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由正则表示式匹配($regex)引起的一次mongo数据库c

某一天，监控到mongo数据库cpu使用率高了很多，查了一下，发现是下面这种语句引起的：

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db.example_collection.find({ "idField" : { "$regex" : "123456789012345678"} , "dateField" : { "$regex" : "2019/10/10"}})

通常，遇到这种情况，我第一反应是缺少相关字段的索引，导致每执行一次这种语句都会全表扫描一次。

但是我用explain( )语句分析了下，发现上面所涉及的两个字段idField、dateField是有索引的，并且该语句也是有使用到索引的。如下为explain( )的结果：

mgset-11111111:PRIMARY> db.example_collection.find({ "idField" : { "$regex" : "123456789012345678"} , "dateField" : { "$regex" : "2019/10/10"}}).explain("queryPlanner")
{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "example_db.example_collection",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "$and" : [
                                {
                                        "idField" : {
                                                "$regex" : "123456789012345678"
                                        }
                                },
                                {
                                        "dateField" : {
                                                "$regex" : "2019/10/10"
                                        }
                                }
                        ]
                },
                "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "filter" : {
                                        "$and" : [
                                                {
                                                        "idField" : {
                                                                "$regex" : "123456789012345678"
                                                        }
                                                },
                                                {
                                                        "dateField" : {
                                                                "$regex" : "2019/10/10"
                                                        }
                                                }
                                        ]
                                },
                                "keyPattern" : {
                                        "idField" : 1,
                                        "dateField" : 1
                                },
                                "indexName" : "idField_1_dateField_1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : {
                                        "idField" : [ ],
                                        "dateField" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : {
                                        "idField" : [
                                                "[\"\", {})",
                                                "[/123456789012345678/, /123456789012345678/]"
                                        ],
                                        "dateField" : [
                                                "[\"\", {})",
                                                "[/2019/10/10/, /2019/10/10/]"
                                        ]
                                }
                        }
                },
                "rejectedPlans" : [ ]
        },
        "ok" : 1
}

查看mongo的日志发现，这种语句执行一次就要800~900ms，的确是比较慢。除非数据库cpu核数很多，要不然只要这种语句每秒并发稍微高一点，cpu很快就被占满了。

之后搜索了下，发现有可能是正则表达式的问题。原来，虽然该语句的确是使用了索引，但是explain( )语句的输出中还有一个字段"indexBounds"，表示执行该语句时所需扫描的索引范围。说实话，上面那个输出中，我始终没看明白它那个索引范围。上面的语句对idField、dateField这两个字段都进行了普通的正则表达式匹配，我猜测它应该是扫描了整个索引树，所以导致索引并未实际提升该语句的查询效率。

我看了下数据库里面的数据，发现idField、dateField这两个字段完全没有必要进行正则匹配，进行普通的文本匹配就行。将正则匹配操作$regex去掉之后，再分析一下，结果是这样的：

mgset-11111111:PRIMARY> db.example_collection.find({ "idField" : "123456789012345678", "dateField" : "2019/10/10"}).explain("queryPlanner")
{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "example_db.example_collection",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "$and" : [
                                {
                                        "idField" : {
                                                "$eq" : "123456789012345678"
                                        }
                                },
                                {
                                        "dateField" : {
                                                "$eq" : "2019/10/10"
                                        }
                                }
                        ]
                },
                "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "keyPattern" : {
                                        "idField" : 1,
                                        "dateField" : 1
                                },
                                "indexName" : "idField_1_dateField_1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : {
                                        "idField" : [ ],
                                        "dateField" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : {
                                        "idField" : [
                                                "[\"123456789012345678\", \"123456789012345678\"]"
                                        ],
                                        "dateField" : [
                                                "[\"2019/10/10\", \"2019/10/10\"]"
                                        ]
                                }
                        }
                },
                "rejectedPlans" : [ ]
        },
        "ok" : 1
}

可以看到，仍然使用到了索引，并且索引扫描范围是仅限于一个值的。

后来跟开发人员确认了下，该语句确实没必要使用正则匹配，就让他把正则匹配去掉了。之后就没有再出现问题了，mongo慢日志中也未再出现该语句。


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