-
Notifications
You must be signed in to change notification settings - Fork 1
Expand file tree
/
Copy pathcount-vs-exists.html
More file actions
229 lines (208 loc) · 14.6 KB
/
count-vs-exists.html
File metadata and controls
229 lines (208 loc) · 14.6 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
<!DOCTYPE html>
<html lang="en-US">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="author" content="Won" />
<meta name="copyright" content="Won" />
<meta name="keywords" content="Mysql, It, " />
<title>count(*) vs exists() · Tugqi Biz
</title>
<link href="http://cdn-images.mailchimp.com/embedcode/slim-081711.css" rel="stylesheet" type="text/css">
<link href="//netdna.bootstrapcdn.com/twitter-bootstrap/2.3.1/css/bootstrap-combined.min.css" rel="stylesheet">
<link rel="stylesheet" type="text/css" href="http://wbowam.github.io/theme/css/style.css" media="screen">
<link rel="stylesheet" type="text/css" href="http://wbowam.github.io/theme/css/solarizedlight.css" media="screen">
<link rel="shortcut icon" href="http://wbowam.github.io/theme/images/favicon.ico" type="image/x-icon" />
<link rel="apple-touch-icon" href="http://wbowam.github.io/theme/images/apple-touch-icon.png" />
<link rel="apple-touch-icon" sizes="57x57" href="http://wbowam.github.io/theme/images/apple-touch-icon-57x57.png" />
<link rel="apple-touch-icon" sizes="72x72" href="http://wbowam.github.io/theme/images/apple-touch-icon-72x72.png" />
<link rel="apple-touch-icon" sizes="114x114" href="http://wbowam.github.io/theme/images/apple-touch-icon-114x114.png" />
<link rel="apple-touch-icon" sizes="144x144" href="http://wbowam.github.io/theme/images/apple-touch-icon-144x144.png" />
<link rel="icon" href="http://wbowam.github.io/theme/images/apple-touch-icon-144x144.png" />
</head>
<body>
<div id="content-sans-footer">
<div class="navbar navbar-static-top">
<div class="navbar-inner">
<div class="container">
<a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</a>
<a class="brand" href="http://wbowam.github.io/"><span class=site-name>Tugqi Biz</span></a>
<div class="nav-collapse collapse">
<ul class="nav pull-right top-menu">
<li ><a href="http://wbowam.github.io">Home</a></li>
<li ><a href="http://wbowam.github.io/categories.html">Categories</a></li>
<li ><a href="http://wbowam.github.io/tags.html">Tags</a></li>
<li ><a href="http://wbowam.github.io/archives.html">Archives</a></li>
<li><form class="navbar-search" action="http://wbowam.github.io/search.html" onsubmit="return validateForm(this.elements['q'].value);"> <input type="text" class="search-query" placeholder="Search" name="q" id="tipue_search_input"></form></li>
</ul>
</div>
</div>
</div>
</div>
<div class="container-fluid">
<div class="row-fluid">
<div class="span1"></div>
<div class="span10">
<article>
<div class="row-fluid">
<header class="page_header span10">
<h1><a href="http://wbowam.github.io/count-vs-exists.html"> count(*) vs exists() </a></h1>
</header>
</div>
<div class="row-fluid">
<div class="span10 article-content">
<blockquote>
<h5 id="exists-vs-count-the-battle-never-ends">Exists Vs. Count(*) - The battle never ends...<a class="headerlink" href="#exists-vs-count-the-battle-never-ends" title="Permanent link">¶</a></h5>
<p>I am still amazed at how many of the database applications written today still disregard some basic rules of thumb when it comes to accessing the data. One in particular is the use of COUNT(*) to check to see if there are any rows that match some criteria.
——Andrew Kelly</p>
</blockquote>
<h4 id="_1">让我们做个了结....<a class="headerlink" href="#_1" title="Permanent link">¶</a></h4>
<hr />
<h3 id="_2">情景：<a class="headerlink" href="#_2" title="Permanent link">¶</a></h3>
<p>判断是否存在<code>cat=”极客“</code>的Article</p>
<h5 id="_3">方法一：<a class="headerlink" href="#_3" title="Permanent link">¶</a></h5>
<div class="highlight"><pre><span class="n">count</span> <span class="o">=</span> <span class="n">Article</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">cat</span><span class="o">=</span><span class="s">"极客"</span><span class="p">).</span><span class="n">count</span><span class="p">()</span>
<span class="k">if</span> <span class="n">count</span><span class="err">：</span>
<span class="err">#</span> <span class="n">balabala</span><span class="p">...</span>
</pre></div>
<p>SQL: <code>SELECT COUNT(*) FROM table_article WHERE cat="极客";</code></p>
<h5 id="_4">方法二:<a class="headerlink" href="#_4" title="Permanent link">¶</a></h5>
<div class="highlight"><pre><span class="n">exist</span> <span class="o">=</span> <span class="n">Article</span><span class="p">.</span><span class="n">objects</span><span class="p">.</span><span class="n">filter</span><span class="p">(</span><span class="n">cat</span><span class="o">=</span><span class="s">"极客"</span><span class="p">).</span><span class="n">exists</span><span class="p">()</span>
<span class="k">if</span> <span class="n">exist</span><span class="o">:</span>
<span class="err">#</span> <span class="n">balabalabala</span><span class="p">....</span>
</pre></div>
<p>SQL: <code>EXISTS (SELECT * FROM table_article WHERE cat="极客")</code> <br />
<a href="http://sqlblog.com/blogs/andrew_kelly/archive/2007/12/15/exists-vs-count-the-battle-never-ends.aspx">查看</a></p>
<h4 id="2cat">当只有2条符合要求的数据(cat="极客")时,查询情况<a class="headerlink" href="#2cat" title="Permanent link">¶</a></h4>
<ul>
<li>方法一:</li>
</ul>
<div class="highlight"><pre><span class="n">Scan</span> <span class="n">count</span> <span class="mi">1</span><span class="p">,</span>
<span class="n">logical</span> <span class="n">reads</span> <span class="mi">3</span><span class="p">,</span>
<span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">logical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">0</span>
</pre></div>
<ul>
<li>方法二 :+1: </li>
</ul>
<div class="highlight"><pre> <span class="n">Scan</span> <span class="n">count</span> <span class="mi">1</span><span class="p">,</span>
<span class="n">logical</span> <span class="n">reads</span> <span class="mi">2</span><span class="p">,</span>
<span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">logical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mf">0.</span>
</pre></div>
<h4 id="4688">当有4688条符合要求的数据时,查询情况如下<a class="headerlink" href="#4688" title="Permanent link">¶</a></h4>
<ul>
<li>方法一:</li>
</ul>
<div class="highlight"><pre> <span class="n">Scan</span> <span class="n">count</span> <span class="mi">1</span><span class="p">,</span>
<span class="n">logical</span> <span class="n">reads</span> <span class="mi">11</span><span class="p">,</span>
<span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">logical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mf">0.</span>
</pre></div>
<ul>
<li>方法二 :+1: </li>
</ul>
<div class="highlight"><pre><span class="n">Scan</span> <span class="n">count</span> <span class="mi">1</span><span class="p">,</span>
<span class="n">logical</span> <span class="n">reads</span> <span class="mi">2</span><span class="p">,</span>
<span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">logical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mf">0.</span>
</pre></div>
<h4 id="357121317">当没有索引,有357条符合要求的数据,总共有121317条数据时,查询情况如下<a class="headerlink" href="#357121317" title="Permanent link">¶</a></h4>
<ul>
<li>方法一:</li>
</ul>
<div class="highlight"><pre> <span class="n">Scan</span> <span class="n">count</span> <span class="mi">1</span><span class="p">,</span>
<span class="n">logical</span> <span class="n">reads</span> <span class="mi">1241</span><span class="p">,</span>
<span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">331</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">logical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mf">0.</span>
</pre></div>
<ul>
<li>方法二 :+1: </li>
</ul>
<div class="highlight"><pre><span class="n">Scan</span> <span class="n">count</span> <span class="mi">1</span><span class="p">,</span>
<span class="n">logical</span> <span class="n">reads</span> <span class="mi">5</span><span class="p">,</span>
<span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">logical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">physical</span> <span class="n">reads</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">lob</span> <span class="n">read</span><span class="o">-</span><span class="n">ahead</span> <span class="n">reads</span> <span class="mf">0.</span>
</pre></div>
<aside>
<hr/>
<nav>
<ul class="articles_timeline">
<li class="next_article"><a href="http://wbowam.github.io/linux-xiao-ji-qiao-2.html" title="Next: linux 小技巧（2）">linux 小技巧（2）</a> »</li>
</ul>
</nav>
</aside>
<!-- Duoshuo Comment BEGIN -->
<div class="ds-thread" data-author-key="4433755"></div>
<script type="text/javascript">
var duoshuoQuery = {short_name:"tlbog"};
(function() {
var ds = document.createElement('script');
ds.type = 'text/javascript';ds.async = true;
ds.src = 'http://static.duoshuo.com/embed.js';
ds.charset = 'UTF-8';
(document.getElementsByTagName('head')[0]
|| document.getElementsByTagName('body')[0]).appendChild(ds);
})();
</script>
<!-- Duoshuo Comment END -->
</div>
<section>
<div class="span2" style="float:right;font-size:0.9em;">
<h4>Published</h4>
<time pubdate="pubdate" datetime="2013-02-04T00:00:00+08:00">Feb 4, 2013</time>
<h4>Category</h4>
<a class="category-link" href="/categories.html#It-ref">It</a>
<h4>Tags</h4>
<ul class="list-of-tags tags-in-article">
<li><a href="/tags.html#Mysql-ref">Mysql
<span>5</span>
</a></li>
</ul>
</div>
</section>
</div>
</article>
</div>
<div class="span1"></div>
</div>
</div>
</div>
<footer>
<div id="footer">
<ul class="footer-content">
<li class="elegant-power">Powered by <a href="http://getpelican.com/" title="Pelican Home Page">Pelican</a>. Theme: <a href="http://oncrashreboot.com/pelican-elegant" title="Theme Elegant Home Page">Elegant</a> by <a href="http://oncrashreboot.com" title="Talha Mansoor Home Page">Talha Mansoor</a></li>
</ul>
</div>
</footer> <script src="http://code.jquery.com/jquery.min.js"></script>
<script src="//netdna.bootstrapcdn.com/twitter-bootstrap/2.3.1/js/bootstrap.min.js"></script>
<script>
function validateForm(query)
{
return (query.length > 0);
}
</script>
</body>
</html>