MapReduce aggregation
Mongo Streams supports mongodbs mapreduce functionality. Similarly with Aggregation Pipeline syntax mapReduce is based
on initial query. It also honors thesort` syntax from query. Following are available syntax for mapReduce operation:
- simple map and reduce operation
query() mapReduce (mapJavaScript reduce reduceJavaScript)- with finalization function
query() mapReduce (mapJavaScript reduce reduceJavaScript finalize finalizeJavaScript)- with sorting of documents first
query() sort ("key" Ascending) mapReduce (mapJavaScript reduce reduceJavaScript ) - with configuring javascript scope
query() mapReduce (mapJavaScript reduce reduceJavaScript scope scopeJavascript) - with configuring in-memory javascript object passing
query() mapReduce (mapJavaScript reduce reduceJavaScript jsModeInMemory) - with configuring increased verbosity
query() mapReduce (mapJavaScript reduce reduceJavaScript verbose) - with specifying maximum number of results returned
query() mapReduce (mapJavaScript reduce reduceJavaScript limit (100))
Specifying output of MapReduce command
Each mapReduce command may eventually specify if results will be stored persistently into database, where these results will be stored and
how the results will be stored. By default results override any results found in target collection. This is controlled with persists syntax:
- in current database under given collection :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist ("results"))- in supplied collection and database :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist ("resultdb.results"))- in supplied collection :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist (dbCollection)) - to merge with current records in result collection :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist (dbCollection) mergeCurrent) - to reduce with current records in result collection :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist (dbCollection) reduceCurrent) - to store results in shard collection :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist (dbCollection) asShard) - to make the store operation nonatomic :
query() mapReduce (mapJavaScript reduce reduceJavaScript persist (dbCollection) nonAtomic)| Total for specification MapReduceSpec | |
|---|---|
| Finished in | 2 seconds, 320 ms |
| Results | 4 examples, 0 failure, 0 error |