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

Operations Unavailable in Sharded Environments分片化环境中不可用的操作

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

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

To use updateOne() and deleteOne() operations for a sharded collection that specify the multi: false or justOne option:要对指定multi:false或justOne选项的分片集合使用updateOne()和deleteOne()操作：

• If you only target one shard, you can use a partial shard key in the query specification or,如果你只针对一个分片，你可以在查询规范中使用部分分片键，或者，
• You can provide the shard key or the _id field in the query specification.您可以在查询规范中提供分片键或_id字段。

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. In these situations MongoDB will enforce uniqueness across the full key, not a single field.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。在这些情况下，MongoDB将强制整个键的唯一性，而不是单个字段。

Consistent Indexes一致的索引

MongoDB does not guarantee consistent indexes across shards. Index creation during addShard operations or chunk migrations may not propagate to new shards.MongoDB不保证跨分片的索引一致。在addShard操作或块迁移期间创建索引可能不会传播到新的分片。

To check a sharded cluster for consistent indexes, use the checkMetadataConsistency command:要检查分片集群的索引是否一致，请使用checkMetadataConsistency命令：

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

Write Concern for DDL Operations编写DDL操作问题

On a sharded cluster, DDL (Data Definition Language) operations run with write concern "majority". 在分片集群上，DDL（数据定义语言）操作以写关注"majority"运行。If you specify a different write concern, the operation overrides the provided write concern with "majority".如果指定了不同的写入关注，则该操作将用"majority"覆盖提供的写入关注。