「生产事故」MongoDB复合索引引发的灾难

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分布式监控系统官网:https://www.zabbix.com/","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e6/e6f2212c69deae3fdfd8353d05689736.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"开始分析","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们研发是没有操控服务器权限的，因此委托运维同学帮助我们抓取了部分查询记录，如下所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"bash"},"content":[{"type":"text","text":"---------------------------------------------------------------------------------------------------------------------------+\nOp | Duration | Query ---------------------------------------------------------------------------------------------------------------------------+\nquery | 5 s | {\"filter\": {\"orgCode\": 350119, \"fixedStatus\": {\"$in\": [1, 2]}}, \"sort\": {\"_id\": -1}, \"find\": \"sku_main\"} \nquery | 5 s | {\"filter\": {\"orgCode\": 350119, \"fixedStatus\": {\"$in\": [1, 2]}}, \"sort\": {\"_id\": -1}, \"find\": \"sku_main\"} query | 4 s | {\"filter\": {\"orgCode\": 346814, \"fixedStatus\": {\"$in\": [1, 2]}}, \"sort\": {\"_id\": -1}, \"find\": \"sku_main\"} query | 4 s | {\"filter\": {\"orgCode\": 346814, \"fixedStatus\": {\"$in\": [1, 2]}}, \"sort\": {\"_id\": -1}, \"find\": \"sku_main\"} query | 4 s | {\"filter\": {\"orgCode\": 346814, \"fixedStatus\": {\"$in\": [1, 2]}}, \"sort\": {\"_id\": -1}, \"find\": \"sku_main\"}\n...","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"查询很慢的话所有研发应该第一时间想到的就是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"索引","attrs":{}}],"attrs":{}},{"type":"text","text":"的使用问题，所以立即检查了一遍索引，如下所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"bash"},"content":[{"type":"text","text":"### 当时的索引\n\ndb.sku_main.ensureIndex({\"orgCode\": 1, \"_id\": -1},{background:true});\ndb.sku_main.ensureIndex({\"orgCode\": 1, \"upcCode\": 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当时场景日志\n\nquery: { $query: { shopCategories.0: { $exists: false }, orgCode: 337451, fixedStatus: { $in: [ 1, 2 ] }, _id: { $lt: 2038092587 } }, $orderby: { _id: -1 } } planSummary: IXSCAN { _id: 1 } ntoreturn:1000 ntoskip:0 keysExamined:37567133 docsExamined:37567133 cursorExhausted:1 keyUpdates:0 writeConflicts:0 numYields:293501 nreturned:659 reslen:2469894 locks:{ Global: { acquireCount: { r: 587004 } }, Database: { acquireCount: { r: 293502 } }, Collection: { acquireCount: { r: 293502 } } } \n\n# 耗时\n179530ms","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"耗时180秒且基于查询的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"执行计划","attrs":{}}],"attrs":{}},{"type":"text","text":"可以看出，它走的是","attrs":{}},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":"id","attrs":{}}],"attrs":{}},{"type":"text","text":"索引，进行了全表扫描，扫描的数据总量为：37567133，不慢才怪。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"迅速解决","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"定位到问题后，没办法立即修改，第一要务是：","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"止损","attrs":{}}],"attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"结合当时的时间也比较晚了，因此我们发了公告，禁止了上述查询的功能并短暂暂停了部分业务，，过了一会之后进行了","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"主从切换","attrs":{}}],"attrs":{}},{"type":"text","text":"，再去看","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Zabbix监控","attrs":{}}],"attrs":{}},{"type":"text","text":"就一切安好了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"分析根源","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们回顾一下查询的语句和我们预期的索引，如下所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"bash"},"content":[{"type":"text","text":"### 原始Query\ndb.getCollection(\"sku_main\").find({ \n \"orgCode\" : NumberLong(337451), \n \"fixedStatus\" : { \n \"$in\" : [\n 1.0, \n 2.0\n ]\n }, \n \"shopCategories\" : { \n \"$exists\" : false\n }, \n \"_id\" : { \n \"$lt\" : NumberLong(2038092587)\n }\n }\n).sort(\n { \n \"_id\" : -1.0\n }\n).skip(1000).limit(1000);\n\n### 期望的索引\ndb.sku_main.ensureIndex({\"orgCode\": 1, \"_id\": -1},{background:true});","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"乍一看，好像一切都很Nice啊，字段","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"orgCode","attrs":{}}],"attrs":{}},{"type":"text","text":"等值查询，字段","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"_id","attrs":{}}],"attrs":{}},{"type":"text","text":"按照创建索引的方向进行倒序排序，为啥会这么慢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是，关键的一点就在 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"$lt","attrs":{}}],"attrs":{}},{"type":"text","text":" 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索引\ndb.records.createIndex({a:1}); \n\n# 查询\ndb.records.find().sort({a:-1});\n\n# 索引为升序，但是我查询要按降序，我只需要从右端开始遍历即可满足需求，反之亦然\nMIN 0 1 2 3 4 5 6 7 MAX","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"MongoDB的复合索引结构","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"官方介绍：MongoDB supports ","attrs":{}},{"type":"text","marks":[{"type":"italic","attrs":{}}],"text":"compound indexes","attrs":{}},{"type":"text","text":", where a single index structure holds references to multiple fields within a collection’s documents. 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查询 一\ndb.getCollection(\"records\").find({ \n \"userid\" : \"ca2\"\n}).sort({\"score\" : -1.0});\n\n\n# 查询 二\ndb.getCollection(\"records\").find({ \n \"userid\" : \"ca2\"\n}).sort({\"score\" : 1.0});","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上述的查询没有任何问题，因为受到","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"score","attrs":{}}],"attrs":{}},{"type":"text","text":"字段排序的影响，只是数据从左侧还是从右侧遍历的问题，那么下面的一个查询呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"bash"},"content":[{"type":"text","text":"# 错误示范\ndb.getCollection(\"records\").find({ \n \"userid\" : \"ca2\",\n \"score\" 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