explain (database command数据库命令)

Definition定义

explain

The explain command provides information on the execution of the following commands: aggregate, count, distinct, find, findAndModify, delete, mapReduce, and update.explain命令提供有关执行以下命令的信息：aggregate、count、distinct、find、findAndModify、delete、mapReduce和update。

Tip

In mongosh, this command can also be run through the db.collection.explain() and cursor.explain() helper methods.在mongosh中，此命令也可以通过db.collection.explain()和cursor.explain()辅助方法运行。

Helper methods are convenient for mongosh users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.助手方法对mongosh用户来说很方便，但它们可能不会返回与数据库命令相同级别的信息。如果不需要便利性或需要额外的返回字段，请使用database命令。

Note

Using explain ignores all existing plan cache entries and prevents the MongoDB query planner from creating a new plan cache entry.使用explain会忽略所有现有的计划缓存条目，并阻止MongoDB查询计划器创建新的计划缓存条目的操作。

Compatibility兼容性

This command is available in deployments hosted in the following environments:此命令在以下环境中托管的部署中可用：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud：云中MongoDB部署的完全托管服务

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB：MongoDB的基于订阅的自我管理版本
• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB：MongoDB的源代码可用、免费使用和自我管理版本

Required Access所需访问权限

To use explain, you must have permission to run the underlying command.要使用explain，您必须具有运行底层命令的权限。

Syntax语法

The command has the following syntax:该命令具有以下语法：

db.runCommand(
   {
     explain: <command>,
     verbosity: <string>,
     comment: <any>
   }
)

Command Fields命令字段

The command takes the following fields:该命令包含以下字段：

Behavior行为

Output输出

explain operations can return information regarding:操作可以返回以下信息：

• explainVersion, the output format version (for example, "1").，输出格式版本（例如"1"）。
• command, which details the command being explained.，其中详细说明了正在解释的命令。
• queryShapeHash, starting in MongoDB 8.0, which is a hexadecimal string with the hash of a query shape. ，从MongoDB 8.0开始，它是一个十六进制字符串，具有查询形状的哈希值。For details, see Query Shapes, Query Shape Hash, and explain.queryShapeHash.有关详细信息，请参阅查询形状、查询形状哈希和explain.queryShapeHash。
• queryPlanner, which details the plan selected by the query optimizer and lists the rejected plans.，其中详细说明了查询优化器选择的计划，并列出了被拒绝的计划。
• executionStats, which details the execution of the winning plan and the rejected plans.，其中详细说明了获胜计划和被拒绝计划的执行情况。
• serverInfo, which provides information on the MongoDB instance.，它提供有关MongoDB实例的信息。
• serverParameters, which details internal parameters.，其中详细说明了内部参数。

The verbosity mode (i.e. queryPlanner, executionStats, allPlansExecution) determines whether the results include executionStats and whether executionStats includes data captured during plan selection.详细程度模式（即queryPlanner、executionStats、allPlansExecution）确定结果是否包括executionStats，以及executionStats是否包括在计划选择过程中捕获的数据。

Explain output is limited by the maximum Nested Depth for BSON Documents, which is 100 levels of nesting. Explain output that exceeds the limit is truncated.解释输出受BSON文档的最大嵌套深度限制，即100级嵌套。解释超出限制的输出被截断。

For details on the output, see Explain Results.有关输出的详细信息，请参阅解释结果。

Examples示例

db.runCommand(
   {
     explain: { count: "products", query: { quantity: { $gt: 50 } } },
     verbosity: "queryPlanner"
   }
)

db.runCommand(
   {
      explain: { count: "products", query: { quantity: { $gt: 50 } } },
      verbosity: "executionStats"
   }
)

db.runCommand(
   {
     explain: {
        update: "products",
        updates: [
           {
               q: { quantity: 1057, category: "apparel" },
               u: { $set: { reorder: true } }
           }
        ]
     }
   }
)