Hash Function in MongoDB

Hash Function in MongoDB

A hash function in MongoDB is used primarily for hashed indexes, which are indexes based on the hashed value of a field. Hashed indexes provide a uniform distribution of values, which can be particularly useful for sharding, ensuring even data distribution across shards.

Can You Apply Your Own Hash Function in MongoDB?

In MongoDB, you cannot directly apply your own custom hash function for indexing. MongoDB's hashed indexes use a built-in hash function that you cannot modify or replace. This built-in hash function is designed to ensure consistent and efficient distribution of data for sharding purposes.

How to Create a Hashed Index

Here’s how you can create a hashed index in MongoDB:

db.collection.createIndex({ field: "hashed" })

Use Cases for Hashed Indexes

• Sharding: Ensures even data distribution across shards.
• Uniform Data Distribution: Ideal for workloads where uniform distribution is required.

Example of Using Hashed Index

1. Creating a Hashed Index

   db.users.createIndex({ user_id: "hashed" })
   

2. Querying with Hashed Index

   db.users.find({ user_id: "someUserId" })
   

Limitations of Hashed Indexes

1. No Range Queries: Hashed indexes do not support range queries, as the hashed values do not preserve order.
2. Single Field Only: Hashed indexes can only be created on single fields, not compound fields.
3. Cannot Customize: As mentioned, you cannot customize the hash function used by MongoDB.

Workarounds for Custom Hash Functions

If you need to apply a custom hash function for some reason, you can:

1. Pre-hash the Data: Pre-process your data with your custom hash function before inserting it into MongoDB.
2. Store Hashed Values: Store both the original and hashed values if you need to query by both.

Example

1. Pre-hashing Data Before Insert

   const customHash = (value) => {
       // Implement your custom hash function here
       return yourHashedValue;
   };
   
   const document = { user_id: "someUserId", hashed_user_id: customHash("someUserId") };
   
   db.users.insert(document);
   

2. Creating an Index on the Hashed Value

   db.users.createIndex({ hashed_user_id: 1 });
   

3. Querying by Hashed Value

   db.users.find({ hashed_user_id: customHash("someUserId") });
   

Conclusion

While MongoDB does not allow for custom hash functions directly within its indexing mechanisms, you can still apply your own hash functions by pre-processing your data and storing the hashed values in the collection. This approach provides flexibility while maintaining the benefits of hashed indexing.

Links: Conclusions from CUP theorem for vectors data sets. What is the CAP Theorem?; Conclusions from CUP theorem for vectors data sets. What is the CAP Theorem?;