Hashed Indexes哈希索引

Embedded Documents嵌入式文档

The hashing function collapses embedded documents and computes the hash for the entire value, but does not support multi-key (i.e. arrays) indexes. 散列函数折叠嵌入的文档并计算整个值的散列，但不支持多关键字（即数组）索引。Specifically, creating a hashed index on a field that contains an array or attempting to insert an array to a hashed indexed field returns an error.具体来说，在包含数组的字段上创建哈希索引或试图将数组插入到哈希索引字段会返回错误。

Unique Constraint唯一约束

MongoDB does not support specifying a unique constraint on a hashed index. MongoDB不支持在hashed索引上指定唯一约束。You can instead create an additional non-hashed index with the unique constraint on that field. 相反，您可以在该字段上创建一个具有唯一约束的附加非哈希索引。MongoDB can use that non-hashed index for enforcing uniqueness on the field.MongoDB可以使用非散列索引来强制字段的唯一性。

2 ⁵³ Limit限度

PowerPC and 2 ⁶³

For hashed indexes, MongoDB 4.2 ensures that the hashed value for the floating point value 2 ⁶³ on PowerPC is consistent with other platforms.对于哈希索引，MongoDB 4.2确保PowerPC上浮点值2 ⁶³的散列值与其他平台一致。

Although hashed indexes on a field that may contain floating point values greater than 2 ⁵³ is an unsupported configuration, clients may still insert documents where the indexed field has the value 2 ⁶³.尽管可能包含大于2⁵³的浮点值的字段上的哈希索引是不受支持的配置，但是客户端仍然可以在索引字段具有值2⁶³的地方插入文档。

To list all hashed indexes for all collections in your deployment, you can use the following operation in mongosh:要列出部署中所有集合的所有hashed索引，可以在mongosh中使用以下操作：

db.adminCommand("listDatabases").databases.forEach(function(d){
   let mdb = db.getSiblingDB(d.name);
   mdb.getCollectionInfos({ type: "collection" }).forEach(function(c){
      let currentCollection = mdb.getCollection(c.name);
      currentCollection.getIndexes().forEach(function(idx){
        let idxValues = Object.values(Object.assign({}, idx.key));

        if (idxValues.includes("hashed")) {
          print("Hashed index: " + idx.name + " on " + d.name + "." + c.name);
          printjson(idx);
        };
      });
   });
});

To check if the indexed field contains the value 2 ⁶³, run the following operation for the collection and the indexed field:要检查索引字段是否包含值2⁶³，请对集合和索引字段运行以下操作：

• If the indexed field type is a scalar and never a document:如果索引字段类型是标量，而不是文档：

  // substitute the actual collection name for <collection>
  // substitute the actual indexed field name for <indexfield>
  
  db.<collection>.find( { <indexfield>: Math.pow(2,63) } );

• If the indexed field type is a document (or a scalar), you can run:如果索引字段类型是文档（或标量），则可以运行：

  // substitute the actual collection name for <collection>
  // substitute the actual indexed field name for <indexfield>
  
  db.<collection>.find({
      $where: function() {
          function findVal(obj, val) {
              if (obj === val)
                  return true;
  
              for (const child in obj) {
                  if (findVal(obj[child], val)) {
                      return true;
                  }
              }
              return false;
          }
          return findVal(this.<indexfield>, Math.pow(2, 63));
      }
  })