Is it possible to create a unique index based on a hash function?.

Yes, it is possible to create a unique index based on a hash function, but with some caveats. A hash function transforms data into a fixed length, and theoretically, different input data can have the same hash (collisions).

In databases, sometimes a hash index is created to speed up searches, but guaranteeing uniqueness solely by hash is not possible without additional checks.

If a unique constraint is needed, it is better to use the original data or combine the hash with additional checks.

Example in Go for creating a hash and using it in an index:

import (
    "crypto/sha256"
    "fmt"
)

func hashKey(key string) string {
    h := sha256.Sum256([]byte(key))
    return fmt.Sprintf("%x", h)
}

// Use hashKey to create an index, but check for collisions during insertion.