Function and Literal expressions can be non-deterministic but in most cases they are safe, and giving them proper toString()'s will help caching quite a bit.

src/java/com/samskivert/depot/expression/FunctionExp.java

@@ -20,6 +20,8 @@
 
 package com.samskivert.depot.expression;
 
+import com.samskivert.util.StringUtil;
+
 import java.util.Collection;
 
 import com.samskivert.depot.PersistentRecord;
@@ -63,6 +65,11 @@ public class FunctionExp implements SQLExpression
         return _arguments;
     }
 
+    public String toString ()
+    {
+        return _function + "(" + StringUtil.join(_arguments, ", ") + ")";
+    }
+
     /** The literal name of this function, e.g. FLOOR */
     protected String _function;
 

src/java/com/samskivert/depot/expression/LiteralExp.java

@@ -53,6 +53,11 @@ public class LiteralExp
         return _text;
     }
 
+    public String toString ()
+    {
+        return _text;
+    }
+
     /** The literal text of this expression, e.g. COUNT(*) */
     protected String _text;
 }