MongoDB Limits and ThresholdsMongoDB限制和阈值¶

BSON Documents文件¶

BSON Document Size文件大小¶

The maximum BSON document size is 16 megabytes.BSON文档的最大大小为16 MB。

The maximum document size helps ensure that a single document cannot use excessive amount of RAM or, during transmission, excessive amount of bandwidth. 最大文档大小有助于确保单个文档不会使用过多的RAM，或在传输过程中使用过多的带宽。To store documents larger than the maximum size, MongoDB provides the GridFS API. 为了存储大于最大大小的文档，MongoDB提供了GridFS API。See mongofiles and the documentation for your driver for more information about GridFS.有关GridFS的更多信息，请参阅mongofiles和驱动程序的文档。

Nested Depth for BSON DocumentsBSON文档的嵌套深度¶

MongoDB supports no more than 100 levels of nesting for BSON documents.MongoDB支持的BSON文档嵌套级别不超过100级。

Naming Restrictions命名限制¶

Database Name Case Sensitivity数据库名称区分大小写¶

Since database names are case insensitive in MongoDB, database names cannot differ only by the case of the characters.由于数据库名称在MongoDB中不区分大小写，因此数据库名称不能仅因字符大小写不同而不同。

Restrictions on Database Names for Windows对Windows数据库名称的限制¶

For MongoDB deployments running on Windows, database names cannot contain any of the following characters:对于在Windows上运行的MongoDB部署，数据库名称不能包含以下任何字符：

/\. "$*<>:|?

Also database names cannot contain the null character.此外，数据库名称不能包含空字符。

Restrictions on Database Names for Unix and Linux SystemsUnix和Linux系统的数据库名称限制¶

For MongoDB deployments running on Unix and Linux systems, database names cannot contain any of the following characters:对于在Unix和Linux系统上运行的MongoDB部署，数据库名称不能包含以下任何字符：

/\. "$

Also database names cannot contain the null character.此外，数据库名称不能包含空字符。

Length of Database Names数据库名称的长度¶

Database names cannot be empty and must have fewer than 64 characters.数据库名称不能为空，且必须少于64个字符。

Restriction on Collection Names对集合名称的限制¶

Collection names should begin with an underscore or a letter character, and cannot:集合名称应以下划线或字母字符开头，不能：

• contain the $.包含$。
• be an empty string (e.g. "").是空字符串（如""）。
• contain the null character.包含空字符。
• begin with the system. prefix. 以system.前缀开始。(Reserved for internal use.)（保留供内部使用。）

If your collection name includes special characters, such as the underscore character, or begins with numbers, then to access the collection use the db.getCollection() method in the mongo shell or a similar method for your driver.如果集合名称包含特殊字符（如下划线字符）或以数字开头，则要访问集合，请使用mongo shell中的db.getCollection()方法或驱动程序的类似方法。

Namespace Length:命名空间长度：

• For featureCompatibilityVersion set to "4.4" or greater, MongoDB raises the limit on collection/view namespace to 255 bytes. 对于设置为“4.4”或更高版本的功能兼容性版本，MongoDB将集合/视图命名空间的限制提高到255字节。For a collection or a view, the namespace includes the database name, the dot (.) separator, and the collection/view name (e.g. <database>.<collection>),对于集合或视图，名称空间包括数据库名称、点（.）分隔符和集合/视图名称（例如<database><collection>），
• For featureCompatibilityVersion set to "4.2" or earlier, the maximum length of the collection/view namespace remains 120 bytes.对于设置为“4.2”或更早版本的功能兼容性版本，集合/视图命名空间的最大长度仍为120字节。

Restrictions on Field Names对字段名的限制¶

• Field names cannot contain the null character.字段名不能包含null字符。
• Top-level field names cannot start with the dollar sign ($) character.顶级字段名不能以美元符号（$）字符开头。

  Otherwise, starting in MongoDB 3.6, the server permits storage of field names that contain dots (i.e. .) and dollar signs (i.e. $).否则，从MongoDB 3.6开始，服务器允许存储包含点（即.）和美元符号（即$）的字段名。

  Important

  The MongoDB Query Language cannot always meaningfully express queries over documents whose field names contain these characters (see SERVER-30575).MongoDB查询语言不能总是有意义地表达对字段名包含这些字符的文档的查询（请参阅SERVER-30575）。

  Until support is added in the query language, the use of $ and . in field names is not recommended and is not supported by the official MongoDB drivers.在查询语言中添加支持之前，不建议在字段名称中使用$和.，官方MongoDB驱动程序也不支持这样做。

MongoDB does not support duplicate field names不支持重复的字段名

The MongoDB Query Language is undefined over documents with duplicate field names. 对于具有重复字段名的文档，MongoDB查询语言未定义。BSON builders may support creating a BSON document with duplicate field names. BSON构建器可能支持创建具有重复字段名的BSON文档。While the BSON builder may not throw an error, inserting these documents into MongoDB is not supported even if the insert succeeds. 虽然BSON builder可能不会抛出错误，但即使插入成功，也不支持将这些文档插入MongoDB。For example, inserting a BSON document with duplicate field names through a MongoDB driver may result in the driver silently dropping the duplicate values prior to insertion.例如，通过MongoDB驱动程序插入具有重复字段名的BSON文档可能会导致驱动程序在插入之前自动删除重复的值。

Namespaces名称空间¶

Namespace Length名称空间长度¶

• For featureCompatibilityVersion set to "4.4" or greater, MongoDB raises the limit on collection/view namespace to 255 bytes. For a collection or a view, the namespace includes the database name, the dot (.) separator, and the collection/view name (e.g. <database>.<collection>),
• For featureCompatibilityVersion set to "4.2" or earlier, the maximum length of the collection/view namespace remains 120 bytes.

See also参阅

Naming Restrictions

Indexes索引¶

Index Key Limit索引键限制¶

Changed in version 4.2在版本4.2中更改

Starting in version 4.2, MongoDB removes the Index Key Limit for featureCompatibilityVersion (fCV) set to "4.2" or greater.从4.2版开始，MongoDB删除了功能兼容性版本(fCV)设置为"4.2"或更高版本的索引键限制。

For MongoDB 2.6 through MongoDB versions with fCV set to "4.0" or earlier, the total size of an index entry, which can include structural overhead depending on the BSON type, must be less than 1024 bytes.

When the Index Key Limit applies:当索引键限制适用时：

• MongoDB will not create an index on a collection if the index entry for an existing document exceeds the index key limit.如果现有文档的索引项超过索引键限制，MongoDB将不会在集合上创建索引。
• Reindexing operations will error if the index entry for an indexed field exceeds the index key limit. Reindexing operations occur as part of the compact command as well as the db.collection.reIndex() method.

  Because these operations drop all the indexes from a collection and then recreate them sequentially, the error from the index key limit prevents these operations from rebuilding any remaining indexes for the collection.

• MongoDB will not insert into an indexed collection any document with an indexed field whose corresponding index entry would exceed the index key limit, and instead, will return an error. Previous versions of MongoDB would insert but not index such documents.
• Updates to the indexed field will error if the updated value causes the index entry to exceed the index key limit.

  If an existing document contains an indexed field whose index entry exceeds the limit, any update that results in the relocation of that document on disk will error.

• mongorestore and mongoimport will not insert documents that contain an indexed field whose corresponding index entry would exceed the index key limit.
• In MongoDB 2.6, secondary members of replica sets will continue to replicate documents with an indexed field whose corresponding index entry exceeds the index key limit on initial sync but will print warnings in the logs.

  Secondary members also allow index build and rebuild operations on a collection that contains an indexed field whose corresponding index entry exceeds the index key limit but with warnings in the logs.

  With mixed version replica sets where the secondaries are version 2.6 and the primary is version 2.4, secondaries will replicate documents inserted or updated on the 2.4 primary, but will print error messages in the log if the documents contain an indexed field whose corresponding index entry exceeds the index key limit.

• For existing sharded collections, chunk migration will fail if the chunk has a document that contains an indexed field whose index entry exceeds the index key limit.

Number of Indexes per Collection¶

A single collection can have no more than 64 indexes.

Index Name Length索引名长度¶

Changed in version 4.2在版本4.2中更改

Starting in version 4.2, MongoDB removes the Index Name Length Limit for MongoDB versions with featureCompatibilityVersion (fCV) set to "4.2" or greater.

In previous versions of MongoDB or MongoDB versions with fCV set to "4.0" or earlier, fully qualified index names, which include the namespace and the dot separators (i.e. <database name>.<collection name>.$<index name>), cannot be longer than 127 bytes.

By default, <index name> is the concatenation of the field names and index type. You can explicitly specify the <index name> to the createIndex() method to ensure that the fully qualified index name does not exceed the limit.

Number of Indexed Fields in a Compound Index复合索引中的索引字段数¶

There can be no more than 32 fields in a compound index.复合索引中的字段不能超过32个。

Queries cannot use both text and Geospatial Indexes查询不能同时使用文本索引和地理空间索引¶

You cannot combine the $text query, which requires a special text index, with a query operator that requires a different type of special index. For example you cannot combine $text query with the $near operator.

Fields with 2dsphere Indexes can only hold Geometries具有2dsphere索引的字段只能保存几何图形¶

Fields with 2dsphere indexes must hold geometry data in the form of coordinate pairs or GeoJSON data. If you attempt to insert a document with non-geometry data in a 2dsphere indexed field, or build a 2dsphere index on a collection where the indexed field has non-geometry data, the operation will fail.

NaN values returned from Covered Queries by the WiredTiger Storage Engine are always of type doubleWiredTiger存储引擎从覆盖查询返回的NaN值始终为double类型¶

If the value of a field returned from a query that is covered by an index is NaN, the type of that NaN value is always double.

Multikey Index多键索引¶

Multikey indexes cannot cover queries over array field(s).

Geospatial Index¶

Geospatial indexes cannot cover a query.

Memory Usage in Index Builds¶

createIndexes supports building one or more indexes on a collection. createIndexes uses a combination of memory and temporary files on disk to complete index builds. The default limit on memory usage for createIndexes is 200 megabytes (for versions 4.2.3 and later) and 500 (for versions 4.2.2 and earlier), shared between all indexes built using a single createIndexes command. Once the memory limit is reached, createIndexes uses temporary disk files in a subdirectory named _tmp within the --dbpath directory to complete the build.

You can override the memory limit by setting the maxIndexBuildMemoryUsageMegabytes server parameter. Setting a higher memory limit may result in faster completion of index builds. However, setting this limit too high relative to the unused RAM on your system can result in memory exhaustion and server shutdown.

Changed in version 4.2.在版本4.2中更改。

• For feature compatibility version (fcv) "4.2", the index build memory limit applies to all index builds.
• For feature compatibility version (fcv) "4.0", the index build memory limit only applies to foreground index builds.

Index builds may be initiated either by a user command such as Create Index or by an administrative process such as an initial sync. Both are subject to the limit set by maxIndexBuildMemoryUsageMegabytes.

An initial sync operation populates only one collection at a time and has no risk of exceeding the memory limit. However, it is possible for a user to start index builds on multiple collections in multiple databases simultaneously and potentially consume an amount of memory greater than the limit set in maxIndexBuildMemoryUsageMegabytes.

Tip

To minimize the impact of building an index on replica sets and sharded clusters with replica set shards, use a rolling index build procedure as described on Rolling Index Builds on Replica Sets.

Collation and Index Types¶

The following index types only support simple binary comparison and do not support collation:

• text indexes,
• 2d indexes, and
• geoHaystack indexes.

Tip

To create a text, a 2d, or a geoHaystack index on a collection that has a non-simple collation, you must explicitly specify {collation: {locale: "simple"} } when creating the index.

Hidden Indexes¶

• You cannot hide the _id index.
• You cannot use hint() on a hidden index.

Data¶

Maximum Number of Documents in a Capped Collection¶

If you specify a maximum number of documents for a capped collection using the max parameter to create, the limit must be less than 2³² documents. If you do not specify a maximum number of documents when creating a capped collection, there is no limit on the number of documents.

Replica Sets¶

Number of Members of a Replica Set¶

Replica sets can have up to 50 members.

Number of Voting Members of a Replica Set¶

Replica sets can have up to 7 voting members. For replica sets with more than 7 total members, see Non-Voting Members.

Maximum Size of Auto-Created Oplog¶

If you do not explicitly specify an oplog size (i.e. with oplogSizeMB or --oplogSize) MongoDB will create an oplog that is no larger than 50 gigabytes. [1]

[1]	Starting in MongoDB 4.0, the oplog can grow past its configured size limit to avoid deleting the majority commit point.

Sharded Clusters¶

Sharded clusters have the restrictions and thresholds described here.

Operations¶

Sort Operations¶

If MongoDB cannot use an index or indexes to obtain the sort order, MongoDB must perform a blocking sort operation on the data. The name refers to the requirement that the SORT stage reads all input documents before returning any output documents, blocking the flow of data for that specific query.

If MongoDB requires using more than 100 megabytes of system memory for the blocking sort operation, MongoDB returns an error unless the query specifies cursor.allowDiskUse() (New in MongoDB 4.4). allowDiskUse() allows MongoDB to use temporary files on disk to store data exceeding the 100 megabyte system memory limit while processing a blocking sort operation.

Changed in version 4.4:For MongoDB 4.2 and prior, blocking sort operations could not exceed 32 megabytes of system memory.

For more information on sorts and index use, see Sort and Index Use.

Aggregation Pipeline Operation¶

Pipeline stages have a limit of 100 megabytes of RAM. If a stage exceeds this limit, MongoDB will produce an error. To allow for the handling of large datasets, use the allowDiskUse option to enable aggregation pipeline stages to write data to temporary files.

Changed in version 3.4.

The $graphLookup stage must stay within the 100 megabyte memory limit. If allowDiskUse: true is specified for the aggregate() operation, the $graphLookup stage ignores the option. If there are other stages in the aggregate() operation, allowDiskUse: true option is in effect for these other stages.

Starting in MongoDB 4.2, the profiler log messages and diagnostic log messages includes a usedDisk indicator if any aggregation stage wrote data to temporary files due to memory restrictions.

See also参阅

$sort and Memory Restrictions and $group Operator and Memory.

Aggregation and Read Concern¶

• Starting in MongoDB 4.2, the $out stage cannot be used in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include the $out stage in the pipeline.
• The $merge stage cannot be used in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include the $merge stage in the pipeline.

2d Geospatial queries cannot use the $or operator¶

See

$or and 2d Index Internals.

Geospatial Queries¶

For spherical queries, use the 2dsphere index result.

The use of 2d index for spherical queries may lead to incorrect results, such as the use of the 2d index for spherical queries that wrap around the poles.

Geospatial Coordinates¶

• Valid longitude values are between -180 and 180, both inclusive.
• Valid latitude values are between -90 and 90, both inclusive.

Area of GeoJSON Polygons¶

For $geoIntersects or $geoWithin, if you specify a single-ringed polygon that has an area greater than a single hemisphere, include the custom MongoDB coordinate reference system in the $geometry expression; otherwise, $geoIntersects or $geoWithin queries for the complementary geometry. For all other GeoJSON polygons with areas greater than a hemisphere, $geoIntersects or $geoWithin queries for the complementary geometry.

Multi-document Transactions¶

For multi-document transactions:

• You can specify read/write (CRUD) operations on existing collections. For a list of CRUD operations, see CRUD Operations.
• When using feature compatibility version (fcv) "4.4" or greater, you can create collections and indexes in transactions. For details, see Create Collections and Indexes In a Transaction
• The collections used in a transaction can be in different databases.

  Note

  You cannot create new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.

• You cannot write to capped collections. (Starting in MongoDB 4.2)
• You cannot read/write to collections in the config, admin, or local databases.
• You cannot write to system.* collections.
• You cannot return the supported operation’s query plan (i.e. explain).

• For cursors created outside of a transaction, you cannot call getMore inside the transaction.
• For cursors created in a transaction, you cannot call getMore outside the transaction.
• Starting in MongoDB 4.2, you cannot specify killCursors as the first operation in a transaction.

Changed in version 4.4.

The following operations are not allowed in transactions:

• Operations that affect the database catalog, such as creating or dropping a collection or an index when using feature compatibility version (fcv) "4.2" or lower. With fcv "4.4" or greater, you can create collections and indexes in transactions unless the transaction is a cross-shard write transaction. For details, see Create Collections and Indexes In a Transaction.
• Creating new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.
• Explicit creation of collections, e.g. db.createCollection() method, and indexes, e.g. db.collection.createIndexes() and db.collection.createIndex() methods, when using a read concern level other than "local".
• The listCollections and listIndexes commands and their helper methods.
• Other non-CRUD and non-informational operations, such as createUser, getParameter, count, etc. and their helpers.

Transactions have a lifetime limit as specified by transactionLifetimeLimitSeconds. The default is 60 seconds.

Write Command Batch Limit Size¶

100,000 writes are allowed in a single batch operation, defined by a single request to the server.

Changed in version 3.6:The limit raises from 1,000 to 100,000 writes. This limit also applies to legacy OP_INSERT messages.

The Bulk() operations in the mongo shell and comparable methods in the drivers do not have this limit.

Views¶

The view definition pipeline cannot include the $out or the $merge stage. If the view definition includes nested pipeline (e.g. the view definition includes $lookup or $facet stage), this restriction applies to the nested pipelines as well.

Views have the following operation restrictions:

• Views are read-only.
• You cannot rename views.
• find() operations on views do not support the following projection operators:
  • $
  • $elemMatch
  • $slice
  • $meta
• Views do not support text search.
• Views do not support map-reduce operations.
• Views do not support geoNear operations (i.e. $geoNear pipeline stage).

Projection Restrictions¶

New in version 4.4:

$-Prefixed Field Path Restriction

Starting in MongoDB 4.4, the find and findAndModify projection cannot project a field that starts with $ with the exception of the DBRef fields.

For example, starting in MongoDB 4.4, the following operation is invalid:

db.inventory.find( {}, { "$instock.warehouse": 0, "$item": 0, "detail.$price": 1 } ) // Invalid starting in 4.4

MongoDB already has a restriction where top-level field names cannot start with the dollar sign ($).

In earlier version, MongoDB ignores the $-prefixed field projections.

$ Positional Operator Placement Restriction

Starting in MongoDB 4.4, the $ projection operator can only appear at the end of the field path; e.g. "field.$" or "fieldA.fieldB.$".

For example, starting in MongoDB 4.4, the following operation is invalid:

db.inventory.find( { }, { "instock.$.qty": 1 } ) // Invalid starting in 4.4

To resolve, remove the component of the field path that follows the $ projection operator.

In previous versions, MongoDB ignores the part of the path that follows the $; i.e. the projection is treated as "instock.$".

Empty Field Name Projection Restriction

Starting in MongoDB 4.4, find and findAndModify projection cannot include a projection of an empty field name.

For example, starting in MongoDB 4.4, the following operation is invalid:

db.inventory.find( { }, { "": 0 } ) // Invalid starting in 4.4

In previous versions, MongoDB treats the inclusion/exclusion of the empty field as it would the projection of non-existing fields.

Path Collision: Embedded Documents and Its Fields

Starting in MongoDB 4.4, it is illegal to project an embedded document with any of the embedded document’s fields.

For example, consider a collection inventory with documents that contain a size field:

{ ..., size: { h: 10, w: 15.25, uom: "cm" }, ... }

Starting in MongoDB 4.4, the following operation fails with a Path collision error because it attempts to project both size document and the size.uom field:

db.inventory.find( {}, { size: 1, "size.uom": 1 } )  // Invalid starting in 4.4

In previous versions, lattermost projection between the embedded documents and its fields determines the projection:

• If the projection of the embedded document comes after any and all projections of its fields, MongoDB projects the embedded document. For example, the projection document { "size.uom": 1, size: 1 } produces the same result as the projection document { size: 1 }.
• If the projection of the embedded document comes before the projection any of its fields, MongoDB projects the specified field or fields. For example, the projection document { "size.uom": 1, size: 1, "size.h": 1 } produces the same result as the projection document { "size.uom": 1, "size.h": 1 }.

Path Collision: $slice of an Array and Embedded Fields

Starting in MongoDB 4.4, find and findAndModify projection cannot contain both a $slice of an array and a field embedded in the array.

For example, consider a collection inventory that contains an array field instock:

{ ..., instock: [ { warehouse: "A", qty: 35 }, { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ], ... }

Starting in MongoDB 4.4, the following operation fails with a Path collision error:

db.inventory.find( {}, { "instock": { $slice: 1 }, "instock.warehouse": 0 } ) // Invalid starting in 4.4

In previous versions, the projection applies both projections and returns the first element ($slice: 1) in the instock array but suppresses the warehouse field in the projected element. Starting in MongoDB 4.4, to achieve the same result, use the db.collection.aggregate() method with two separate $project stages.

$ Positional Operator and $slice Restriction

Starting in MongoDB 4.4, find and findAndModify projection cannot include $slice projection expression as part of a $ projection expression.

For example, starting in MongoDB 4.4, the following operation is invalid:

db.inventory.find( { "instock.qty": { $gt: 25 } }, { "instock.$": { $slice: 1 } } ) // Invalid starting in 4.4

MongoDB already has a restriction where top-level field names cannot start with the dollar sign ($).

In previous versions, MongoDB returns the first element (instock.$) in the instock array that matches the query condition; i.e. the positional projection "instock.$" takes precedence and the $slice:1 is a no-op. The "instock.$": { $slice: 1 } does not exclude any other document field.

Sessions¶

Sessions and $external Username Limit¶

Changed in version 3.6.3:To use sessions with $external authentication users (i.e. Kerberos, LDAP, x.509 users), the usernames cannot be greater than 10k bytes.

Session Idle Timeout¶

Sessions that receive no read or write operations for 30 minutes or that are not refreshed using refreshSessions within this threshold are marked as expired and can be closed by the MongoDB server at any time. Closing a session kills any in-progress operations and open cursors associated with the session. This includes cursors configured with noCursorTimeout or a maxTimeMS greater than 30 minutes.

Consider an application that issues a db.collection.find(). The server returns a cursor along with a batch of documents defined by the cursor.batchSize() of the find(). The session refreshes each time the application requests a new batch of documents from the server. However, if the application takes longer than 30 minutes to process the current batch of documents, the session is marked as expired and closed. When the application requests the next batch of documents, the server returns an error as the cursor was killed when the session was closed.

For operations that return a cursor, if the cursor may be idle for longer than 30 minutes, issue the operation within an explicit session using Session.startSession() and periodically refresh the session using the refreshSessions command. For example:例如：

var session = db.getMongo().startSession()
var sessionId = session.getSessionId().id

var cursor = session.getDatabase("examples").getCollection("data").find().noCursorTimeout()
var refreshTimestamp = new Date() // take note of time at operation start

while (cursor.hasNext()) {

  // Check if more than 5 minutes have passed since the last refresh
  if ( (new Date()-refreshTimestamp)/1000 > 300 ) {
    print("refreshing session")
    db.adminCommand({"refreshSessions" : [sessionId]})
    refreshTimestamp = new Date()
  }

  // process cursor normally

}

In the example operation, the db.collection.find() method is associated with an explicit session. The cursor is configured with noCursorTimeout() to prevent the server from closing the cursor if idle. The while loop includes a block that uses refreshSessions to refresh the session every 5 minutes. Since the session will never exceed the 30 minute idle timeout, the cursor can remain open indefinitely.

For MongoDB drivers, defer to the driver documentation for instructions and syntax for creating sessions.

Shell¶

The mongo shell prompt has a limit of 4095 codepoints for each line. If you enter a line with more than 4095 codepoints, the shell will truncate it.