{"id":251,"date":"2023-02-24T06:09:10","date_gmt":"2023-02-23T22:09:10","guid":{"rendered":"https:\/\/www.appblog.cn\/?p=251"},"modified":"2023-04-30T15:19:16","modified_gmt":"2023-04-30T07:19:16","slug":"mysql-explain-details","status":"publish","type":"post","link":"https:\/\/www.appblog.cn\/index.php\/2023\/02\/24\/mysql-explain-details\/","title":{"rendered":"MySQL Explain\u8be6\u89e3"},"content":{"rendered":"<p>\u5728\u65e5\u5e38\u5de5\u4f5c\u4e2d\uff0c\u6211\u4eec\u6709\u65f6\u4f1a\u5f00\u6162\u67e5\u8be2\u53bb\u8bb0\u5f55\u4e00\u4e9b\u6267\u884c\u65f6\u95f4\u6bd4\u8f83\u4e45\u7684SQL\u8bed\u53e5\uff0c\u627e\u51fa\u8fd9\u4e9bSQL\u8bed\u53e5\u5e76\u4e0d\u610f\u5473\u7740\u5b8c\u4e8b\u4e86\uff0c\u6709\u65f6\u6211\u4eec\u5e38\u5e38\u7528\u5230explain\u8fd9\u4e2a\u547d\u4ee4\u6765\u67e5\u770b\u4e00\u4e2a\u8fd9\u4e9bSQL\u8bed\u53e5\u7684\u6267\u884c\u8ba1\u5212\uff0c\u67e5\u770b\u8be5SQL\u8bed\u53e5\u6709\u6ca1\u6709\u4f7f\u7528\u4e0a\u4e86\u7d22\u5f15\uff0c\u6709\u6ca1\u6709\u505a\u5168\u8868\u626b\u63cf\uff0c\u8fd9\u90fd\u53ef\u4ee5\u901a\u8fc7explain\u547d\u4ee4\u6765\u67e5\u770b\u3002\u6240\u4ee5\u6211\u4eec\u6df1\u5165\u4e86\u89e3MySQL\u7684\u57fa\u4e8e\u5f00\u9500\u7684\u4f18\u5316\u5668\uff0c\u8fd8\u53ef\u4ee5\u83b7\u5f97\u5f88\u591a\u53ef\u80fd\u88ab\u4f18\u5316\u5668\u8003\u8651\u5230\u7684\u8bbf\u95ee\u7b56\u7565\u7684\u7ec6\u8282\uff0c\u4ee5\u53ca\u5f53\u8fd0\u884cSQL\u8bed\u53e5\u65f6\u54ea\u79cd\u7b56\u7565\u9884\u8ba1\u4f1a\u88ab\u4f18\u5316\u5668\u91c7\u7528\u3002<\/p>\n<pre><code class=\"language-sql\">-- \u5b9e\u9645SQL\uff0c\u67e5\u627e\u7528\u6237\u540d\u4e3aJefabc\u7684\u5458\u5de5\nselect * from emp where name = &#039;Jefabc&#039;;\n-- \u67e5\u770bSQL\u662f\u5426\u4f7f\u7528\u7d22\u5f15\uff0c\u524d\u9762\u52a0\u4e0aexplain\u5373\u53ef\nexplain select * from emp where name = &#039;Jefabc&#039;;<\/code><\/pre>\n<p><!-- more --><\/p>\n<p><img decoding=\"async\" src=\"http:\/\/www.yezhou.me\/AppBlog\/images\/\u6570\u636e\u5e93\/MySQL-Explain\u57fa\u672c\u4f7f\u7528.png\" alt=\"MySQL-Explain\u57fa\u672c\u4f7f\u7528\" \/><\/p>\n<p>expain\u51fa\u6765\u7684\u4fe1\u606f\u670910\u5217\uff0c\u5206\u522b\u662fid\u3001select_type\u3001table\u3001type\u3001possible_keys\u3001key\u3001key_len\u3001ref\u3001rows\u3001Extra<\/p>\n<p>\u6982\u8981\u63cf\u8ff0\uff1a<\/p>\n<ul>\n<li>id: \u9009\u62e9\u6807\u8bc6\u7b26<\/li>\n<li>select_type: \u8868\u793a\u67e5\u8be2\u7684\u7c7b\u578b\u3002<\/li>\n<li>table: \u8f93\u51fa\u7ed3\u679c\u96c6\u7684\u8868<\/li>\n<li>partitions: \u5339\u914d\u7684\u5206\u533a<\/li>\n<li>type: \u8868\u793a\u8868\u7684\u8fde\u63a5\u7c7b\u578b<\/li>\n<li>possible_keys: \u8868\u793a\u67e5\u8be2\u65f6\uff0c\u53ef\u80fd\u4f7f\u7528\u7684\u7d22\u5f15<\/li>\n<li>key: \u8868\u793a\u5b9e\u9645\u4f7f\u7528\u7684\u7d22\u5f15<\/li>\n<li>key_len: \u7d22\u5f15\u5b57\u6bb5\u7684\u957f\u5ea6<\/li>\n<li>ref: \u5217\u4e0e\u7d22\u5f15\u7684\u6bd4\u8f83<\/li>\n<li>rows: \u626b\u63cf\u51fa\u7684\u884c\u6570(\u4f30\u7b97\u7684\u884c\u6570)<\/li>\n<li>filtered: \u6309\u8868\u6761\u4ef6\u8fc7\u6ee4\u7684\u884c\u767e\u5206\u6bd4<\/li>\n<li>Extra: \u6267\u884c\u60c5\u51b5\u7684\u63cf\u8ff0\u548c\u8bf4\u660e<\/li>\n<\/ul>\n<p>\u4e0b\u9762\u5bf9\u8fd9\u4e9b\u5b57\u6bb5\u51fa\u73b0\u7684\u53ef\u80fd\u8fdb\u884c\u89e3\u91ca\uff1a<\/p>\n<h3>id<\/h3>\n<p>SELECT\u8bc6\u522b\u7b26\u3002\u8fd9\u662fSELECT\u7684\u67e5\u8be2\u5e8f\u5217\u53f7<\/p>\n<p><strong>\u6211\u7684\u7406\u89e3\u662fSQL\u6267\u884c\u7684\u987a\u5e8f\u7684\u6807\u8bc6\uff0cSQL\u4ece\u5927\u5230\u5c0f\u7684\u6267\u884c<\/strong><\/p>\n<ol>\n<li>id\u76f8\u540c\u65f6\uff0c\u6267\u884c\u987a\u5e8f\u7531\u4e0a\u81f3\u4e0b<\/li>\n<li>\u5982\u679c\u662f\u5b50\u67e5\u8be2\uff0cid\u7684\u5e8f\u53f7\u4f1a\u9012\u589e\uff0cid\u503c\u8d8a\u5927\u4f18\u5148\u7ea7\u8d8a\u9ad8\uff0c\u8d8a\u5148\u88ab\u6267\u884c<\/li>\n<li>id\u5982\u679c\u76f8\u540c\uff0c\u53ef\u4ee5\u8ba4\u4e3a\u662f\u4e00\u7ec4\uff0c\u4ece\u4e0a\u5f80\u4e0b\u987a\u5e8f\u6267\u884c\uff1b\u5728\u6240\u6709\u7ec4\u4e2d\uff0cid\u503c\u8d8a\u5927\uff0c\u4f18\u5148\u7ea7\u8d8a\u9ad8\uff0c\u8d8a\u5148\u6267\u884c<\/li>\n<\/ol>\n<pre><code class=\"language-sql\">-- \u67e5\u770b\u5728\u7814\u53d1\u90e8\u5e76\u4e14\u540d\u5b57\u4ee5Jef\u5f00\u5934\u7684\u5458\u5de5\uff0c\u7ecf\u5178\u67e5\u8be2\nexplain select e.no, e.name from emp e left join dept d on e.dept_no = d.no where e.name like &#039;Jef%&#039; and d.name = &#039;\u7814\u53d1\u90e8&#039;;<\/code><\/pre>\n<p><img decoding=\"async\" src=\"http:\/\/www.yezhou.me\/AppBlog\/images\/\u6570\u636e\u5e93\/MySQL-Explain\u5173\u8054\u67e5\u8be2.png\" alt=\"MySQL-Explain\u5173\u8054\u67e5\u8be2\" \/><\/p>\n<h3>select_type<\/h3>\n<p><strong>\u6307\u793a\u67e5\u8be2\u4e2d\u6bcf\u4e2aselect\u5b50\u53e5\u7684\u7c7b\u578b<\/strong><\/p>\n<ul>\n<li>(1) SIMPLE(\u7b80\u5355SELECT\uff0c\u4e0d\u4f7f\u7528UNION\u6216\u5b50\u67e5\u8be2\u7b49)<\/li>\n<li>(2) PRIMARY(\u5b50\u67e5\u8be2\u4e2d\u6700\u5916\u5c42\u67e5\u8be2\uff0c\u67e5\u8be2\u4e2d\u82e5\u5305\u542b\u4efb\u4f55\u590d\u6742\u7684\u5b50\u90e8\u5206\uff0c\u6700\u5916\u5c42\u7684select\u88ab\u6807\u8bb0\u4e3aPRIMARY)<\/li>\n<li>(3) UNION(UNION\u4e2d\u7684\u7b2c\u4e8c\u4e2a\u6216\u540e\u9762\u7684SELECT\u8bed\u53e5)<\/li>\n<li>(4) DEPENDENT UNION(UNION\u4e2d\u7684\u7b2c\u4e8c\u4e2a\u6216\u540e\u9762\u7684SELECT\u8bed\u53e5\uff0c\u53d6\u51b3\u4e8e\u5916\u9762\u7684\u67e5\u8be2)<\/li>\n<li>(5) UNION RESULT(UNION\u7684\u7ed3\u679c\uff0cUNION\u8bed\u53e5\u4e2d\u7b2c\u4e8c\u4e2aselect\u5f00\u59cb\u540e\u9762\u6240\u6709select)<\/li>\n<li>(6) SUBQUERY(\u5b50\u67e5\u8be2\u4e2d\u7684\u7b2c\u4e00\u4e2aSELECT\uff0c\u7ed3\u679c\u4e0d\u4f9d\u8d56\u4e8e\u5916\u90e8\u67e5\u8be2)<\/li>\n<li>(7) DEPENDENT SUBQUERY(\u5b50\u67e5\u8be2\u4e2d\u7684\u7b2c\u4e00\u4e2aSELECT\uff0c\u4f9d\u8d56\u4e8e\u5916\u90e8\u67e5\u8be2)<\/li>\n<li>(8) DERIVED(\u6d3e\u751f\u8868\u7684SELECT, FROM\u5b50\u53e5\u7684\u5b50\u67e5\u8be2)<\/li>\n<li>(9) UNCACHEABLE SUBQUERY(\u4e00\u4e2a\u5b50\u67e5\u8be2\u7684\u7ed3\u679c\u4e0d\u80fd\u88ab\u7f13\u5b58\uff0c\u5fc5\u987b\u91cd\u65b0\u8bc4\u4f30\u5916\u94fe\u63a5\u7684\u7b2c\u4e00\u884c)<\/li>\n<\/ul>\n<h3>table<\/h3>\n<p>\u663e\u793a\u8fd9\u4e00\u6b65\u6240\u8bbf\u95ee\u6570\u636e\u5e93\u4e2d\u8868\u540d\u79f0\uff08\u663e\u793a\u8fd9\u4e00\u884c\u7684\u6570\u636e\u662f\u5173\u4e8e\u54ea\u5f20\u8868\u7684\uff09\uff0c\u6709\u65f6\u4e0d\u662f\u771f\u5b9e\u7684\u8868\u540d\u5b57\uff0c\u53ef\u80fd\u662f\u7b80\u79f0\uff0c\u4f8b\u5982\u4e0a\u9762\u7684e\uff0cd\uff0c\u4e5f\u53ef\u80fd\u662f\u7b2c\u51e0\u6b65\u6267\u884c\u7684\u7ed3\u679c\u7684\u7b80\u79f0<\/p>\n<h3>type<\/h3>\n<p>\u5bf9\u8868\u8bbf\u95ee\u65b9\u5f0f\uff0c\u8868\u793aMySQL\u5728\u8868\u4e2d\u627e\u5230\u6240\u9700\u884c\u7684\u65b9\u5f0f\uff0c\u53c8\u79f0\u201c\u8bbf\u95ee\u7c7b\u578b\u201d\u3002<\/p>\n<p>\u5e38\u7528\u7684\u7c7b\u578b\u6709\uff1a<strong>ALL\u3001index\u3001range\u3001 ref\u3001eq_ref\u3001const\u3001system\u3001NULL\uff08\u4ece\u5de6\u5230\u53f3\uff0c\u6027\u80fd\u4ece\u5dee\u5230\u597d\uff09<\/strong><\/p>\n<ul>\n<li>ALL\uff1aFull Table Scan\uff0cMySQL\u5c06\u904d\u5386\u5168\u8868\u4ee5\u627e\u5230\u5339\u914d\u7684\u884c<\/li>\n<li>index: Full Index Scan\uff0cindex\u4e0eALL\u533a\u522b\u4e3aindex\u7c7b\u578b\u53ea\u904d\u5386\u7d22\u5f15\u6811<\/li>\n<li>range: \u53ea\u68c0\u7d22\u7ed9\u5b9a\u8303\u56f4\u7684\u884c\uff0c\u4f7f\u7528\u4e00\u4e2a\u7d22\u5f15\u6765\u9009\u62e9\u884c<\/li>\n<li>ref: \u8868\u793a\u4e0a\u8ff0\u8868\u7684\u8fde\u63a5\u5339\u914d\u6761\u4ef6\uff0c\u5373\u54ea\u4e9b\u5217\u6216\u5e38\u91cf\u88ab\u7528\u4e8e\u67e5\u627e\u7d22\u5f15\u5217\u4e0a\u7684\u503c<\/li>\n<li>eq_ref: \u7c7b\u4f3cref\uff0c\u533a\u522b\u5c31\u5728\u4f7f\u7528\u7684\u7d22\u5f15\u662f\u552f\u4e00\u7d22\u5f15\uff0c\u5bf9\u4e8e\u6bcf\u4e2a\u7d22\u5f15\u952e\u503c\uff0c\u8868\u4e2d\u53ea\u6709\u4e00\u6761\u8bb0\u5f55\u5339\u914d\uff0c\u7b80\u5355\u6765\u8bf4\uff0c\u5c31\u662f\u591a\u8868\u8fde\u63a5\u4e2d\u4f7f\u7528<code>primary key<\/code>\u6216\u8005<code>unique key<\/code>\u4f5c\u4e3a\u5173\u8054\u6761\u4ef6<\/li>\n<li>const\u3001system: \u5f53MySQL\u5bf9\u67e5\u8be2\u67d0\u90e8\u5206\u8fdb\u884c\u4f18\u5316\uff0c\u5e76\u8f6c\u6362\u4e3a\u4e00\u4e2a\u5e38\u91cf\u65f6\uff0c\u4f7f\u7528\u8fd9\u4e9b\u7c7b\u578b\u8bbf\u95ee\u3002\u5982\u5c06\u4e3b\u952e\u7f6e\u4e8ewhere\u5217\u8868\u4e2d\uff0cMySQL\u5c31\u80fd\u5c06\u8be5\u67e5\u8be2\u8f6c\u6362\u4e3a\u4e00\u4e2a\u5e38\u91cf\uff0csystem\u662fconst\u7c7b\u578b\u7684\u7279\u4f8b\uff0c\u5f53\u67e5\u8be2\u7684\u8868\u53ea\u6709\u4e00\u884c\u7684\u60c5\u51b5\u4e0b\uff0c\u4f7f\u7528system<\/li>\n<li>NULL: MySQL\u5728\u4f18\u5316\u8fc7\u7a0b\u4e2d\u5206\u89e3\u8bed\u53e5\uff0c\u6267\u884c\u65f6\u751a\u81f3\u4e0d\u7528\u8bbf\u95ee\u8868\u6216\u7d22\u5f15\uff0c\u4f8b\u5982\u4ece\u4e00\u4e2a\u7d22\u5f15\u5217\u91cc\u9009\u53d6\u6700\u5c0f\u503c\u53ef\u4ee5\u901a\u8fc7\u5355\u72ec\u7d22\u5f15\u67e5\u627e\u5b8c\u6210<\/li>\n<\/ul>\n<h3>possible_keys<\/h3>\n<p><strong>\u6307\u51faMySQL\u80fd\u4f7f\u7528\u54ea\u4e2a\u7d22\u5f15\u5728\u8868\u4e2d\u627e\u5230\u8bb0\u5f55\uff0c\u67e5\u8be2\u6d89\u53ca\u5230\u7684\u5b57\u6bb5\u4e0a\u82e5\u5b58\u5728\u7d22\u5f15\uff0c\u5219\u8be5\u7d22\u5f15\u5c06\u88ab\u5217\u51fa\uff0c\u4f46\u4e0d\u4e00\u5b9a\u88ab\u67e5\u8be2\u4f7f\u7528\uff08\u8be5\u67e5\u8be2\u53ef\u4ee5\u5229\u7528\u7684\u7d22\u5f15\uff0c\u5982\u679c\u6ca1\u6709\u4efb\u4f55\u7d22\u5f15\u663e\u793a null\uff09<\/strong><\/p>\n<p>\u8be5\u5217\u5b8c\u5168\u72ec\u7acb\u4e8eEXPLAIN\u8f93\u51fa\u6240\u793a\u7684\u8868\u7684\u6b21\u5e8f\u3002\u8fd9\u610f\u5473\u7740\u5728<code>possible_keys<\/code>\u4e2d\u7684\u67d0\u4e9b\u952e\u5b9e\u9645\u4e0a\u4e0d\u80fd\u6309\u751f\u6210\u7684\u8868\u6b21\u5e8f\u4f7f\u7528\u3002<br \/>\n\u5982\u679c\u8be5\u5217\u662fNULL\uff0c\u5219\u6ca1\u6709\u76f8\u5173\u7684\u7d22\u5f15\u3002\u5728\u8fd9\u79cd\u60c5\u51b5\u4e0b\uff0c\u53ef\u4ee5\u901a\u8fc7\u68c0\u67e5WHERE\u5b50\u53e5\u770b\u662f\u5426\u5b83\u5f15\u7528\u67d0\u4e9b\u5217\u6216\u9002\u5408\u7d22\u5f15\u7684\u5217\u6765\u63d0\u9ad8\u4f60\u7684\u67e5\u8be2\u6027\u80fd\u3002\u5982\u679c\u662f\u8fd9\u6837\uff0c\u521b\u9020\u4e00\u4e2a\u9002\u5f53\u7684\u7d22\u5f15\u5e76\u4e14\u518d\u6b21\u7528EXPLAIN\u68c0\u67e5\u67e5\u8be2<\/p>\n<h3>Key<\/h3>\n<p><strong>key\u5217\u663e\u793aMySQL\u5b9e\u9645\u51b3\u5b9a\u4f7f\u7528\u7684\u952e\uff08\u7d22\u5f15\uff09\uff0c\u5fc5\u7136\u5305\u542b\u5728<code>possible_keys<\/code>\u4e2d<\/strong><\/p>\n<p>\u5982\u679c\u6ca1\u6709\u9009\u62e9\u7d22\u5f15\uff0c\u952e\u662fNULL\u3002\u8981\u60f3\u5f3a\u5236MySQL\u4f7f\u7528\u6216\u5ffd\u89c6<code>possible_keys<\/code>\u5217\u4e2d\u7684\u7d22\u5f15\uff0c\u5728\u67e5\u8be2\u4e2d\u4f7f\u7528<code>FORCE INDEX<\/code>\u3001<code>USE INDEX<\/code>\u6216\u8005<code>IGNORE INDEX<\/code>\u3002<\/p>\n<h3>key_len<\/h3>\n<p><strong>\u8868\u793a\u7d22\u5f15\u4e2d\u4f7f\u7528\u7684\u5b57\u8282\u6570\uff0c\u53ef\u901a\u8fc7\u8be5\u5217\u8ba1\u7b97\u67e5\u8be2\u4e2d\u4f7f\u7528\u7684\u7d22\u5f15\u7684\u957f\u5ea6\uff08<code>key_len<\/code>\u663e\u793a\u7684\u503c\u4e3a\u7d22\u5f15\u5b57\u6bb5\u7684\u6700\u5927\u53ef\u80fd\u957f\u5ea6\uff0c\u5e76\u975e\u5b9e\u9645\u4f7f\u7528\u957f\u5ea6\uff0c\u5373<code>key_len<\/code>\u662f\u6839\u636e\u8868\u5b9a\u4e49\u8ba1\u7b97\u800c\u5f97\uff0c\u4e0d\u662f\u901a\u8fc7\u8868\u5185\u68c0\u7d22\u51fa\u7684\uff09<\/strong><\/p>\n<p>\u4e0d\u635f\u5931\u7cbe\u786e\u6027\u7684\u60c5\u51b5\u4e0b\uff0c\u957f\u5ea6\u8d8a\u77ed\u8d8a\u597d <\/p>\n<h3>ref<\/h3>\n<p><strong>\u5217\u4e0e\u7d22\u5f15\u7684\u6bd4\u8f83\uff0c\u8868\u793a\u4e0a\u8ff0\u8868\u7684\u8fde\u63a5\u5339\u914d\u6761\u4ef6\uff0c\u5373\u54ea\u4e9b\u5217\u6216\u5e38\u91cf\u88ab\u7528\u4e8e\u67e5\u627e\u7d22\u5f15\u5217\u4e0a\u7684\u503c<\/strong><\/p>\n<h3>rows<\/h3>\n<p><strong>\u4f30\u7b97\u51fa\u7ed3\u679c\u96c6\u884c\u6570\uff0c\u8868\u793aMySQL\u6839\u636e\u8868\u7edf\u8ba1\u4fe1\u606f\u53ca\u7d22\u5f15\u9009\u7528\u60c5\u51b5\uff0c\u4f30\u7b97\u7684\u627e\u5230\u6240\u9700\u7684\u8bb0\u5f55\u6240\u9700\u8981\u8bfb\u53d6\u7684\u884c\u6570<\/strong><\/p>\n<h3>Extra<\/h3>\n<p><strong>\u8be5\u5217\u5305\u542bMySQL\u89e3\u51b3\u67e5\u8be2\u7684\u8be6\u7ec6\u4fe1\u606f\uff0c\u6709\u4ee5\u4e0b\u51e0\u79cd\u60c5\u51b5<\/strong>\uff1a<\/p>\n<ul>\n<li>\n<p><code>Using where<\/code>: \u4e0d\u7528\u8bfb\u53d6\u8868\u4e2d\u6240\u6709\u4fe1\u606f\uff0c\u4ec5\u901a\u8fc7\u7d22\u5f15\u5c31\u53ef\u4ee5\u83b7\u53d6\u6240\u9700\u6570\u636e\uff0c\u8fd9\u53d1\u751f\u5728\u5bf9\u8868\u7684\u5168\u90e8\u7684\u8bf7\u6c42\u5217\u90fd\u662f\u540c\u4e00\u4e2a\u7d22\u5f15\u7684\u90e8\u5206\u7684\u65f6\u5019\uff0c\u8868\u793amysql\u670d\u52a1\u5668\u5c06\u5728\u5b58\u50a8\u5f15\u64ce\u68c0\u7d22\u884c\u540e\u518d\u8fdb\u884c\u8fc7\u6ee4<\/p>\n<\/li>\n<li>\n<p><code>Using temporary<\/code>\uff1a\u8868\u793aMySQL\u9700\u8981\u4f7f\u7528\u4e34\u65f6\u8868\u6765\u5b58\u50a8\u7ed3\u679c\u96c6\uff0c\u5e38\u89c1\u4e8e\u6392\u5e8f\u548c\u5206\u7ec4\u67e5\u8be2\uff0c\u5e38\u89c1<code>group by; order by<\/code><\/p>\n<\/li>\n<li>\n<p><code>Using filesort<\/code>\uff1a\u5f53Query\u4e2d\u5305\u542b<code>order by<\/code>\u64cd\u4f5c\uff0c\u800c\u4e14\u65e0\u6cd5\u5229\u7528\u7d22\u5f15\u5b8c\u6210\u7684\u6392\u5e8f\u64cd\u4f5c\u79f0\u4e3a\u201c\u6587\u4ef6\u6392\u5e8f\u201d<\/p>\n<\/li>\n<\/ul>\n<pre><code class=\"language-sql\">-- \u6d4b\u8bd5Extra\u7684filesort\nexplain select * from emp order by name;<\/code><\/pre>\n<ul>\n<li>\n<p><code>Using join buffer<\/code>\uff1a\u6539\u503c\u5f3a\u8c03\u4e86\u5728\u83b7\u53d6\u8fde\u63a5\u6761\u4ef6\u65f6\u6ca1\u6709\u4f7f\u7528\u7d22\u5f15\uff0c\u5e76\u4e14\u9700\u8981\u8fde\u63a5\u7f13\u51b2\u533a\u6765\u5b58\u50a8\u4e2d\u95f4\u7ed3\u679c\u3002\u5982\u679c\u51fa\u73b0\u4e86\u8fd9\u4e2a\u503c\uff0c\u90a3\u5e94\u8be5\u6ce8\u610f\uff0c\u6839\u636e\u67e5\u8be2\u7684\u5177\u4f53\u60c5\u51b5\u53ef\u80fd\u9700\u8981\u6dfb\u52a0\u7d22\u5f15\u6765\u6539\u8fdb\u80fd<\/p>\n<\/li>\n<li>\n<p><code>Impossible where<\/code>\uff1a\u8fd9\u4e2a\u503c\u5f3a\u8c03\u4e86where\u8bed\u53e5\u4f1a\u5bfc\u81f4\u6ca1\u6709\u7b26\u5408\u6761\u4ef6\u7684\u884c\uff08\u901a\u8fc7\u6536\u96c6\u7edf\u8ba1\u4fe1\u606f\u4e0d\u53ef\u80fd\u5b58\u5728\u7ed3\u679c\uff09<\/p>\n<\/li>\n<li>\n<p><code>Select tables optimized away<\/code>\uff1a\u8fd9\u4e2a\u503c\u610f\u5473\u7740\u4ec5\u901a\u8fc7\u4f7f\u7528\u7d22\u5f15\uff0c\u4f18\u5316\u5668\u53ef\u80fd\u4ec5\u4ece\u805a\u5408\u51fd\u6570\u7ed3\u679c\u4e2d\u8fd4\u56de\u4e00\u884c<\/p>\n<\/li>\n<li>\n<p><code>No tables used<\/code>\uff1aQuery\u8bed\u53e5\u4e2d\u4f7f\u7528from dual \u6216\u4e0d\u542b\u4efb\u4f55from\u5b50\u53e5<\/p>\n<\/li>\n<\/ul>\n<pre><code class=\"language-sql\">-- explain select now() from dual;<\/code><\/pre>\n<p>\u603b\u7ed3\uff1a<\/p>\n<ul>\n<li><strong>\u2022 EXPLAIN\u4e0d\u4f1a\u544a\u8bc9\u4f60\u5173\u4e8e\u89e6\u53d1\u5668\u3001\u5b58\u50a8\u8fc7\u7a0b\u7684\u4fe1\u606f\u6216\u7528\u6237\u81ea\u5b9a\u4e49\u51fd\u6570\u5bf9\u67e5\u8be2\u7684\u5f71\u54cd\u60c5\u51b5<\/strong><\/li>\n<li><strong>\u2022 EXPLAIN\u4e0d\u8003\u8651\u5404\u79cdCache<\/strong><\/li>\n<li><strong>\u2022 EXPLAIN\u4e0d\u80fd\u663e\u793aMySQL\u5728\u6267\u884c\u67e5\u8be2\u65f6\u6240\u4f5c\u7684\u4f18\u5316\u5de5\u4f5c<\/strong><\/li>\n<li><strong>\u2022 \u90e8\u5206\u7edf\u8ba1\u4fe1\u606f\u662f\u4f30\u7b97\u7684\uff0c\u5e76\u975e\u7cbe\u786e\u503c<\/strong><\/li>\n<li><strong>\u2022 EXPALIN\u53ea\u80fd\u89e3\u91caSELECT\u64cd\u4f5c\uff0c\u5176\u4ed6\u64cd\u4f5c\u8981\u91cd\u5199\u4e3aSELECT\u540e\u67e5\u770b\u6267\u884c\u8ba1\u5212<\/strong><\/li>\n<\/ul>\n<p>\u901a\u8fc7\u6536\u96c6\u7edf\u8ba1\u4fe1\u606f\u4e0d\u53ef\u80fd\u5b58\u5728\u7ed3\u679c<\/p>\n<blockquote>\n<p>\u8f6c\u8f7d\u81f3\uff1a<a target=\"_blank\" rel=\"noopener\" href=\"https:\/\/www.cnblogs.com\/tufujie\/p\/9413852.html\">https:\/\/www.cnblogs.com\/tufujie\/p\/9413852.html<\/a><\/p>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"<p>\u5728\u65e5\u5e38\u5de5\u4f5c\u4e2d\uff0c\u6211\u4eec\u6709\u65f6\u4f1a\u5f00\u6162\u67e5\u8be2\u53bb\u8bb0\u5f55\u4e00\u4e9b\u6267\u884c\u65f6\u95f4\u6bd4\u8f83\u4e45\u7684SQL\u8bed\u53e5\uff0c\u627e\u51fa\u8fd9\u4e9bSQL\u8bed\u53e5\u5e76\u4e0d\u610f\u5473\u7740\u5b8c\u4e8b\u4e86\uff0c\u6709\u65f6 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[13],"tags":[],"class_list":["post-251","post","type-post","status-publish","format-standard","hentry","category-mysql"],"_links":{"self":[{"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/posts\/251","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/comments?post=251"}],"version-history":[{"count":0,"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/posts\/251\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/media?parent=251"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/categories?post=251"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.appblog.cn\/index.php\/wp-json\/wp\/v2\/tags?post=251"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}