diff --git a/src/debug.rs b/src/debug.rs
new file mode 100644
index 00000000..491ffdf1
--- /dev/null
+++ b/src/debug.rs
@@ -0,0 +1,28 @@
+//! Debugging APIs: these are meant for use when unit-testing or
+//! debugging your application but aren't ordinarily needed.
+
+use crate::Database;
+use crate::Query;
+use crate::QueryStorageOps;
+use crate::QueryTable;
+
+pub trait DebugQueryTable {
+    type Key;
+
+    /// True if salsa thinks that the value for `key` is a
+    /// **constant**, meaning that it can never change, no matter what
+    /// values the inputs take on from this point.
+    fn is_constant(&self, key: Self::Key) -> bool;
+}
+
+impl<DB, Q> DebugQueryTable for QueryTable<'_, DB, Q>
+where
+    DB: Database,
+    Q: Query<DB>,
+{
+    type Key = Q::Key;
+
+    fn is_constant(&self, key: Q::Key) -> bool {
+        self.storage.is_constant(self.db, &key)
+    }
+}
diff --git a/src/derived.rs b/src/derived.rs
index 49ca1e5d..e827f71b 100644
--- a/src/derived.rs
+++ b/src/derived.rs
@@ -396,6 +396,15 @@ where
 
         true
     }
+
+    fn is_constant(&self, _db: &DB, key: &Q::Key) -> bool {
+        let map_read = self.map.read();
+        match map_read.get(key) {
+            None => false,
+            Some(QueryState::InProgress) => panic!("query in progress"),
+            Some(QueryState::Memoized(memo)) => memo.changed_at.is_constant(),
+        }
+    }
 }
 
 impl<DB, Q, MP> UncheckedMutQueryStorageOps<DB, Q> for DerivedStorage<DB, Q, MP>
@@ -443,6 +452,19 @@ where
 
     fn verify_inputs(&self, db: &DB) -> bool {
         match self.changed_at {
+            ChangedAt::Constant(_) => {
+                // If we know that the value is constant, it had
+                // better not change, but in that case, we ought not
+                // to have any inputs. Using `debug_assert` because
+                // this is on the fast path.
+                debug_assert!(match &self.inputs {
+                    QueryDescriptorSet::Tracked(inputs) => inputs.is_empty(),
+                    QueryDescriptorSet::Untracked => false,
+                });
+
+                true
+            }
+
             ChangedAt::Revision(revision) => match &self.inputs {
                 QueryDescriptorSet::Tracked(inputs) => inputs
                     .iter()
diff --git a/src/input.rs b/src/input.rs
index 80678bfa..a90c92ea 100644
--- a/src/input.rs
+++ b/src/input.rs
@@ -4,7 +4,7 @@ use crate::runtime::Revision;
 use crate::runtime::StampedValue;
 use crate::CycleDetected;
 use crate::Database;
-use crate::MutQueryStorageOps;
+use crate::InputQueryStorageOps;
 use crate::Query;
 use crate::QueryDescriptor;
 use crate::QueryStorageOps;
@@ -46,6 +46,8 @@ where
     }
 }
 
+struct IsConstant(bool);
+
 impl<DB, Q> InputStorage<DB, Q>
 where
     Q: Query<DB>,
@@ -70,6 +72,67 @@ where
             changed_at: ChangedAt::Revision(Revision::ZERO),
         })
     }
+
+    fn set_common(&self, db: &DB, key: &Q::Key, value: Q::Value, is_constant: IsConstant) {
+        let mut map = self.map.write();
+
+        // If this value was previously stored, check if this is an
+        // *actual change* before we do anything.
+        if let Some(old_value) = map.get_mut(key) {
+            if old_value.value == value {
+                // If the value did not change, but it is now
+                // considered constant, we can just update
+                // `changed_at`. We don't have to trigger a new
+                // revision for this case: all the derived values are
+                // still intact, they just have conservative
+                // dependencies. The next revision, they may wind up
+                // with something more precise.
+                if is_constant.0 && !old_value.changed_at.is_constant() {
+                    old_value.changed_at =
+                        ChangedAt::Constant(db.salsa_runtime().current_revision());
+                }
+
+                return;
+            }
+        }
+
+        let key = key.clone();
+
+        // The value is changing, so even if we are setting this to a
+        // constant, we still need a new revision.
+        let next_revision = db.salsa_runtime().increment_revision();
+
+        // Do this *after* we acquire the lock, so that we are not
+        // racing with somebody else to modify this same cell.
+        // (Otherwise, someone else might write a *newer* revision
+        // into the same cell while we block on the lock.)
+        let changed_at = if is_constant.0 {
+            ChangedAt::Constant(next_revision)
+        } else {
+            ChangedAt::Revision(next_revision)
+        };
+
+        let stamped_value = StampedValue { value, changed_at };
+
+        match map.entry(key) {
+            Entry::Occupied(mut entry) => {
+                assert!(
+                    !entry.get().changed_at.is_constant(),
+                    "modifying `{:?}({:?})`, which was previously marked as constant (old value `{:?}`, new value `{:?}`)",
+                    Q::default(),
+                    entry.key(),
+                    entry.get().value,
+                    stamped_value.value,
+                );
+
+                entry.insert(stamped_value);
+            }
+
+            Entry::Vacant(entry) => {
+                entry.insert(stamped_value);
+            }
+        }
+    }
 }
 
 impl<DB, Q> QueryStorageOps<DB, Q> for InputStorage<DB, Q>
@@ -115,26 +178,28 @@ where
 
         changed_at.changed_since(revision)
     }
+
+    fn is_constant(&self, _db: &DB, key: &Q::Key) -> bool {
+        let map_read = self.map.read();
+        map_read
+            .get(key)
+            .map(|v| v.changed_at.is_constant())
+            .unwrap_or(false)
+    }
 }
 
-impl<DB, Q> MutQueryStorageOps<DB, Q> for InputStorage<DB, Q>
+impl<DB, Q> InputQueryStorageOps<DB, Q> for InputStorage<DB, Q>
 where
     Q: Query<DB>,
     DB: Database,
     Q::Value: Default,
 {
     fn set(&self, db: &DB, key: &Q::Key, value: Q::Value) {
-        let key = key.clone();
+        self.set_common(db, key, value, IsConstant(false))
+    }
 
-        let mut map_write = self.map.write();
-
-        // Do this *after* we acquire the lock, so that we are not
-        // racing with somebody else to modify this same cell.
-        // (Otherwise, someone else might write a *newer* revision
-        // into the same cell while we block on the lock.)
-        let changed_at = ChangedAt::Revision(db.salsa_runtime().increment_revision());
-
-        map_write.insert(key, StampedValue { value, changed_at });
+    fn set_constant(&self, db: &DB, key: &Q::Key, value: Q::Value) {
+        self.set_common(db, key, value, IsConstant(true))
     }
 }
 
diff --git a/src/lib.rs b/src/lib.rs
index 56b6faca..80653de8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -12,6 +12,7 @@ use std::fmt::Display;
 use std::fmt::Write;
 use std::hash::Hash;
 
+pub mod debug;
 pub mod derived;
 pub mod input;
 pub mod runtime;
@@ -119,17 +120,22 @@ where
         key: &Q::Key,
         descriptor: &DB::QueryDescriptor,
     ) -> bool;
+
+    /// Check if `key` is (currently) believed to be a constant.
+    fn is_constant(&self, db: &DB, key: &Q::Key) -> bool;
 }
 
 /// An optional trait that is implemented for "user mutable" storage:
 /// that is, storage whose value is not derived from other storage but
 /// is set independently.
-pub trait MutQueryStorageOps<DB, Q>: Default
+pub trait InputQueryStorageOps<DB, Q>: Default
 where
     DB: Database,
     Q: Query<DB>,
 {
     fn set(&self, db: &DB, key: &Q::Key, new_value: Q::Value);
+
+    fn set_constant(&self, db: &DB, key: &Q::Key, new_value: Q::Value);
 }
 
 /// An optional trait that is implemented for "user mutable" storage:
@@ -146,8 +152,8 @@ where
 #[derive(new)]
 pub struct QueryTable<'me, DB, Q>
 where
-    DB: Database,
-    Q: Query<DB>,
+    DB: Database + 'me,
+    Q: Query<DB> + 'me,
 {
     db: &'me DB,
     storage: &'me Q::Storage,
@@ -170,15 +176,25 @@ where
             })
     }
 
-    /// Assign a value to an "input queries". Must be used outside of
+    /// Assign a value to an "input query". Must be used outside of
     /// an active query computation.
     pub fn set(&self, key: Q::Key, value: Q::Value)
     where
-        Q::Storage: MutQueryStorageOps<DB, Q>,
+        Q::Storage: InputQueryStorageOps<DB, Q>,
     {
         self.storage.set(self.db, &key, value);
     }
 
+    /// Assign a value to an "input query", with the additional
+    /// promise that this value will **never change**. Must be used
+    /// outside of an active query computation.
+    pub fn set_constant(&self, key: Q::Key, value: Q::Value)
+    where
+        Q::Storage: InputQueryStorageOps<DB, Q>,
+    {
+        self.storage.set_constant(self.db, &key, value);
+    }
+
     /// Assigns a value to the query **bypassing the normal
     /// incremental checking** -- this value becomes the value for the
     /// query in the current revision. This can even be used on
diff --git a/src/runtime.rs b/src/runtime.rs
index e032453d..41c6cba7 100644
--- a/src/runtime.rs
+++ b/src/runtime.rs
@@ -134,7 +134,11 @@ where
     /// - `descriptor`: the query whose result was read
     /// - `changed_revision`: the last revision in which the result of that
     ///   query had changed
-    pub(crate) fn report_query_read(&self, descriptor: &DB::QueryDescriptor, changed_at: ChangedAt) {
+    pub(crate) fn report_query_read(
+        &self,
+        descriptor: &DB::QueryDescriptor,
+        changed_at: ChangedAt,
+    ) {
         if let Some(top_query) = self.local_state.borrow_mut().query_stack.last_mut() {
             top_query.add_read(descriptor, changed_at);
         }
@@ -194,14 +198,22 @@ impl<DB: Database> ActiveQuery<DB> {
     fn new(descriptor: DB::QueryDescriptor) -> Self {
         ActiveQuery {
             descriptor,
-            changed_at: ChangedAt::Revision(Revision::ZERO),
+            changed_at: ChangedAt::Constant(Revision::ZERO),
             subqueries: QueryDescriptorSet::default(),
         }
     }
 
     fn add_read(&mut self, subquery: &DB::QueryDescriptor, changed_at: ChangedAt) {
-        self.subqueries.insert(subquery.clone());
-        self.changed_at = self.changed_at.max(changed_at);
+        match changed_at {
+            ChangedAt::Constant(_) => {
+                // When we read constant values, we don't need to
+                // track the source of the value.
+            }
+            ChangedAt::Revision(_) => {
+                self.subqueries.insert(subquery.clone());
+                self.changed_at = self.changed_at.max(changed_at);
+            }
+        }
     }
 
     fn add_untracked_read(&mut self, changed_at: ChangedAt) {
@@ -232,14 +244,28 @@ impl std::fmt::Debug for Revision {
 /// changed.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum ChangedAt {
+    /// Will never change again (and the revision in which we became a
+    /// constant).
+    Constant(Revision),
+
+    /// Last changed in the given revision. May change in the future.
     Revision(Revision),
 }
 
 impl ChangedAt {
-    /// True if this value has changed after `revision`.
+    pub fn is_constant(self) -> bool {
+        match self {
+            ChangedAt::Constant(_) => true,
+            ChangedAt::Revision(_) => false,
+        }
+    }
+
+    /// True if a value is stored with this `ChangedAt` value has
+    /// changed after `revision`. This is invoked by query storage
+    /// when their dependents are asking them if they have changed.
     pub fn changed_since(self, revision: Revision) -> bool {
         match self {
-            ChangedAt::Revision(r) => r > revision,
+            ChangedAt::Constant(r) | ChangedAt::Revision(r) => r > revision,
         }
     }
 }
diff --git a/tests/incremental/constants.rs b/tests/incremental/constants.rs
new file mode 100644
index 00000000..a7391bda
--- /dev/null
+++ b/tests/incremental/constants.rs
@@ -0,0 +1,127 @@
+use crate::implementation::{TestContext, TestContextImpl};
+use salsa::debug::DebugQueryTable;
+use salsa::Database;
+
+salsa::query_group! {
+    pub(crate) trait ConstantsDatabase: TestContext {
+        fn constants_input(key: char) -> usize {
+            type ConstantsInput;
+            storage input;
+        }
+
+        fn constants_add(keys: (char, char)) -> usize {
+            type ConstantsAdd;
+        }
+    }
+}
+
+fn constants_add(db: &impl ConstantsDatabase, (key1, key2): (char, char)) -> usize {
+    db.log().add(format!("add({}, {})", key1, key2));
+    db.constants_input(key1) + db.constants_input(key2)
+}
+
+#[test]
+#[should_panic]
+fn invalidate_constant() {
+    let db = &TestContextImpl::default();
+    db.query(ConstantsInput).set_constant('a', 44);
+    db.query(ConstantsInput).set_constant('a', 66);
+}
+
+#[test]
+#[should_panic]
+fn invalidate_constant_1() {
+    let db = &TestContextImpl::default();
+
+    // Not constant:
+    db.query(ConstantsInput).set('a', 44);
+
+    // Becomes constant:
+    db.query(ConstantsInput).set_constant('a', 44);
+
+    // Invalidates:
+    db.query(ConstantsInput).set_constant('a', 66);
+}
+
+/// Test that use can still `set` an input that is constant, so long
+/// as you don't change the value.
+#[test]
+fn set_after_constant_same_value() {
+    let db = &TestContextImpl::default();
+    db.query(ConstantsInput).set_constant('a', 44);
+    db.query(ConstantsInput).set('a', 44);
+}
+
+#[test]
+fn not_constant() {
+    let db = &TestContextImpl::default();
+
+    db.query(ConstantsInput).set('a', 22);
+    db.query(ConstantsInput).set('b', 44);
+    assert_eq!(db.constants_add(('a', 'b')), 66);
+    assert!(!db.query(ConstantsAdd).is_constant(('a', 'b')));
+}
+
+#[test]
+fn is_constant() {
+    let db = &TestContextImpl::default();
+
+    db.query(ConstantsInput).set_constant('a', 22);
+    db.query(ConstantsInput).set_constant('b', 44);
+    assert_eq!(db.constants_add(('a', 'b')), 66);
+    assert!(db.query(ConstantsAdd).is_constant(('a', 'b')));
+}
+
+#[test]
+fn mixed_constant() {
+    let db = &TestContextImpl::default();
+
+    db.query(ConstantsInput).set_constant('a', 22);
+    db.query(ConstantsInput).set('b', 44);
+    assert_eq!(db.constants_add(('a', 'b')), 66);
+    assert!(!db.query(ConstantsAdd).is_constant(('a', 'b')));
+}
+
+#[test]
+fn becomes_constant() {
+    let db = &TestContextImpl::default();
+
+    db.query(ConstantsInput).set('a', 22);
+    db.query(ConstantsInput).set('b', 44);
+    assert_eq!(db.constants_add(('a', 'b')), 66);
+    assert!(!db.query(ConstantsAdd).is_constant(('a', 'b')));
+
+    db.query(ConstantsInput).set_constant('a', 23);
+    assert_eq!(db.constants_add(('a', 'b')), 67);
+    assert!(!db.query(ConstantsAdd).is_constant(('a', 'b')));
+
+    db.query(ConstantsInput).set_constant('b', 45);
+    assert_eq!(db.constants_add(('a', 'b')), 68);
+    assert!(db.query(ConstantsAdd).is_constant(('a', 'b')));
+}
+
+#[test]
+fn becomes_constant_no_change() {
+    let db = &TestContextImpl::default();
+
+    db.query(ConstantsInput).set('a', 22);
+    db.query(ConstantsInput).set('b', 44);
+    assert_eq!(db.constants_add(('a', 'b')), 66);
+    assert!(!db.query(ConstantsAdd).is_constant(('a', 'b')));
+    db.assert_log(&["add(a, b)"]);
+
+    // 'a' is now constant, but the value did not change; this
+    // should not in and of itself trigger a new revision.
+    db.query(ConstantsInput).set_constant('a', 22);
+    assert_eq!(db.constants_add(('a', 'b')), 66);
+    assert!(!db.query(ConstantsAdd).is_constant(('a', 'b')));
+    db.assert_log(&[]); // no new revision, no new log entries
+
+    // 'b' is now constant, and its value DID change. This triggers a
+    // new revision, and at that point we figure out that we are
+    // constant.
+    db.query(ConstantsInput).set_constant('b', 45);
+    assert_eq!(db.constants_add(('a', 'b')), 67);
+    assert!(db.query(ConstantsAdd).is_constant(('a', 'b')));
+    db.assert_log(&["add(a, b)"]);
+}
diff --git a/tests/incremental/implementation.rs b/tests/incremental/implementation.rs
index 868e49c9..36809bef 100644
--- a/tests/incremental/implementation.rs
+++ b/tests/incremental/implementation.rs
@@ -1,3 +1,4 @@
+use crate::constants;
 use crate::counter::Counter;
 use crate::log::Log;
 use crate::memoized_dep_inputs;
@@ -43,6 +44,11 @@ impl TestContextImpl {
 
 salsa::database_storage! {
     pub(crate) struct TestContextImplStorage for TestContextImpl {
+        impl constants::ConstantsDatabase {
+            fn constants_input() for constants::ConstantsInput;
+            fn constants_derived() for constants::ConstantsAdd;
+        }
+
         impl memoized_dep_inputs::MemoizedDepInputsContext {
             fn dep_memoized2() for memoized_dep_inputs::Memoized2;
             fn dep_memoized1() for memoized_dep_inputs::Memoized1;
diff --git a/tests/incremental/main.rs b/tests/incremental/main.rs
index 33a623cd..bcd13c75 100644
--- a/tests/incremental/main.rs
+++ b/tests/incremental/main.rs
@@ -1,3 +1,4 @@
+mod constants;
 mod counter;
 mod implementation;
 mod log;
diff --git a/tests/incremental/memoized_inputs.rs b/tests/incremental/memoized_inputs.rs
index 0fdc9b79..23e690f3 100644
--- a/tests/incremental/memoized_inputs.rs
+++ b/tests/incremental/memoized_inputs.rs
@@ -60,3 +60,20 @@ fn revalidate() {
     assert_eq!(v, 66);
     db.assert_log(&[]);
 }
+
+/// Test that invoking `set` on an input with the same value does not
+/// trigger a new revision.
+#[test]
+fn set_after_no_change() {
+    let db = &TestContextImpl::default();
+
+    db.query(Input1).set((), 44);
+    let v = db.max(());
+    assert_eq!(v, 44);
+    db.assert_log(&["Max invoked"]);
+
+    db.query(Input1).set((), 44);
+    let v = db.max(());
+    assert_eq!(v, 44);
+    db.assert_log(&[]);
+}