Secondary indexes support

CHANGELOG.md

@@ -45,6 +45,14 @@
   namespace in `PrefixedStorage::new`, `PrefixedStorage::multilevel`,
   `ReadonlyPrefixedStorage::new`, `ReadonlyPrefixedStorage::multilevel`,
   `prefixed` and `prefixed_read`.
+- Remove `Bucket::multilevel` as it was a misnomer. Provide support for
+  composite `PrimaryKey`s in storage, using `Pk2` and `Pk3` for 2 or 3 items
+  keys. Add `Bucket::range_prefixed` to provide the first section(s) of the
+  composite key and iterate under that.
+- Bucket stores all data under `(namespace, "_pk", key)`, where the first two
+  are length-prefixed. This is a different layout from previous form
+  where it was `(namespace, key)` and this is intended to support co-existence
+  with `IndexedBucket`. (It only changes the raw storage layout, APIs don't change).
 
 **cosmwasm-vm**
 

MIGRATING.md

@@ -162,6 +162,52 @@ major releases of `cosmwasm`. Note that you can also view the
   })
   ```
 
+- Remove `Bukcet::multilevel` constructor. The bucket now has exactly one namespace
+  and if we need `multilevel`, it was really to provide composite primary keys.
+  We now do that explicitly in a way that allows better compatibility
+  with `IndexedBucket`. The test `bucket_with_composite_pk` in `bucket.rs` is
+  a good (tested) example of the new API.
+
+  ```rust
+  // before (from cw20-base)
+  pub fn allowances<'a, S: Storage>(
+      storage: &'a mut S,
+      owner: &CanonicalAddr,
+  ) -> Bucket<'a, S, AllowanceResponse> {
+      Bucket::multilevel(&[PREFIX_ALLOWANCE, owner.as_slice()], storage)
+  }
+
+  allowances(&mut deps.storage, owner_raw).update(spender_raw.as_slice(), |allow| {
+      // code omitted...
+  })?;
+
+  let allowances: StdResult<Vec<AllowanceInfo>> = allowances_read(&deps.storage, &owner_raw)
+      .range(start.as_deref(), None, Order::Ascending)
+      .take(limit)
+      .map(|item| {
+         // code omitted...
+      })
+      .collect();
+
+  // after
+  pub fn allowances<S: Storage>(storage: &mut S) -> Bucket<S, AllowanceResponse> {
+      Bucket::new(PREFIX_ALLOWANCE, storage)
+  }
+
+  allowances(&mut deps.storage).update(&Pk2(owner_raw.as_slice(), spender_raw.as_slice()).pk(), |allow| {
+      // code omitted...
+  })?;
+
+  let allowances: StdResult<Vec<AllowanceInfo>> = allowances(&deps.storage)
+      .range_prefixed(owner_raw.as_slice(), start.as_deref(), None, Order::Ascending)
+      .take(limit)
+      .map(|item| {
+         // code omitted...
+      })
+      .collect();
+  ```
+
+
 ## 0.9 -> 0.10
 
 Integration tests: