db.collection.explain()

Verbosity Modes详细信息模式

The behavior of db.collection.explain() and the amount of information returned depend on the verbosity mode.db.collection.explain()的行为和返回的信息量取决于verbosity模式。

Explain and Write Operations解释和写入操作

For write operations, db.collection.explain() returns information about the write operation that would be performed but does not actually modify the database.对于写操作，db.collection.explain()返回关于将要执行但实际上不会修改数据库的写操作的信息。

Restrictions限制

Starting in MongoDB 4.2, you cannot run the explain command/db.collection.explain() in executionStats mode or allPlansExecution mode for an aggregation pipeline that contains the $out stage. 从MongoDB 4.2开始，对于包含$out阶段的聚合管道，不能在executionStats模式或allPlansExecution模式下运行explain命令/db.collection.explain()。Instead, you can either:但是，您可以：

• run the explain in queryPlanner mode or在queryPlanner模式下运行解释，或
• run the explain in executionStats mode or allPlansExecution mode but without the $out stage to return information for the stages that precede the $out stage.在executionStats模式或allPlansExecution模式下运行explain，但不运行$out阶段，以返回$out阶段之前的阶段的信息。

explain() Mechanics机制

The db.collection.explain() method wraps the explain command and is the preferred way to run explain.db.collection.explain()方法包装了explain命令，是运行explain的首选方法。

db.collection.explain().find() is similar to db.collection.find().explain() with the following key differences:与db.collection.find().explain()类似，但有以下主要区别：

• The db.collection.explain().find() construct allows for the additional chaining of query modifiers. db.collection.explain().find()构造允许额外链接查询修饰符。For list of query modifiers, see db.collection.explain().find().help().有关查询修饰符的列表，请参阅db.collection.explain().find().help()。
• The db.collection.explain().find() returns a cursor, which requires a call to .next(), or its alias .finish(), to return the explain() results. db.collection.explain().find()返回一个游标，该游标需要调用.next()或其别名.finish()来返回explain()结果。If run interactively in mongosh, mongosh automatically calls .finish() to return the results. 如果在mongosh中以交互方式运行，mongosh会自动调用.finish()返回结果。For scripts, however, you must explicitly call .next(), or .finish(), to return the results. 但是，对于脚本，必须显式调用.next()或.finish()才能返回结果。For list of cursor-related methods, see db.collection.explain().find().help().有关游标相关方法的列表，请参阅db.collection.explain().find().help()。

db.collection.explain().aggregate() is equivalent to passing the explain option to the db.collection.aggregate() method.等效于将explain选项传递给db.collection.aggregate()方法。

help()

To see the list of operations supported by db.collection.explain(), run:要查看db.collection.explain()支持的操作列表，请运行：

db.collection.explain().help()

db.collection.explain().find() returns a cursor, which allows for the chaining of query modifiers. 返回一个游标，该游标允许查询修饰符的链接。To see the list of query modifiers supported by db.collection.explain().find() as well as cursor-related methods, run:要查看db.collection.explain().find()支持的查询修饰符列表以及与游标相关的方法，请运行：

db.collection.explain().find().help()

You can chain multiple modifiers to db.collection.explain().find(). 可以将多个修饰符链接到db.collection.explain().find()。For an example, see Explain find() with Modifiers.有关示例，请参阅解释find()配合修饰符。

queryPlanner Mode模式

By default, db.collection.explain() runs in "queryPlanner" verbosity mode.默认情况下，db.collection.explain()以"queryPlanner"详细模式运行。

The following example runs db.collection.explain() in "queryPlanner" verbosity mode to return the query planning information for the specified count() operation:以下示例以"queryPlanner"详细模式运行db.collection.explain()，以返回指定count()操作的查询计划信息：

db.products.explain().count( { quantity: { $gt: 50 } } )

executionStats Mode模式

The following example runs db.collection.explain() in "executionStats" verbosity mode to return the query planning and execution information for the specified find() operation:以下示例在"executionStats"详细模式下运行db.collection.explain()，以返回指定find()操作的查询计划和执行信息：

db.products.explain("executionStats").find(
   { quantity: { $gt: 50 }, category: "apparel" }
)

allPlansExecution Mode模式

The following example runs db.collection.explain() in "allPlansExecution" verbosity mode. 以下示例以"allPlansExecution"详细模式运行db.collection.explain()。db.collection.explain() returns the queryPlanner and executionStats for all considered plans for the specified findAndModify() operation:db.collection.explain()返回指定findAndModify()操作的所有考虑计划的queryPlanner和executionStats：

db.products.explain( "allPlansExecution" ).findAndModify( {
   query: { name: "Tom", state: "active", rating: { $gt: 10 } },
   sort: { rating: 1 },
   update: { $inc: { score: 1 } }
} )

Explain find() with Modifiers用修饰符解释find()

db.collection.explain().find() construct allows for the chaining of query modifiers. construct允许链接查询修饰符。For example, the following operation provides information on the find() method with sort() and hint() query modifiers.例如，以下操作提供关于带有sort()和hint()查询修饰符的find()方法的信息。

db.products.explain("executionStats").find(
   { quantity: { $gt: 50 }, category: "apparel" }
).sort( { quantity: -1 } ).hint( { category: 1, quantity: -1 } )

For a list of query modifiers available, run the following in mongosh:要获得可用的查询修饰符列表，请在mongosh中运行以下命令：

db.collection.explain().find().help()

Iterate the explain().find() Return Cursor迭代explain().find()返回游标

db.collection.explain().find() returns a cursor to the explain results. 将游标返回到解释结果。If run interactively in mongosh, mongosh automatically iterates the cursor using the .next() method. 如果在mongosh中以交互方式运行，mongosh会使用.next()方法自动迭代游标。For scripts, however, you must explicitly call .next() (or its alias .finish()) to return the results:但是，对于脚本，必须显式调用.next()（或其别名.finish()）以返回结果：

var explainResult = db.products.explain().find( { category: "apparel" } ).next();