reader-map: Add threshold-based B-tree storage option

Because the vector is stored in sorted order, insertions during startup
are O(n^2).  We add optional B-tree storage, and we cut over to a B-tree
at a hardcoded threshold of 10 values.

Change-Id: Ifb161a77c23d7e9669791415f73aad605c9ff2e2
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/7959
Tested-by: Buildkite CI
Reviewed-by: Jason Brown <[email protected]>

Author: rs-sac

partial-map/src/lib.rs

@@ -504,7 +504,7 @@ where
 }
 
 /// A trait that is required to keep the inner values in the correct order
-pub trait InsertionOrder<V>: Clone + Default {
+pub trait InsertionOrder<V>: Clone + fmt::Debug + Default {
     /// Compare two elements, like [`Ord::cmp`], but the comparison can be based on anything,
     /// not necessarily the same as `a.cmp(b)`.
     fn cmp(&self, a: &V, b: &V) -> std::cmp::Ordering;

reader-map/src/lib.rs

@@ -460,3 +460,15 @@ where
 {
     Options::default().construct()
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn default_size() {
+        // By ensuring this type is a ZST, we ensure that BTreeValue is a zero-cost wrapper
+        // around the wrapped value.
+        assert_eq!(std::mem::size_of::<DefaultInsertionOrder>(), 0);
+    }
+}

reader-map/src/read.rs

@@ -167,6 +167,7 @@ where
     pub fn first<'rh, Q>(&'rh self, key: &'_ Q) -> Result<Option<ReadGuard<'rh, V>>>
     where
         K: Borrow<Q>,
+        V: Ord,
         Q: Ord + Clone + Hash,
     {
         let vs = if let Some(vs) = self.get_raw(key.borrow())? {

reader-map/src/read/read_ref.rs

@@ -156,6 +156,7 @@ where
     pub fn first<'a, Q>(&'a self, key: &'_ Q) -> Option<&'a V>
     where
         K: Borrow<Q>,
+        V: Ord,
         Q: Ord + Hash + ?Sized,
     {
         self.guard.data.get(key).and_then(|values| values.first())