MongoDB性能查询优化工具:explain()

MongoDB 3.0之后,explain()的返回与使用方法与之前版本有了不少变化,介于3.0之后的优秀特色,本文仅针对MongoDB 3.0+的explain进行讨论

explain()用法

$Mongo
>use blog
>db.system_user.find().explain(true);

现版本explain有三种模式,分别如下:
queryPlanner
executionStats
allPlansExecution

queryPlanner

{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "game_db.game_user",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "w" : {
                                "$eq" : 1
                        }
                },
                "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "keyPattern" : {
                                        "w" : 1,
                                        "n" : 1
                                },
                                "indexName" : "w_1_n_1",
                                "isMultiKey" : false,
                                "direction" : "forward",
                                "indexBounds" : {
                                        "w" : [
                                                "[1.0, 1.0]"
                                        ],
                                        "n" : [
                                                "[MinKey, MaxKey]"
                                        ]
                                }
                        }
                },
                "rejectedPlans" : [
                        {
                                "stage" : "FETCH",
                                "inputStage" : {
                                        "stage" : "IXSCAN",
                                        "keyPattern" : {
                                                "w" : 1,
                                                "v" : 1
                                        },
                                        "indexName" : "w_1_v_1",
                                        "isMultiKey" : false,
                                        "direction" : "forward",
                                        "indexBounds" : {
                                                "w" : [
                                                        "[1.0, 1.0]"
                                                ],
                                                "v" : [
                                                        "[MinKey, MaxKey]"
                                                ]
                                        }
                                }
                        }
                ]
        }

先来看queryPlanner模式的各个返回意义

explain.queryPlanner

queryPlanner的返回。

explain.queryPlanner.namespace

顾名思义,该值返回的是该query所查询的表。

explain.queryPlanner.indexFilterSet

针对该query是否有indexfilter(会在后文进行详细解释)。

explain.queryPlanner.winningPlan

查询优化器针对该query所返回的最优执行计划的详细内容。

explain.queryPlanner.winningPlan.stage

最优执行计划的stage,这里返回是FETCH,可以理解为通过返回的index位置去检索具体的文档。

explain.queryPlanner.winningPlan.inputStage

explain.queryPlanner.winningPlan.stage的child stage,此处是IXSCAN,表示进行的是index scanning。

explain.queryPlanner.winningPlan.keyPattern

所扫描的index内容,此处是w:1与n:1。

explain.queryPlanner.winningPlan.indexName

winning plan所选用的index。

explain.queryPlanner.winningPlan.isMultiKey

是否是Multikey,此处返回是false,如果索引建立在array上,此处将是true。

explain.queryPlanner.winningPlan.direction

此query的查询顺序,此处是forward,如果用了.sort({w:-1})将显示backward。

explain.queryPlanner.winningPlan.indexBounds

winningplan所扫描的索引范围,此处查询条件是w:1,使用的index是w与n的联合索引,故w是[1.0,1.0]而n没有指定在查询条件中,故是[MinKey,MaxKey]。

explain.queryPlanner.rejectedPlans

其他执行计划(非最优而被查询优化器reject的)的详细返回,其中具体信息与winningPlan的返回中意义相同,故不在此赘述。

executionStats

executionStats的返回中多了如下:

 "executionStats" : {
                "executionSuccess" : true,
                "nReturned" : 29861,
                "executionTimeMillis" : 23079,
                "totalKeysExamined" : 29861,
                "totalDocsExamined" : 29861,
                "executionStages" : {
                        "stage" : "FETCH",
                        "nReturned" : 29861,
                        "executionTimeMillisEstimate" : 22685,
                        "works" : 29862,
                        "advanced" : 29861,
                        "needTime" : 0,
                        "needFetch" : 0,
                        "saveState" : 946,
                        "restoreState" : 946,
                        "isEOF" : 1,
                        "invalidates" : 0,
                        "docsExamined" : 29861,
                        "alreadyHasObj" : 0,
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "nReturned" : 29861,
                                "executionTimeMillisEstimate" : 70,
                                "works" : 29862,
                                "advanced" : 29861,
                                "needTime" : 0,
                                "needFetch" : 0,
                                "saveState" : 946,
                                "restoreState" : 946,
                                "isEOF" : 1,
                                "invalidates" : 0,
                                "keyPattern" : {
                                        "w" : 1,
                                        "n" : 1
                                },
                                "indexName" : "w_1_n_1",
                                "isMultiKey" : false,
                                "direction" : "forward",
                                "indexBounds" : {
                                        "w" : [
                                                "[1.0, 1.0]"
                                        ],
                                        "n" : [
                                                "[MinKey, MaxKey]"
                                        ]
                                },
                                "keysExamined" : 29861,
                                "dupsTested" : 0,
                                "dupsDropped" : 0,
                                "seenInvalidated" : 0,
                                "matchTested" : 0
                        }
                }
        }

executionStats模式中,我们主要需要注意的返回有如下几个

executionStats.executionSuccess

是否执行成功

executionStats.nReturned

查询的返回条数

executionStats.executionTimeMillis

整体执行时间

executionStats.totalKeysExamined

索引扫描次数

executionStats.totalDocsExamined

document扫描次数

executionStats.executionStages.stage

这里是FETCH去扫描对于documents

executionStats.executionStages.nReturned

由于是FETCH,所以这里该值与executionStats.nReturned一致

executionStats.executionStages.docsExamined

与executionStats.totalDocsExamined一致

executionStats.inputStage中的与上述理解方式相同

还有一些文档中没有描述的返回如:

“works” : 29862,

“advanced” : 29861,

“isEOF” : 1,

这些值都会在explan之初初始化:

mongo/src/mongo/db/exec/plan_stats.h

struct CommonStats {
    CommonStats(const char* type)
        : stageTypeStr(type),
          works(0),
          yields(0),
          unyields(0),
          invalidates(0),
          advanced(0),
          needTime(0),
          needYield(0),
          executionTimeMillis(0),
          isEOF(false) {}

以works为例,查看源码中发现,每次操作会加1,且会把执行时间记录在executionTimeMillis中。

mongo/src/mongo/db/exec/fetch.cpp

++_commonStats.works;

// Adds the amount of time taken by work() to executionTimeMillis.
ScopedTimer timer(&_commonStats.executionTimeMillis);

而在查询结束EOF,works又会加1,advanced不加。

mongo/src/mongo/db/exec/eof.cpp

PlanStage::StageState EOFStage::work(WorkingSetID* out) {
    ++_commonStats.works;
    // Adds the amount of time taken by work() to executionTimeMillis.
    ScopedTimer timer(&_commonStats.executionTimeMillis);
    return PlanStage::IS_EOF;
}

故正常的返回works会比nReturned多1,这时候isEOF为true(1):

mongo/src/mongo/db/exec/eof.cpp

bool EOFStage::isEOF() {
    return true;
}

unique_ptr<PlanStageStats> EOFStage::getStats() {
    _commonStats.isEOF = isEOF();
    return make_unique<PlanStageStats>(_commonStats, STAGE_EOF);
}

advanced的返回值在命中的时候+1,在skip,eof的时候不会增加如:

mongo/src/mongo/db/exec/skip.cpp

if (PlanStage::ADVANCED == status) {
        // If we're still skipping results...
        if (_toSkip > 0) {
            // ...drop the result.
            --_toSkip;
            _ws->free(id);
            ++_commonStats.needTime;
            return PlanStage::NEED_TIME;
        }

        *out = id;
        ++_commonStats.advanced;
        return PlanStage::ADVANCED;

更多参数解释参考:

http://www.mongoing.com/eshu_explain3
https://docs.mongodb.com/manual/reference/explain-results

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