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<h1>
<a href="./mysqlxi-lie-si.html">
MySQL系列四
<small class="subtitle">
分区表和主从复制
</small>
</a>
</h1>
</header>
</div>
<div class="row-fluid">
<div class="span8 offset2 article-content">
<h2>前述</h2>
<p>本篇前面讲解分区表，最好讲解主从复制的一些原理和架构思想。这里不打算讲主从复制的搭建。</p>
<h2>分区表概述</h2>
<p>分区表对用户来说仍然是一个独立的逻辑表，但是底层由多个物理子表组成。首先我们使用下面的语句创建一个分区表，来看看分区表的组成。对于分区表的创建类型，后面会讲。</p>
<div class="highlight"><pre><span></span><span class="k">CREATE</span> <span class="k">TABLE</span> <span class="nf">user_partition</span><span class="p">(</span>
<span class="n">id</span> <span class="kt">INT</span><span class="p">,</span>
<span class="n">name</span> <span class="kt">char</span><span class="p">(</span><span class="mi">20</span><span class="p">),</span>
<span class="n">age</span> <span class="kt">int</span>
<span class="p">)</span>
<span class="n">PARTITION</span> <span class="k">BY</span> <span class="nf">RANGE</span> <span class="p">(</span><span class="n">age</span><span class="p">)</span> <span class="p">(</span>
<span class="n">PARTITION</span> <span class="n">u1</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">6</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">u2</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">18</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">u3</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">30</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">u4</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">50</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">u5</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">65</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">u6</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">80</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">u7</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">100</span><span class="p">),</span>
<span class="n">PARTITION</span> <span class="n">uother</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="n">MAXVALUE</span><span class="p">)</span>
<span class="p">);</span>
</pre></div>
<p>我在测试库<code>partition_db</code>创建了一个分区表<code>user_partition</code>，并根据年龄范围存储在8个子分区表中。用<code>show tables</code>看看建表情况，显示的情况和普通表一样，只有一个。这也说明了分区表对用户逻辑上只有一个。</p>
<div class="highlight"><pre><span></span>mysql> show tables<span class="p">;</span>
+------------------------+
<span class="p">|</span> Tables_in_partition_db <span class="p">|</span>
+------------------------+
<span class="p">|</span> my_range_datetime <span class="p">|</span>
<span class="p">|</span> user_partition <span class="p">|</span>
+------------------------+
<span class="m">2</span> rows in <span class="nb">set</span> <span class="o">(</span><span class="m">0</span>.01 sec<span class="o">)</span>
</pre></div>
<p>接下来我们看看底层存储情况，进入MySQL数据存储目录，在Linux系统上，默认是/var/lib/mysql，具体位置要根据你的配置。找到<code>partition_db</code>库目录，通过<code>ls -l | grep user</code>查看表文件情况。</p>
<div class="highlight"><pre><span></span>-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u1.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u2.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u3.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u4.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u5.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u6.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#u7.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">98304</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition#P#uother.ibd
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">8614</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition.frm
-rw-rw---- <span class="m">1</span> xufan staff <span class="m">52</span> <span class="m">3</span> <span class="m">19</span> <span class="m">19</span>:43 user_partition.par
</pre></div>
<p>上面是我的机器上显示结果。可以看到，分区表底层存储对应的是多个分区子表，这里是8个，就是上面展示的以#分隔命名的标文件。<code>user_partition.frm</code>文件存储表结构，<code>user_partition.par</code>存储分区信息。</p>
<p>由于分区表是对底层表的封装，所以索引等也是独立建立在分区表子表上，而没有全局索引。对分区表的请求，都会通过句柄对象，转换为对存储引擎接口的调用，最后落实到具体的分区子表上。</p>
<h2>分区表的操作</h2>
<h5>1: 查看分区</h5>
<div class="highlight"><pre><span></span><span class="n">mysql</span><span class="o">></span> <span class="k">SELECT</span> <span class="n">PARTITION_NAME</span><span class="p">,</span><span class="n">TABLE_ROWS</span> <span class="k">FROM</span> <span class="n">INFORMATION_SCHEMA</span><span class="p">.</span><span class="n">PARTITIONS</span> <span class="k">WHERE</span> <span class="n">TABLE_NAME</span> <span class="o">=</span> <span class="s1">'user_partition'</span><span class="p">;</span>
<span class="o">+----------------+------------+</span>
<span class="o">|</span> <span class="n">PARTITION_NAME</span> <span class="o">|</span> <span class="n">TABLE_ROWS</span> <span class="o">|</span>
<span class="o">+----------------+------------+</span>
<span class="o">|</span> <span class="n">u1</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">u2</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">u3</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">u4</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">u5</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">u6</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">u7</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">|</span> <span class="n">uother</span> <span class="o">|</span> <span class="mi">0</span> <span class="o">|</span>
<span class="o">+----------------+------------+</span>
<span class="mi">8</span> <span class="n">rows</span> <span class="k">in</span> <span class="kt">set</span> <span class="p">(</span><span class="mi">0</span><span class="p">.</span><span class="mi">01</span> <span class="n">sec</span><span class="p">)</span>
</pre></div>
<p>我们看到，<code>user_partition</code>分区表有8个分区子表组成。</p>
<h5>2: 增加和删除分区子表</h5>
<div class="highlight"><pre><span></span><span class="k">ALTER</span> <span class="k">TABLE</span> <span class="n">user_partition</span> <span class="k">ADD</span> <span class="nf">PARTITION</span> <span class="p">(</span><span class="n">PARTITION</span> <span class="n">u6_1</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">90</span><span class="p">));</span>
</pre></div>
<p>执行上面的命令，会报错<code>MAXVALUE can only be used in last partition definition</code>。可以看到，增加分区只能递增的形式，新分区不能再包含已有分区的值。</p>
<p>用下面的命令先删除最后一个分区</p>
<div class="highlight"><pre><span></span><span class="k">ALTER</span> <span class="k">TABLE</span> <span class="ss">`user_partition`</span> <span class="k">DROP</span> <span class="n">PARTITION</span> <span class="n">uother</span><span class="p">;</span>
</pre></div>
<p>再执行上面的命令，现在报<code>VALUES LESS THAN value must be strictly increasing for each partitio</code>，这是因为上面所说的，新增加的子分区不能包含已有分区的值。</p>
<p>将增加分区的数值改改，如下，执行成功。</p>
<div class="highlight"><pre><span></span><span class="k">ALTER</span> <span class="k">TABLE</span> <span class="n">user_partition</span> <span class="k">ADD</span> <span class="nf">PARTITION</span> <span class="p">(</span><span class="n">PARTITION</span> <span class="n">u6_1</span> <span class="k">VALUES</span> <span class="n">LESS</span> <span class="nf">THAN</span> <span class="p">(</span><span class="mi">120</span><span class="p">));</span>
</pre></div>
<blockquote>
<p>如果对于已经存在的一个表，且已经有数据，需要把它变成分区表，可执行下面的命令。</p>
<p>alter table user partition by range(age) (
PARTITION u1 VALUES LESS THAN (6),
PARTITION u2 VALUES LESS THAN (18)
);</p>
</blockquote>
<h5>3: 分区表的使用</h5>
<p>分区表的使用和普通表一样，直接插入就行，查询也是。</p>
<div class="highlight"><pre><span></span><span class="k">INSERT</span> <span class="k">INTO</span> <span class="nf">user_partion</span><span class="p">(</span><span class="n">name</span><span class="p">,</span><span class="n">age</span><span class="p">)</span> <span class="k">VALUES</span><span class="p">(</span><span class="s2">"zhangsan"</span><span class="p">,</span> <span class="mi">30</span><span class="p">);</span>
<span class="k">INSERT</span> <span class="k">INTO</span> <span class="nf">user_partion</span><span class="p">(</span><span class="n">name</span><span class="p">,</span><span class="n">age</span><span class="p">)</span> <span class="k">VALUES</span><span class="p">(</span><span class="s2">"wanger"</span><span class="p">,</span> <span class="mi">105</span><span class="p">);</span>
</pre></div>
<div class="highlight"><pre><span></span><span class="n">mysql</span><span class="o">></span> <span class="k">select</span> <span class="o">*</span> <span class="k">from</span> <span class="n">user_partition</span><span class="p">;</span>
<span class="o">+------+----------+------+</span>
<span class="o">|</span> <span class="n">id</span> <span class="o">|</span> <span class="n">name</span> <span class="o">|</span> <span class="n">age</span> <span class="o">|</span>
<span class="o">+------+----------+------+</span>
<span class="o">|</span> <span class="no">NULL</span> <span class="o">|</span> <span class="n">zhangsan</span> <span class="o">|</span> <span class="mi">30</span> <span class="o">|</span>
<span class="o">|</span> <span class="no">NULL</span> <span class="o">|</span> <span class="n">wanger</span> <span class="o">|</span> <span class="mi">105</span> <span class="o">|</span>
<span class="o">+------+----------+------+</span>
<span class="mi">2</span> <span class="n">rows</span> <span class="k">in</span> <span class="kt">set</span> <span class="p">(</span><span class="mi">0</span><span class="p">.</span><span class="mi">00</span> <span class="n">sec</span><span class="p">)</span>
</pre></div>
<p>对应到底层，会先锁住分区表的所有底层表，再调用存储引擎接口操作对应的底层表。</p>
<h2>分区表的创建类型</h2>
<h5>1: RANGE分区</h5>
<p>基于属于一个给定连续区间的列值，把多行分配给分区。基于分区的列判断数值需是整型，如果不是需要用函数转换。如时间类型，可以用to_days函数转换为整型。</p>
<h5>2: LIST分区</h5>
<p>LIST分区和RANGE分区类似，区别在于LIST是枚举值列表的集合，RANGE是连续的区间值的集合。二者在语法方面非常的相似。</p>
<h5>3: HASH分区</h5>
<p>基于给定的分区个数，将数据分配到不同的分区，HASH分区只能针对整数进行HASH，对于非整形的字段只能通过表达式将其转换成整数。</p>
<h5>3: KEY分区</h5>
<p>KEY分区其实跟HASH分区差不多，不同点如下：</p>
<ol>
<li>
<p>KEY分区允许多列，而HASH分区只允许一列。</p>
</li>
<li>
<p>如果在有主键或者唯一键的情况下，key中分区列可不指定，默认为主键或者唯一键，如果没有，则必须显性指定列。</p>
</li>
<li>
<p>KEY分区对象必须为列，而不能是基于列的表达式。</p>
</li>
<li>
<p>KEY分区和HASH分区的算法不一样，PARTITION BY HASH (expr)，MOD取值的对象是expr返回的值，而PARTITION BY KEY (column_list)，基于的是列的MD5值。</p>
</li>
</ol>
<p>更多分区类型详情，请参考<a href="http://mysql.taobao.org/monthly/2017/11/09/">分区表基本类型</a></p>
<h2>分区表如何跨存储</h2>
<p>创建分区表时，可以分别通过<code>INDEX DIRECTORY</code>和<code>DATA DIRECTORY</code>指定分区子表数据文件和索引文件存储路径。如果路径是挂载的磁盘、RAID或者NFS，可以将数据保持到多个存储介质上。</p>
<h2>分表和分区表</h2>
<p>分表，是将一张大表拆分成多张逻辑表，用户层面看到多张表，而MySQL分区表用户层面只看到一张。对于分表，客户端应用可以根据id或其他一些条件判断读哪一张表，或者加一层代理。</p>
<p>分表实现比较灵活，现在有Merge存储引擎支持分表操作。</p>
<hr>
<p><em>后述：</em></p>
<p>对分区表的使用有限，没有太多实战经验，因此这里就不谈使用场景等了，先把基础的知识记录下来，后面有更多的使用经验或心得，再来更新。</p>
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