Operational Restrictions in Sharded Clusters分片集群中的操作限制

Operations Unavailable in Sharded Environments在共享环境中不可用的操作

$where does not permit references to the db object from the $where function. 不允许从$where函数引用db对象。This is uncommon in un-sharded collections.这在未分片的集合中并不常见。

The geoSearch command is not supported in sharded environments.在分片环境中不支持geoSearch命令。

In MongoDB 5.0 and earlier, you cannot specify sharded collections in the from parameter of $lookup stages.在MongoDB 5.0及更早版本中，不能在$lookup阶段的from参数中指定分片集合。

Single Document Modification Operations in Sharded Collections分片集合中的单个文档修改操作

All updateOne() and deleteOne() operations for a sharded collection that specify the multi: false or justOne option must include the shard key or the _id field in the query specification.指定multi: false或justOne选项的分片集合的所有updateOne()和deleteOne()操作都必须在查询规范中包括分片键或_id字段。

updateOne() and deleteOne() operations specifying multi: false or justOne in a sharded collection which do not contain either the shard key or the _id field return an error.在不包含分片键或_id字段的分片集合中指定multi:false或justOne的updateOne()和deleteOne()操作返回错误。

To use findOneAndUpdate() with a sharded collection, your query filter must include an equality condition on the shard key to compare the key and value in either of these formats:要将findOneAndUpdate()与分片集合一起使用，查询筛选器必须在分片键上包含一个相等条件，以便以以下任一格式比较键和值：

{ key: value }
{ key: { $eq: value } }

Unique Indexes in Sharded Collections分片集合中的唯一索引

MongoDB does not support unique indexes across shards, except when the unique index contains the full shard key as a prefix of the index. MongoDB不支持跨分片的唯一索引，除非唯一索引包含完整的分片键作为索引的前缀。In these situations MongoDB will enforce uniqueness across the full key, not a single field.在这些情况下，MongoDB将在整个键中强制执行唯一性，而不是单个字段。

Consistent Indexes一致性索引

MongoDB does not guarantee consistent indexes across shards. MongoDB不能保证分片之间的索引一致。Index creation during addShard operations or chunk migrations may not propagate to new shards.addShard操作或区块迁移期间的索引创建可能不会传播到新的分片。

To check a sharded cluster for consistent indexes, use the checkMetadataConsistency command:要检查分片集群的一致索引，请使用checkMetadataConsistency命令：

db.runCommand( {
   checkMetadataConsistency: 1,
   checkIndexes: true
} )