<blockquote>要查看mysql索引字段的类型，需先用SHOW INDEX FROM表名获取索引列，再通过DESCRIBE或SHOW CREATE table查看对应列的数据类型，两者结合即可确定索引字段类型。</blockquote> <p><img src=”https://img.php.cn/upload/article/001/503/042/175564986613494.jpeg” alt=”mysql如何查看索引字段类型 mysql表索引字段类型查询教程”></p> <p>要查看mysql索引字段的类型，最直接的方法是先用 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW INDEX FROM your_table_name;</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 命令获取索引对应的列名，然后结合 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DESCRIBE your_table_name;</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 或 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW CREATE TABLE your_table_name;</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 来查找这些列的具体数据类型。毕竟，索引本身是建立在表字段上的，它的类型自然就是对应字段的类型。</p> <h3>解决方案</h3> <p>其实，要搞清楚一个MySQL表的索引字段到底是什么类型，并没有一个单一的、直接的SQL命令能一步到位地告诉你“这个索引是VARchar类型的”。它更像是一个两步走的过程，或者说，需要你理解索引和表结构之间的关系。</p> <p>首先，你需要知道你的表上都有哪些索引，以及这些索引都包含了哪些列。这个可以用 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW INDEX FROM your_table_name;</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 来查看。比如，我们有一个 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>products</pre><div class=”contentsignin”></div></div> 表：</p><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=’brush:sql;toolbar:false;’>SHOW INDEX FROM products;</pre><div class=”contentsignin”></div></div><p>你可能会看到类似这样的结果：</p> <table> <thead><tr> <th style=”text-align: left;”>Table</th> <th style=”text-align: left;”>Non_unique</th> <th style=”text-align: left;”>Key_name</th> <th style=”text-align: left;”>Seq_in_index</th> <th style=”text-align: left;”>column_name</th> <th style=”text-align: left;”>Collation</th> <th style=”text-align: left;”>Cardinality</th> <th style=”text-align: left;”>Sub_part</th> <th style=”text-align: left;”>Packed</th> <th style=”text-align: left;”>NULL</th> <th style=”text-align: left;”>Index_type</th> <th style=”text-align: left;”>Comment</th> <th style=”text-align: left;”>Index_comment</th> </tr></thead> <tbody> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>0</td> <td style=”text-align: left;”>PRIMARY</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>id</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>10000</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>idx_name</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>product_name</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>5000</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>idx_price_category</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>price</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>1000</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>idx_price_category</td> <td style=”text-align: left;”>2</td> <td style=”text-align: left;”>category_id</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>100</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>idx_status_created</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>status</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>3</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>idx_status_created</td> <td style=”text-align: left;”>2</td> <td style=”text-align: left;”>created_at</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>10000</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>idx_desc_prefix</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>description</td> <td style=”text-align: left;”>A</td> <td style=”text-align: left;”>1000</td> <td style=”text-align: left;”>50</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>BTREE</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>products</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>fulltext_desc</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>description</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>FULLTEXT</td> <td style=”text-align: left;”></td> <td style=”text-align: left;”></td> </tr> </tbody> </table> <p>从这个结果里，我们能看到 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Key_name</pre><div class=”contentsignin”></div></div> 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Column_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>，比如 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>idx_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 索引在 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>product_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 列上，<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>idx_price_category</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 索引在 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>price</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>category_id</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 列上。但它没直接告诉我 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>product_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>VARCHAR</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 还是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TEXT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>，<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>price</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DECIMAL</pre><div class=”contentsignin”></div></div> 还是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Float</pre><div class=”contentsignin”></div></div>。</p> <p>接着，你需要查询这些列在表中的实际数据类型。最常用的就是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DESCRIBE your_table_name;</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 或者 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW COLUMNS FROM your_table_name;</pre><div class=”contentsignin”></div></div>。</p><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=’brush:sql;toolbar:false;’>DESCRIBE products;</pre><div class=”contentsignin”></div></div><p>或者</p><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=’brush:sql;toolbar:false;’>SHOW COLUMNS FROM products;</pre><div class=”contentsignin”></div></div><p>这会给你一个更详细的列信息，包括 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Field</pre><div class=”contentsignin”></div></div> (列名) 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Type</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> (数据类型)：</p> <table> <thead><tr> <th style=”text-align: left;”>Field</th> <th style=”text-align: left;”>Type</th> <th style=”text-align: left;”>Null</th> <th style=”text-align: left;”>Key</th> <th style=”text-align: left;”>Default</th> <th style=”text-align: left;”>Extra</th> </tr></thead> <tbody> <tr> <td style=”text-align: left;”>id</td> <td style=”text-align: left;”>int(11)</td> <td style=”text-align: left;”>NO</td> <td style=”text-align: left;”>PRI</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”>auto_increment</td> </tr> <tr> <td style=”text-align: left;”>product_name</td> <td style=”text-align: left;”>varchar(255)</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>MUL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>description</td> <td style=”text-align: left;”>text</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>MUL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>price</td> <td style=”text-align: left;”>decimal(10,2)</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>MUL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>category_id</td> <td style=”text-align: left;”>int(11)</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>MUL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>status</td> <td style=”text-align: left;”>tinyint(4)</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>MUL</td> <td style=”text-align: left;”>1</td> <td style=”text-align: left;”></td> </tr> <tr> <td style=”text-align: left;”>created_at</td> <td style=”text-align: left;”>datetime</td> <td style=”text-align: left;”>YES</td> <td style=”text-align: left;”>MUL</td> <td style=”text-align: left;”>NULL</td> <td style=”text-align: left;”></td> </tr> </tbody> </table> <p>现在，你就可以把 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW INDEX</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 查到的 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Column_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DESCRIBE</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 查到的 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Type</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 对应起来了。比如，<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>idx_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 索引在 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>product_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 列上，而 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>product_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 的类型是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>varchar(255)</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>。<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>idx_price_category</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 索引涉及 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>price</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> (decimal(10,2)) 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>category_id</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> (int(11))。</p> <p>其实，我个人更喜欢用 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW CREATE TABLE your_table_name;</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>，因为它能一次性把表的创建语句、包括所有列定义和索引定义都展示出来。这样，你可以直接在输出中找到索引定义的那一行，然后往上找对应的列定义，更直观一些。</p><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=’brush:sql;toolbar:false;’>SHOW CREATE TABLE products;</pre><div class=”contentsignin”></div></div><p>这个命令会返回一个 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>CREATE TABLE</pre><div class=”contentsignin”></div></div> 语句，里面清晰地列出了每个字段的类型和所有索引的定义，比如：</p><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=’brush:sql;toolbar:false;’>CREATE TABLE `products` ( `id` int(11) NOT NULL AUTO_INCREMENT, `product_name` varchar(255) DEFAULT NULL, `description` text, `price` decimal(10,2) DEFAULT NULL, `category_id` int(11) DEFAULT NULL, `status` tinyint(4) DEFAULT ‘1’, `created_at` datetime DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_name` (`product_name`), KEY `idx_price_category` (`price`,`category_id`), KEY `idx_status_created` (`status`,`created_at`), KEY `idx_desc_prefix` (`description`(50)), FULLTEXT KEY `fulltext_desc` (`description`) ) ENGINE=InnoDB AUTO_INCREMENT=10001 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci</pre><div class=”contentsignin”></div></div><p>从这里一眼就能看出 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>idx_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 是在 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>product_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> (<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>varchar(255)</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>) 上，<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>idx_price_category</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 是在 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>price</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> (<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>decimal(10,2)</pre><div class=”contentsignin”></div></div>) 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>category_id</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> (<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>int(11)</pre><div class=”contentsignin”></div></div>) 上。这个方法，说白了，就是直接看源头，最直接也最不容易出错。</p> <h3> <a >为什么</a>了解索引字段类型很重要？</h3> <p>我发现，很多时候我们只关注索引“有没有”，却很少深入思考索引所基于的字段“是什么类型”。但这其实是个挺要命的细节。对我来说，理解索引字段类型的重要性，主要体现在以下几个方面：</p> <p>首先，它直接关系到查询性能。不同数据类型在存储、比较和检索效率上差异巨大。比如，对 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 类型的索引进行等值查询或范围查询，通常会比对 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>VARCHAR</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 类型（尤其是长字符串）的索引快得多。这是因为整数比较是CPU原生的，而字符串比较涉及到字符集、排序规则，甚至可能需要逐字节比较，开销自然就大了。如果你的索引字段类型选得不好，或者和你的查询条件类型不匹配（比如拿字符串去查一个数字字段，导致<a >隐式类型转换</a>），那这个索引可能就白建了，甚至会拖慢查询。我见过不少因为<a >隐式转换</a>导致索引失效的案例，那真是让人头疼。</p> <p>其次，存储空间和内存消耗。一个 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>BIGINT</pre><div class=”contentsignin”></div></div> 索引占用的空间肯定比 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TINYINT</pre><div class=”contentsignin”></div></div> 大，一个 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>varchar(255)</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 索引又比 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 大得多。索引是需要占用磁盘空间和内存的，尤其是当你的表数据量非常大时，索引的大小直接影响到你的数据库性能（比如，更多的索引数据需要从磁盘加载到内存，可能导致更多的I/O操作）。我总觉得，能用小类型就用小类型，这是数据库设计的一个基本原则，也同样适用于索引字段。</p> <p>再者，索引的有效性和选择性。某些数据类型天生就不适合做索引，比如 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TEXT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 或 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>BLOB</pre><div class=”contentsignin”></div></div>。虽然MySQL允许你对它们创建前缀索引（<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>description(50)</pre><div class=”contentsignin”></div></div> 这种），但你得清楚，这种索引只覆盖了字段的前一部分内容。如果你的查询条件超出了这个前缀范围，或者需要全文本搜索，那这个前缀索引可能就帮不上忙了。了解字段类型能帮助我们判断当前索引的设计是否合理，或者是否需要考虑全文索引、哈希索引等其他方案。</p> <p>最后，也是我个人比较关注的一点，就是数据一致性和规范性。当你清楚索引字段的类型时，在编写SQL查询时，你会自然而然地注意数据类型匹配。这不仅能避免索引失效，还能减少潜在的数据类型转换错误，让你的查询更健壮。比如，如果我知道 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>product_id</pre><div class=”contentsignin”></div></div> 是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 类型，我就会避免写 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>WHERE product_id = ‘123’</pre><div class=”contentsignin”></div></div> 这种带引号的查询，虽然MySQL多数时候能处理，但这种习惯性的严谨能避免很多不必要的麻烦。</p> <h3>如何通过SQL查询更全面地获取索引信息，包括字段类型？</h3> <p>刚才我们提到了 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SHOW INDEX</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DESCRIBE</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 分开查，然后手动匹配。但如果表很多，或者要批量分析，这种方式效率就太低了。这时候，我就倾向于直接查询 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INFORMATION_SCHEMA</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 数据库。这是MySQL提供的一个元数据数据库，里面包含了关于数据库、表、列、索引等等所有的元信息。</p> <p>要一次性获取某个表的所有索引及其对应的字段类型，我们可以通过连接 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INFORMATION_SCHEMA.STATISTICS</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 表（它包含了索引信息）和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INFORMATION_SCHEMA.COLUMNS</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 表（它包含了列的详细信息，包括类型）来实现。</p> <p>这是一个我常用的SQL查询，它能帮你把这些信息整合起来：</p><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=’brush:sql;toolbar:false;’>SELECT s.TABLE_SCHEMA, s.TABLE_NAME, s.INDEX_NAME, s.SEQ_IN_INDEX, s.COLUMN_NAME, c.COLUMN_TYPE, c.DATA_TYPE, c.CHARACTER_MAXIMUM_LENGTH, s.INDEX_TYPE, s.SUB_PART, s.NON_UNIQUE, s.CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS s JOIN INFORMATION_SCHEMA.COLUMNS c ON s.TABLE_SCHEMA = c.TABLE_SCHEMA AND s.TABLE_NAME = c.TABLE_NAME AND s.COLUMN_NAME = c.COLUMN_NAME WHERE s.TABLE_SCHEMA = ‘your_database_name’ AND s.TABLE_NAME = ‘your_table_name’ ORDER BY s.TABLE_NAME, s.INDEX_NAME, s.SEQ_IN_INDEX;</pre><div class=”contentsignin”></div></div><p>这个查询的逻辑是这样的：</p> <ul> <li><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INFORMATION_SCHEMA.STATISTICS</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 表提供了关于索引的详细信息，包括 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INDEX_NAME</pre><div class=”contentsignin”></div></div>（索引名）、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Column_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>（索引列名）、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>SEQ_IN_INDEX</pre><div class=”contentsignin”></div></div>（列在索引中的顺序）、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INDEX_TYPE</pre><div class=”contentsignin”></div></div>（索引类型，如BTREE、FULLTEXT）等。</li> <li><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INFORMATION_SCHEMA.COLUMNS</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 表则包含了所有列的元数据，比如 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>COLUMN_TYPE</pre><div class=”contentsignin”></div></div>（完整的列类型，如<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>varchar(255)</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>）、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DATA_TYPE</pre><div class=”contentsignin”></div></div>（基本数据类型，如<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>VARCHAR</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>）、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>CHARACTER_MAXIMUM_LENGTH</pre><div class=”contentsignin”></div></div>（字符类型最大长度）等。</li> <li>我们通过 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TABLE_SCHEMA</pre><div class=”contentsignin”></div></div>、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TABLE_NAME</pre><div class=”contentsignin”></div></div> 和 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>Column_name</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 将这两个表连接起来，这样就能把索引信息和它所基于的列的类型信息对应上。</li> <li>最后，用 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>WHERE</pre><div class=”contentsignin”></div></div> 子句过滤出你感兴趣的数据库和表。</li> </ul> <p>这样一来，你就能得到一个非常详细的列表，清晰地展示了每个索引、它包含的列、以及这些列的具体数据类型。这对于进行数据库性能分析、索引优化或者做数据库审计都非常有帮助。我个人觉得，掌握这种通过 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INFORMATION_SCHEMA</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 查询元数据的方法，是深入理解MySQL的一个重要技能点。</p> <h3>索引字段类型对查询性能有哪些具体影响？</h3> <p>索引字段的类型，远不止是定义数据存储格式那么简单，它对查询性能的影响是多维度且深远的。从我的经验来看，以下几个方面是特别值得关注的：</p> <ol> <li> <p><strong>整数类型（INT, BIGINT, TINYINT等）：</strong></p> <ul> <li> <strong>优点：</strong> 这是我最喜欢用作索引的类型。它们占用空间小，比较操作非常高效，因为CPU处理整数是原生的。在B-树索引中，整数的比较和排序速度极快，查找路径短。</li> <li> <strong>影响：</strong> 如果你的主键是自增整数，那插入性能通常也很好，因为新数据总是追加到索引的末尾，避免了页分裂和碎片化。范围查询（<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>BETWEEN</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>></pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”><</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>）在整数索引上表现也极佳。</li> </ul> </li> <li> <p><strong>字符串类型（VARCHAR, CHAR, TEXT等）：</strong></p> <ul> <li> <strong>优点：</strong> 灵活，能存储各种文本信息。</li> <li> <strong>影响：</strong><ul> <li> <strong>比较开销：</strong> 字符串比较比整数复杂得多，涉及到字符集、排序规则。即使是等值比较，也可能比整数慢好几倍。</li> <li> <strong>存储空间：</strong> <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>VARCHAR</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 理论上按需存储，但索引存储的还是实际内容。长字符串索引会占用更多空间，导致B-树的每个节点能存储的键值数量减少，树的高度增加，查找I/O次数增多。</li> <li> <strong>前缀索引：</strong> 对于 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TEXT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 或非常长的 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>VARCHAR</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>，通常只能创建前缀索引（例如 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>INDEX(description(50))</pre><div class=”contentsignin”></div></div>）。这意味着只有查询条件能匹配到前缀时才能使用索引，如果查询条件超出了前缀范围，或者需要全文本匹配，索引就失效了。我经常看到有人对 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TEXT</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 字段建了索引但查询还是慢，一查才发现是前缀索引没覆盖到查询范围。</li> <li> <strong>字符集和排序规则：</strong> 如果字符串列的字符集或排序规则不一致，或者与查询条件不匹配，可能导致索引失效或性能下降。</li> </ul> </li> </ul> </li> <li> <p><strong>日期时间类型（DATE, DATETIME, timestamp）：</strong></p> <ul> <li> <strong>优点：</strong> 适合时间范围查询。MySQL对日期时间类型的索引优化做得很好，可以高效地进行 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>BETWEEN</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>></pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div>、<div class=”code” style=”position:relative; padding:0px; margin:0px;”><div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”><</pre><div class=”contentsignin”></div></div><div class=”contentsignin”></div></div> 等范围查询。</li> <li> <strong>影响：</strong><ul> <li> <strong>格式：</strong> 查询时如果日期格式不匹配（比如把 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>DATETIME</pre><div class=”contentsignin”></div></div> 字段用字符串 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>2023-01-01</pre><div class=”contentsignin”></div></div> 查，而不是 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>CAST</pre><div class=”contentsignin”></div></div> 成日期类型），可能导致隐式转换，进而使索引失效。</li> <li> <strong>时区：</strong> <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>TIMESTAMP</pre><div class=”contentsignin”></div></div> 类型会受时区影响，在跨时区应用中需要特别注意，否则可能导致数据不一致或查询结果不符预期。</li> </ul> </li> </ul> </li> <li> <p><strong>NULL值：</strong></p> <ul><li> <strong>影响：</strong> 大多数B-树索引都能包含NULL值，但对NULL值的处理方式可能因存储引擎而异。例如，<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>WHERE column IS NULL</pre><div class=”contentsignin”></div></div> 或 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>WHERE column IS NOT NULL</pre><div class=”contentsignin”></div></div> 这样的查询，在有索引的情况下通常能有效利用索引。但如果你的业务逻辑经常需要过滤NULL值，且该列允许NULL，那么你需要确认索引是否能有效支持。</li></ul> </li> <li> <p><strong>隐式类型转换：</strong></p> <ul> <li> <strong>致命影响：</strong> 这是我见过最常见的索引失效原因之一。如果查询条件的数据类型与索引列的数据类型不匹配，MySQL可能会尝试进行隐式类型转换。例如，<div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>WHERE int_column = ‘123’</pre><div class=”contentsignin”></div></div>（将整数列与字符串比较），MySQL可能会将 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>int_column</pre><div class=”contentsignin”></div></div> 转换为字符串再进行比较，这就导致索引无法使用，变成了全表扫描。</li> <li> <strong>避免方法：</strong> 始终确保查询条件的数据类型与索引列的数据类型严格匹配。必要时使用 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>CAST()</pre><div class=”contentsignin”></div></div> 或 <div class=”code” style=”position:relative; padding:0px; margin:0px;”><pre class=”brush:php;toolbar:false”>CONVERT()</pre><div class=”contentsignin”></div></div> 进行显式转换，但最好是在查询条件侧转换，而不是让数据库转换索引列。</li> </ul> </li> </ol> <p>说到底，选择正确的索引字段类型，并确保查询时数据类型匹配，是优化MySQL查询性能的基础。这不是什么玄学，而是实实在在的技术细节。</p>
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