.2、 Look at the collection index ：

> db.test2.getIndexes() // Look at the collection index 
[
	{
		"v" : 2,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "jia_test.test2"
	},
	{
		"v" : 2,
		"unique" : true,     // unique index 
		"key" : { 
			"cr_user" : 1
		},
		"name" : "cr_user_1",
		"ns" : "jia_test.test2"
	}
]

     
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1.3、 After reading the official documents, it says : Interested parties can click on the text to view the official documents

If a document has no index field value in the unique index , Then the index will store a null value for this document . Due to unique constraints ,MongoDB Only one document with missing index fields is allowed . If there are multiple documents without the value of the index field or missing the index field , Then the index building will fail due to duplicate key errors .
operation ： Delete documents without unique index fields , The update is successful .

> db.test2.remove({"us_amount" : NumberLong(104857600)})
WriteResult({ "nRemoved" : 1 })
> db.test2.update({"cr_user":"0d69c51de9ea464da4b3aff397fa3d0a"}, {"us_amount": NumberLong(104857600)});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

     
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2、 unique index ：

You can create... In a single field , Create unique index command db.collection.createIndex() take unique Set to true

db.collection.createIndex( <key and index type specification>, { unique: true } )

> db.members.createIndex( { "user_id": 1 }, { unique: true } )   //user_id Fields create unique indexes 
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}

     
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Insert a piece of data

The unique field is repeatedly inserted

> db.members.insert({"user_id":1,"name":"lisi"})
WriteResult({
	"nInserted" : 0,
	"writeError" : {
		"code" : 11000,
		"errmsg" : "E11000 duplicate key error collection: jia_test.members index: user_id_1 dup key: { user_id: 1.0 }"
	}
})

     
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Insert data that does not contain unique index fields

> db.members.insert({"name":"lisi"})
WriteResult({ "nInserted" : 1 })   // Inserting the second item was unsuccessful 
> db.members.insert({"name":"wangwu"})
WriteResult({
	"nInserted" : 0,
	"writeError" : {
		"code" : 11000,
		"errmsg" : "E11000 duplicate key error collection: jia_test.members index: user_id_1 dup key: { user_id: null }"
	}
})
>

     
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You can also create a composite unique index

db.members.createIndex( { groupNumber: 1, lastname: 1, firstname: 1 }, { unique: true } )
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}

     
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Another example is , Consider a collection with the following documents

{ _id: 1, a: [ { loc: “A”, qty: 5 }, { qty: 10 } ] }

db.collection.createIndex( { “a.loc”: 1, “a.qty”: 1 }, { unique: true } )

Unique indexes allow the following documents to be inserted into the collection ,

Because the index is for a.loc and a.qty The combination of values enforces uniqueness , So the following data insertion is successful

> db.collection.createIndex( { "a.loc": 1, "a.qty": 1 }, { unique: true } )
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.collection.insertMany( [
    { _id: 2, a: [ { loc: "A" }, { qty: 5 } ] },
    { _id: 3, a: [ { loc: "A", qty: 10 } ] }
] )
{ "acknowledged" : true, "insertedIds" : [ 2, 3 ] }

> db.collection.insertMany( [   
    { _id: 4, a: [ { loc: "C" }, { qty: 10 } ] },  
    { _id: 5, a: [ { loc: "D", qty: 10 } ] } ] )
{ "acknowledged" : true, "insertedIds" : [ 4, 5 ] }







     
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