{"id":46,"date":"2018-10-15T14:22:30","date_gmt":"2018-10-15T06:22:30","guid":{"rendered":"http:\/\/www.anthonylee.cn\/myblog\/?p=46"},"modified":"2018-12-04T13:52:05","modified_gmt":"2018-12-04T05:52:05","slug":"%e5%85%b3%e4%ba%8emysql%e4%b8%adrand%e7%9a%84%e4%bd%bf%e7%94%a8","status":"publish","type":"post","link":"http:\/\/www.anthonylee.cn\/myblog\/?p=46","title":{"rendered":"\u5173\u4e8emysql\u4e2d\u7528rand()\u968f\u673a\u53d6\u6570\u636e\u7684\u95ee\u9898"},"content":{"rendered":"<p>\u8981\u505a\u968f\u673a\u53d6\u6570\u636e\uff0c\u60f3\u5230\u4e86SELECT * FROM tablename ORDER BY RAND() LIMIT<\/p>\n<p>\u4f46\u662f\u5b98\u65b9\u4e0d\u5efa\u8bae\u5728order by\u4e2d\u4f7f\u7528rand()\uff0c\u5982ORDER BY RAND()\u3002\u67e5\u8be2\u6548\u7387\u6781\u5176\u4f4e\u4e0b\uff0c\u89e3\u91ca\u5982\u4e0b\uff1a<\/p>\n<p>works for small tables, but once the tables grow larger than 300,000<br \/>\nrecords or so this will be very slow because MySQL will have to process<br \/>\nALL the entries from the table, order them randomly and then return the<br \/>\nfirst row of the ordered result, and this sorting takes long time.<br \/>\nInstead you can do it like this (atleast if you have an auto_increment PK):<\/p>\n<p>\u5927\u6982\u610f\u601d\u5c31\u662f\uff0c\u4f1a\u626b\u63cf\u5168\u8868\u518d\u505a\u968f\u673a\u6392\u5e8f\u3002<\/p>\n<p>\u7f51\u4e0a\u7684\u89e3\u51b3\u529e\u6cd5<\/p>\n<p>SELECT * FROM &#8216;table&#8217; WHERE id &gt;= (SELECT floor(RAND() * (SELECT MAX(id) FROM &#8216;table&#8217;)))\u3000 ORDER BY id LIMIT 100;<\/p>\n<p>\u4f46\u662f\u8fd9\u79cd\u65b9\u5f0f\u4e5f\u4f1a\u4ea7\u751f\u95ee\u9898\uff0c\u5b9e\u9645\u6548\u679c\u5e76\u4e0d\u662f\u968f\u673a\u53d6\u3002\u5177\u4f53\u53ef\u4ee5\u81ea\u5df1\u8bd5\u8bd5\uff0c5.7.11\u4ee5\u4e0a\u4f1a\u51fa\u73b0\u3002\u8003\u8651\u5230\u6709\u53ef\u80fd\u662fmysql\u7684\u4e00\u4e2abug\uff0c\u6211\u53bb\u5b98\u7f51\u4e0a\u67e5issue\u3002\u53d1\u73b0\u5df2\u7ecf\u6709\u4eba\u63d0bug\u5355\u4e86\u3002https:\/\/bugs.mysql.com\/bug.php?id=86624\u3002\u90a3\u4e2a\u4eba\u4e5f\u53d1\u73b0\uff0c\u5f53\u5b50\u67e5\u8be2\u4e2d\u6709rand()\u65f6\uff0c\u6bcf\u6b21\u5f15\u7528\u5230rand()\u90fd\u4f1a\u91cd\u7b97\u4e00\u6b21\u3002\u4f46\u662f\u8fd9\u4e2abug\u5355\uff0c\u5b98\u65b9\u4f3c\u4e4e\u4e0d\u4ee5\u4e3a\u610f\uff0c\u5df2\u7ecf\u4e00\u5e74\u591a\u6ca1\u4eba\u56de\u590d\u4e86\u3002\u53ea\u662fMySQL\u7684\u9ad8\u7ea7\u9996\u5e2d\u5de5\u7a0b\u5e08Roy Lyseng\u5728\u7ed3\u5c3e\u63d0\u51fa\u4e86\u6743\u8861\u7684\u89e3\u51b3\u65b9\u6848\uff1a\u5bf9\u4e8e5.7\u7684\u7248\u672c\uff0c\u5728\u5b50\u67e5\u8be2\u4e2d\u52a0\u4e0alimit\uff0c\u8fd9\u4e2a\u95ee\u9898\u5c31\u53ef\u4ee5\u89e3\u51b3\u4e86\u3002\u5bf9\u4e8e8.0\u53ca\u4ee5\u4e0a\u7684\u7248\u672c\uff0c\u53ef\u4ee5\u4f7f\u7528NO_MERGE\u7279\u6027\u6765\u89e3\u51b3\u8fd9\u4e2a\u95ee\u9898\u3002<\/p>\n<p>\u6700\u540e\u89e3\u51b3\u65b9\u5f0f\u5982\u4e0b<\/p>\n<p>SELECT * FROM table , (SELECT RAND()*MAX(id) as id FROM table) AS p WHERE table.id &gt;p.id LIMIT 100<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u8981\u505a\u968f\u673a\u53d6\u6570\u636e\uff0c\u60f3\u5230\u4e86SELECT * FROM tablename ORDER BY RAND()&hellip; <a href=\"http:\/\/www.anthonylee.cn\/myblog\/?p=46\" class=\"more-link read-more\" rel=\"bookmark\">\u7ee7\u7eed\u9605\u8bfb <span class=\"screen-reader-text\">\u5173\u4e8emysql\u4e2d\u7528rand()\u968f\u673a\u53d6\u6570\u636e\u7684\u95ee\u9898<\/span><i class=\"fa fa-arrow-right\"><\/i><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"aside","meta":[],"categories":[2],"tags":[],"_links":{"self":[{"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=\/wp\/v2\/posts\/46"}],"collection":[{"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=46"}],"version-history":[{"count":3,"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=\/wp\/v2\/posts\/46\/revisions"}],"predecessor-version":[{"id":56,"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=\/wp\/v2\/posts\/46\/revisions\/56"}],"wp:attachment":[{"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=46"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=46"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.anthonylee.cn\/myblog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=46"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}