Find a Shard Key
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Every shared collection has a shard key. To
display the shard key, connect to a mongos instance and run
the db.printShardingStatus() method:
db.printShardingStatus()
The output resembles:
<dbname>.<collection> shard key: { <shard key> : <1 or hashed> } unique: <boolean> balancing: <boolean> chunks: <shard name1> <number of chunks> <shard name2> <number of chunks> ... { <shard key>: <min range1> } -->> { <shard key> : <max range1> } on : <shard name> <last modified timestamp> { <shard key>: <min range2> } -->> { <shard key> : <max range2> } on : <shard name> <last modified timestamp> ... tag: <tag1> { <shard key> : <min range1> } -->> { <shard key> : <max range1> } ...
For more details on the db.printShardingStatus() output, see the
sharded collection section
on the sh.status() page.