Release Notes for MongoDB 5.2

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Patch Releases

Aggregation
Time Series Collections
Sharding
General Improvements
Replica Sets
Report an Issue

This version of the documentation is archived and no longer supported. View the current documentation to learn how to upgrade your version of MongoDB server.

Note

MongoDB 5.2 Released January 19, 2022

Patch Releases

5.2.1 - February 24, 2022

Issues fixed:

SERVER-63145 Query $densify produces incorrect results with optimizations enabled
SERVER-62981 Make SBE multi-planner's trial period termination condition independent of collection size
SERVER-62277 Performance regression from dbstats due to occupied disk space calculation
SERVER-62065 Upgrade path from 3.6 to 4.0 can leave chunk entries without history on the shards
SERVER-59754 Incorrect logging of queryHash/planCacheKey for operations that share the same $lookup shape
All JIRA issues closed in 5.2.1
5.2.1 Changelog

Aggregation

New Aggregation Operators

MongoDB 5.2 introduces the following aggregation operators:

Operator	Description
`$bottom`	Returns the bottom element within a group according to the specified sort order.
`$bottomN`	Returns an aggregation of the bottom `n` elements within a group, according to the specified sort order.
`$firstN`	Returns an aggregation of the first `n` elements within a group. Distinct from the `$firstN` array operator.
`$firstN (array operator)`	Returns a specified number of elements from the beginning of an array. Distinct from the `$firstN` accumulator.
`$lastN`	Returns an aggregation of the last `n` elements within a group. Distinct from the `$lastN` array operator.
`$lastN (array operator)`	Returns a specified number of elements from the end of an array. Distinct from the `$lastN` accumulator.
`$locf`	Last observation carried forward. Sets values for `null` and missing fields in a window to the last non-null value for the field.
`$maxN`	Returns an aggregation of the `n` maximum valued elements within a group. Distinct from the `$maxN` array operator.
`$maxN (array operator)`	Returns the `n` largest values in an array. Distinct from the `$maxN` accumulator.
`$minN`	Returns an aggregation of the `n` minimum valued elements within a group. Distinct from the `$minN` array operator.
`$minN (array operator)`	Returns the `n` smallest values in an array. Distinct from the `$minN` accumulator.
`$top`	Returns the top element within a group according to the specified sort order. Distinct from the command `top`.
`$topN`	Returns an aggregation of the top `n` elements within a group, according to the specified sort order.
`$sortArray`	Sorts an array based on its elements.

General Aggregation Improvements

`$convert` Supports Timestamp Conversion to Date

Starting in MongoDB 5.2, you can convert timestamps to dates using the $convert operator. The $toDate operator can also convert timestamps.

Improved Precision for Operator Counters

Starting in MongoDB 5.2 and 5.0.6, aggregation operator metrics are more precise: internal processing does not inflate the counters.

Slot-Based Query Execution Engine Can Execute `$group` Stages

Starting in MongoDB 5.2, MongoDB uses the slot-based execution query engine to execute $group stages when $group is either:

The first stage in the pipeline.
Part of a series of stages executed by the slot-based engine that occurs at the beginning of the pipeline. For example, if a pipeline begins with $match followed by $group, the $match and $group stages are executed by the slot-based engine.

In most cases, the slot-based engine provides improved performance and lower CPU and memory costs compared to the classic query engine.

See $group Optimization.

Time Series Collections

Data Compression

Starting in MongoDB 5.2, time series collection data is further compressed to save database space. This compression does not affect query results, nor does it negatively affect performance.

See time series collection behavior.

Sharding Administration Commands

Starting in MongoDB 5.2, you can run sharding administration commands (such as moveChunk) on the system.buckets collection.

Sharding

Starting in MongoDB 5.2, the time that a mongos was started is included in the:

Output of the sh.status() method (when verbose is set to true)
config.mongos collection

Default Chunk Size Increased from 64 MB to 128 MB

Starting in MongoDB 5.2, the default chunk size is 128 megabytes. In earlier versions of MongoDB, the default chunk size is 64 megabytes.

Prevent Migrations on a Sharded Collection with `setAllowMigrations`

Starting in MongoDB 5.2, the setAllowMigrations command:

Prevents the start of new automatic migrations on a collection
Prevents in-flight manual migrations from committing
Excludes the collection from new balancer rounds

Limit Wait Time For Critical Section

Starting in MongoDB 5.2 (and 5.0.4, 5.1.0)

The metadataRefreshInTransactionMaxWaitBehindCritSecMS parameter limits the time a shard waits for a critical section within a transaction.

General Improvements

Configure Refresh Interval for Cached LDAP User Information

Starting in MongoDB 5.2, you can use the following new server parameters to configure the refresh interval for cached LDAP user information:

Starting in MongoDB 5.2, the update interval for cached user information retrieved from an LDAP server depends on ldapShouldRefreshUserCacheEntries:

If true, use ldapUserCacheRefreshInterval.
If false, use ldapUserCacheInvalidationInterval.

Omit Custom Data from User Output

Starting in MongoDB 5.2, you can omit customData from the response of the:

usersInfo database command
db.getUser() method
db.getUsers() method

To omit custom data from command output, specify the showCustomData: false option.

Replica Sets

`initialSyncMethod` Parameter

Starting in MongoDB 5.2, the initialSyncMethod determines whether initial sync is a logical initial sync or a file copy based initial sync.

initialSyncMethod is only available in MongoDB Enterprise Server.

Report an Issue

To report an issue, see https://github.com/mongodb/mongo/wiki/Submit-Bug-Reports for instructions on how to file a JIRA ticket for the MongoDB server or one of the related projects.

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