treat parameters as "constant" within a query

core/translate/optimizer/mod.rs

@@ -1071,7 +1071,7 @@ impl Optimizable for ast::Expr {
             Expr::Register(..) => false, // Register values can be null
         }
     }
-    /// Returns true if the expression is a constant i.e. does not depend on variables or columns etc.
+    /// Returns true if the expression is a constant i.e. does not depend on columns and can be evaluated only once during the execution
     fn is_constant(&self, resolver: &Resolver<'_>) -> bool {
         match self {
             Expr::SubqueryResult { .. } => false,
@@ -1142,8 +1142,8 @@ impl Optimizable for ast::Expr {
             Expr::Raise(_, expr) => expr.as_ref().is_none_or(|expr| expr.is_constant(resolver)),
             Expr::Subquery(_) => false,
             Expr::Unary(_, expr) => expr.is_constant(resolver),
-            Expr::Variable(_) => false,
-            Expr::Register(_) => false, // Register values are not constants
+            Expr::Variable(_) => true,
+            Expr::Register(_) => true,
         }
     }
     /// Returns true if the expression is a constant expression that, when evaluated as a condition, is always true or false

tests/integration/query_processing/test_read_path.rs

@@ -1018,3 +1018,47 @@ fn test_many_columns() {
         ]]
     );
 }
+
+#[test]
+fn test_eval_param_only_once() {
+    let tmp_db = TempDatabase::new("test_eval_param_only_once");
+    let conn = tmp_db.connect_limbo();
+    conn.execute("CREATE TABLE t(x)").unwrap();
+    conn.execute("INSERT INTO t SELECT value FROM generate_series(1, 10000)")
+        .unwrap();
+    let mut stmt = conn
+        .query("SELECT COUNT(*) FROM t WHERE LENGTH(zeroblob(?)) = ?")
+        .unwrap()
+        .unwrap();
+    stmt.bind_at(
+        1.try_into().unwrap(),
+        turso_core::Value::Integer(100_000_000),
+    );
+    stmt.bind_at(
+        2.try_into().unwrap(),
+        turso_core::Value::Integer(100_000_000),
+    );
+    let start_time = std::time::Instant::now();
+    loop {
+        match stmt.step().unwrap() {
+            StepResult::IO => {
+                stmt.run_once().unwrap();
+            }
+            StepResult::Done => break,
+            StepResult::Row => {
+                let values = stmt
+                    .row()
+                    .unwrap()
+                    .get_values()
+                    .cloned()
+                    .collect::<Vec<_>>();
+                assert_eq!(values, vec![turso_core::Value::Integer(10000)]);
+            }
+            _ => unreachable!(),
+        }
+    }
+    let end_time = std::time::Instant::now();
+    let elapsed = end_time.duration_since(start_time);
+    // the test will allocate 10^8 * 10^4 bytes in case if parameter will be evaluated for every row
+    assert!(elapsed < std::time::Duration::from_millis(100));
+}