### Current State:
MongoDB currently lacks a native `$hash` operator in the aggregation/query language. While certain helpers like `convertShardKeyToHashed` or `$toHashedIndexKey` exist, they are limited in scope:
* Not available as true query operators (e.g., inaccessible from MongoDB drivers).
* Return `Long` values, which aren’t suitable for use cases like generating `ObjectId`-compatible hashes.
* Not guaranteed to be stable across versions — limiting their use in persisted or version-sensitive scenarios.
* Lack of flexibility in algorithm choice (e.g., MD5 only).
The deprecation of `$function` further limits users’ ability to implement custom hashing via JS.
---
### Impact:
Developers working with large datasets (dozens of GB) are unable to:
* Perform deterministic hashing inside MongoDB queries/aggregations.
* Use hash values as `_id` or compact document identifiers for deduplication, fingerprinting, or efficient lookups.
* Build predictable, cross-version-stable hashes within MQL.
* Avoid extra client-side computation or complex workarounds involving bitwise operations, slow modulo arithmetic, or full client-side aggregation to generate a hash.
Additionally, non-driver-accessible helpers lead to more application logic overhead and additional round-trips to the database, hurting latency and performance.
---
### Potential Future State:
Introduce a first-class `$hash` operator in the MongoDB query and aggregation language:
```js
{
$hash: {
algorithm: "SHA384", // e.g., SHA256, SHA512, etc.
data: ,
// Optional: key (for HMAC support in the future)
}
}
```
* Returns a hexadecimal string (or binary, or ObjectId-compatible format).
* Deterministic and version-stable.
* Accepts MQL expressions for the input.
* Could include support for standard NIST algorithms (see: [https://csrc.nist.gov/projects/hash-functions](https://csrc.nist.gov/projects/hash-functions)).
Initial implementation could focus on non-HMAC hashes only to reduce scope.
---
### Additional Info or Links:
* Use case: compact storage of hash-based `_id`s for large datasets.
* Common workaround (not ideal): [https://stackoverflow.com/questions/67426169/how-do-you-convert-a-hexadecimal-string-into-a-number-in-mongodb](https://stackoverflow.com/questions/67426169/how-do-you-convert-a-hexadecimal-string-into-a-number-in-mongodb)
* NIST-approved algorithms list: [https://csrc.nist.gov/projects/hash-functions](https://csrc.nist.gov/projects/hash-functions)
* Workarounds using `convertShardKeyToHashed` introduce compatibility risks, limited hash size (64-bit), and type conversion issues (e.g., can’t easily convert Long to hex string/ObjectId).
* Customer is open to discussing use cases in more depth.
