前言
關於indexedDB的基礎知識,可以查閱 Indexed Database API (W3C) s Using_IndexedDB (MDN) 和 使用IndexedDB (中文).
關於indexedDB的索引,我仔細翻看了W3C API才弄明白是怎麼回事,在上面另外2個鏈接的教程中,都沒有介紹索引的使用。
索引,就是以數據的快速查詢爲目的。
本文展示如何在indexedDB中創建和使用索引來優化數據查詢。網上搜索到的indexedDB入門教程中,都沒有介紹索引的使用,或是隻帶過了一下怎麼創建索引,沒提可以用來幹什麼。我在這裏給出一個簡潔完整的例子,示範一下indexedDB索引的使用。
索引需求場景示例
假設現在有一組如下的數據:
id 姓 姓名
1 張 張三
2 張 張四
3 張 張五
4 張 張六
5 王 王五
6 王 王六
其中包含4個姓張,2個姓王。
如何快速方便的查詢出所有姓張的記錄呢?
針對需求示例使用索引
如果不用索引,代碼上就要查詢出全部記錄,然後對每個記錄進行判斷過濾。
如果用索引,就能直接按指定參數查處某姓的全部記錄。
創建索引的代碼片段示例:
store.createIndex("ix_familyName", "familyName", {unique: false});
插入測試數據的代碼片段示例:store.put({familyName: "張", name: "張三"});
store.put({familyName: "張", name: "張四"});
store.put({familyName: "張", name: "張五"});
store.put({familyName: "張", name: "張六"});
store.put({familyName: "王", name: "王五"});
store.put({familyName: "王", name: "王六"});
查詢所有姓張的記錄的代碼片段示例:var keyRange = IDBKeyRange.only("張");
var req = store.index("ix_familyName").openCursor(keyRange);
這樣得到的結果中,將不會包含其他姓的記錄。使用索引的完整demo代碼
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>IndexedDB's index usage demo</title>
</head>
<body>
<h1>indexedDB中索引的使用demo</h1>
<pre>
姓:<input type="text" id="tx1" value="張">名:<input type="text" id="tx2"><button id="btnAdd">add</button>
result:
<textarea id="taResult" rows="10" cols="90"></textarea>
<button id="btnRead">read</button>
<button id="btnDelDB">deleteDatabase</button>
</pre>
<script>
function $$(id){return document.getElementById(id);}
var dbName = 'testDB1', storeName = 'testStore1', dbVersion = 1;
$$('btnAdd').onclick = add;
$$('btnRead').onclick = read;
$$('btnDelDB').onclick = function (){
indexedDB.deleteDatabase(dbName);
};
function read(){
openDB(function (db){
var transaction = db.transaction(storeName,"readonly");
transaction.oncomplete = function(event) {
console.log("transaction oncomplete");
};
var store = transaction.objectStore(storeName);
//使用索引查詢所有姓張的記錄
var keyRange = IDBKeyRange.only("張");
var req = store.index("ix_familyName").openCursor(keyRange, "next");
var records = [];
$$('taResult').value = '';
req.onsuccess = function(event) {
var cursor = event.target.result;
if (!cursor) {
//打印查詢結果集
$$('taResult').value = records.join('\n');
return;
}
records.push(cursor.primaryKey + ' ' + cursor.key + ' ' + cursor.value.familyName + ' ' + cursor.value.name);
cursor.continue();
};
});
}
function add(){
openDB(function (db){
var transaction = db.transaction(storeName,"readwrite");
transaction.oncomplete = read;
var store = transaction.objectStore(storeName);
var req = store.put({familyName: $$('tx1').value, name: $$('tx2').value});
req.onsuccess = function(event) {
read();
};
});
}
function openDB(callback){
if(openDB.db){
callback(openDB.db);
return;
}
var openRequest = indexedDB.open(dbName, dbVersion);
openRequest.onupgradeneeded = function (e) {
console.log('openRequest.onupgradeneeded');
var db = e.target.result;
if(!db.objectStoreNames.contains(storeName)) {
var store = db.createObjectStore(storeName, {keyPath:"id", autoIncrement: true});
//創建索引
store.createIndex("ix_familyName", "familyName", {unique: false});
//插入初始化測試數據
store.put({familyName: "張", name: "張三"});
store.put({familyName: "張", name: "張四"});
store.put({familyName: "張", name: "張五"});
store.put({familyName: "張", name: "張六"});
store.put({familyName: "王", name: "王五"});
store.put({familyName: "王", name: "王六"});
}
};
openRequest.onsuccess = function(e) {
console.log('openRequest.onsuccess');
if(callback){
openDB.db = e.target.result;
callback(e.target.result);
}
};
}
</script>
</body>
</html>