{"id":200969,"date":"2025-05-06T13:30:30","date_gmt":"2025-05-06T05:30:30","guid":{"rendered":"https:\/\/server.hk\/cnblog\/200969\/"},"modified":"2025-05-06T13:30:30","modified_gmt":"2025-05-06T05:30:30","slug":"mysql-%e7%99%be%e4%b8%87%e7%ba%a7%e6%95%b0%e6%8d%ae%e7%bb%9f%e8%ae%a1%e6%80%a7%e8%83%bd%e5%b7%ae%ef%bc%9acount-%e6%98%af%e7%bd%aa%e9%ad%81%e7%a5%b8%e9%a6%96%ef%bc%9f%e5%a6%82%e4%bd%95%e4%bc%98","status":"publish","type":"post","link":"https:\/\/server.hk\/cnblog\/200969\/","title":{"rendered":"MySQL \u767e\u4e07\u7ea7\u6570\u636e\u7edf\u8ba1\u6027\u80fd\u5dee\uff1acount(*) \u662f\u7f6a\u9b41\u7978\u9996\uff1f\u5982\u4f55\u4f18\u5316\uff1f"},"content":{"rendered":"<p><b><\/b> <\/p>\n<h1>MySQL \u767e\u4e07\u7ea7\u6570\u636e\u7edf\u8ba1\u6027\u80fd\u5dee\uff1acount(*) \u662f\u7f6a\u9b41\u7978\u9996\uff1f\u5982\u4f55\u4f18\u5316\uff1f<\/h1>\n<p>\u672c\u7bc7\u6587\u7ae0\u5411\u5927\u5bb6\u4ecb\u7ecd\u300aMySQL \u767e\u4e07\u7ea7\u6570\u636e\u7edf\u8ba1\u6027\u80fd\u5dee\uff1acount(*) \u662f\u7f6a\u9b41\u7978\u9996\uff1f\u5982\u4f55\u4f18\u5316\uff1f\u300b\uff0c\u4e3b\u8981\u5305\u62ec\uff0c\u5177\u6709\u4e00\u5b9a\u7684\u53c2\u8003\u4ef7\u503c\uff0c\u9700\u8981\u7684\u670b\u53cb\u53ef\u4ee5\u53c2\u8003\u4e00\u4e0b\u3002<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/www.17golang.com\/uploads\/20241123\/1732352432674199b06fa47.jpg\" class=\"aligncenter\"><\/p>\n<p><strong>\u63d0\u95ee\uff1aMYSQL \u767e\u4e07\u7ea7\u6570\u636e\u7edf\u8ba1\u6027\u80fd\u8f83\u5dee<\/strong><\/p>\n<p><strong>\u95ee\u9898\u63cf\u8ff0\uff1a<\/strong><\/p>\n<p>\u4f7f\u7528\u4ee5\u4e0b SQL \u8bed\u53e5\u7edf\u8ba1 29 \u4e07\u6761\u6570\u636e\u7684\u8868 t_order_old\uff0c\u6267\u884c\u65f6\u95f4\u957f\u8fbe 13.96 \u79d2\u3002\u8be2\u95ee\u8fd9\u6837\u7684\u6267\u884c\u65f6\u95f4\u662f\u5426\u6b63\u5e38\uff0c\u4ee5\u53ca\u662f\u5426\u5b58\u5728\u8fdb\u4e00\u6b65\u4f18\u5316\u7684\u53ef\u80fd\u3002<\/p>\n<p><strong>\u89e3\u7b54\uff1a<\/strong><\/p>\n<p><strong>count(*) \u7684\u6027\u80fd\u5f00\u9500<\/strong><\/p>\n<p>\u4f7f\u7528 count(*) \u7edf\u8ba1\u5927\u6570\u636e\u8868\u65f6\uff0c\u6027\u80fd\u5f80\u5f80\u6bd4\u8f83\u4f4e\u3002\u8fd9\u662f\u56e0\u4e3a\uff1a<\/p>\n<ul>\n<li>count(*) \u4f1a\u626b\u63cf\u6574\u4e2a\u8868\uff0c\u5bfc\u81f4 I\/O \u5f00\u9500\u8f83\u5927\u3002<\/li>\n<li>count(*) \u4e0d\u4f1a\u5229\u7528\u7d22\u5f15\uff0c\u53ea\u80fd\u901a\u8fc7\u5168\u8868\u626b\u63cf\u9010\u884c\u7edf\u8ba1\u3002<\/li>\n<\/ul>\n<p><strong>\u4f18\u5316\u5efa\u8bae\uff1a<\/strong><\/p>\n<ul>\n<li><strong>\u4f7f\u7528\u7d22\u5f15\u7edf\u8ba1\uff1a<\/strong>\u9488\u5bf9\u9700\u8981\u7edf\u8ba1\u7684\u5b57\u6bb5\u521b\u5efa\u7d22\u5f15\uff0c\u7136\u540e\u4f7f\u7528 count(column) \u4ee3\u66ff count(*)\uff0c\u8fd9\u6837\u53ef\u4ee5\u5229\u7528\u7d22\u5f15\u52a0\u901f\u7edf\u8ba1\u3002<\/li>\n<li><strong>\u7ef4\u62a4\u7edf\u8ba1\u8868\uff1a<\/strong>\u5efa\u7acb\u4e00\u4e2a\u5305\u542b\u7edf\u8ba1\u6570\u636e\u7684\u5355\u72ec\u8868\uff0c\u5b9a\u671f\u66f4\u65b0\u8868\u7684\u6570\u636e\u4ee5\u4fdd\u6301\u51c6\u786e\u3002\u8fd9\u6837\u53ef\u4ee5\u5728\u9700\u8981\u65f6\u5feb\u901f\u67e5\u8be2\u7edf\u8ba1\u4fe1\u606f\uff0c\u800c\u65e0\u9700\u5bf9\u4e3b\u8868\u8fdb\u884c\u5168\u8868\u626b\u63cf\u3002<\/li>\n<li><strong>\u4f7f\u7528\u89e6\u53d1\u5668\u7ef4\u62a4\u7edf\u8ba1\u6570\u636e\uff1a<\/strong>\u5728\u4e3b\u8868\u4e0a\u521b\u5efa\u89e6\u53d1\u5668\uff0c\u5728\u6570\u636e\u66f4\u65b0\u65f6\u81ea\u52a8\u66f4\u65b0\u7edf\u8ba1\u8868\u3002\u8fd9\u6837\u505a\u53ef\u4ee5\u786e\u4fdd\u7edf\u8ba1\u4fe1\u606f\u59cb\u7ec8\u662f\u6700\u65b0\u7684\uff0c\u5e76\u907f\u514d\u5168\u8868\u626b\u63cf\u5e26\u6765\u7684\u6027\u80fd\u5f00\u9500\u3002<\/li>\n<li><strong>\u4f7f\u7528 EXPLAIN \u5206\u6790\u67e5\u8be2\uff1a<\/strong>\u6267\u884c EXPLAIN SELECT count(*) FROM t_order_old&#8220; 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