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According to the documentation of split with find, it tries to find a median point for the intersecting chunk (which is similar to how the auto splitter works).
With only 1 shard key element there is no way to calculate the median of the chunk, this is reported on the shard logs:
d35993| 2020-01-20T17:05:47.231+0100 W SHARDING [conn20] possible low cardinality key detected in test.user - range { : 0.0 } -->> { : 100.0 } contains only the key { x: 1.0 }
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And also on the router:
s35995| 2020-01-20T17:05:47.241+0100 I COMMAND [conn6] command test.user appName: "MongoDB Shell" command: split { split: "test.user", find: { x: 1.0 }, lsid: { id: UUID("3b658803-7baf-4438-a103-23c828711b2b") }, $clusterTime: { clusterTime: Timestamp(1579536337, 1), signature: { hash: BinData(0, 0000000000000000000000000000000000000000), keyId: 0 } }, $db: "admin" } numYields:0 ok:0 errMsg:"Unable to find median in chunk, possibly because chunk is empty."
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Closing this ticket as won't fix because it is not possible to calculate the median with only one shard key on the chunk range. In case a specific split is needed, splitAt should be used.
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