Set of Keys from a Storage Map

Question

Is it possible to get the set of keys from a Storage Map on Substrate. It doesn’t seem like Substrate exposes a method to do so: https://docs.substrate.io/rustdocs/latest/frame_support/storage/trait.StorageMap.html#required-methods.

Answer 1

The FRAME StorageMap also implements IterableStorageMap which allows you to iterate over the keys and/or values of the map:

pub trait IterableStorageMap<K: FullEncode, V: FullCodec>: StorageMap<K, V> {
    type Iterator: Iterator<Item = (K, V)>;
    type KeyIterator: Iterator<Item = K>;
    fn iter() -> Self::Iterator;
    fn iter_from(starting_raw_key: Vec<u8>) -> Self::Iterator;
    fn iter_keys() -> Self::KeyIterator;
    fn iter_keys_from(starting_raw_key: Vec<u8>) -> Self::KeyIterator;
    fn drain() -> Self::Iterator;
    fn translate<O: Decode, F: FnMut(K, O) -> Option<V>>(f: F);
}

However, you should probably never use this in your runtime in an unbounded way. Iterating over the N keys of your storage map would require N database reads, which can be extremely expensive for your runtime weight.

Instead you should use something like fn iter_keys_from where you can pick a specific key to start iterating from, and control how many keys you iterate in a single call.