Indexeddb 简明教程

IndexedDB - Indexes

索引是一种用于从由指定属性存储的参考对象检索数据的对象存储。即使索引在参考对象存储中并且包含相同数据，但它不使用参考存储的主键，而是使用指定的属性作为其关键路径。

Indexes are a kind of object store used to retrieve data from the reference object stored by a specified property. Even though an index is inside the reference object store and contains the same data, instead of the reference store’s primary key it uses the specified property as its key path.

索引用于定义针对数据唯一约束，它们在创建对象存储时创建。要创建索引，请在对象存储实例上调用 createIndex 方法 -

Indexes are used to define a unique constraint on your data and they are made when the object stores are created. To create an index, call the createIndex method on an object store instance −

Syntax

var myIDBIndex = objectStore.createIndex(indexName, keyPath);
var myIDBIndex = objectStore.createIndex(indexName, keyPath, objectParameters);

此方法创建一个索引对象并返回它。该方法创建包含以下参数的索引 -

This method creates and returns an index object. The method creates an index that takes the following arguments −

  1. Index name − The name of the index.

  2. Keypath − We mention the primary key here.

  3. Object Parameters − There are two object parameters.

  4. Unique − Duplicate values cannot be added.

  5. Multi entry − If true, the index will add an entry in the index for each array element when the keyPath resolves to an Array. If false, it will add one single entry containing the Array.

Example

以下示例显示了在对象存储中实现索引 -

The following example shows the implementation of indexes in an object store −

<!DOCTYPE html>
<html lang="en">
<head>
   <title>Document</title>
</head>
<body>
   <script>
      const request = indexedDB.open("botdatabase",1);
      request.onupgradeneeded = function(){
         const db = request.result;
         const store = db.createObjectStore("bots",{ keyPath: "id"});
         store.createIndex("branch_db",["branch"],{unique: false});
      }
      request.onsuccess = function(){
         document.write("database opened successfully");
         const db = request.result;
         const transaction=db.transaction("bots","readwrite");
         const store = transaction.objectStore("bots");
         const branchIndex = store.index("branch_db");
         store.add({id: 1, name: "jason",branch: "IT"});
         store.add({id: 2, name: "praneeth",branch: "CSE"});
         store.add({id: 3, name: "palli",branch: "EEE"});
         store.add({id: 4, name: "abdul",branch: "IT"});
         store.put({id: 4, name: "deevana",branch: "CSE"});
         transaction.oncomplete = function(){
            db.close;
         }
      }
   </script>
</body>
</html>

Output

branchIndex:
1
['CSE']
0: "CSE"
length: 1
4
{id: 4, name: 'deevana', branch: 'CSE'}
branch: "CSE"
id: 4
name: "deevana"
2
['EEE']
0: "EEE"
length: 1
3
{id: 3, name: 'palli', branch: 'EEE'}
branch: "EEE"
id: 3
name: "palli"
3
['IT']
0: "IT"
length: 1
1
{id: 1, name: 'jason', branch: 'IT'}
branch: "IT"
id: 1
name: "jason"