improve sql wall, support rand

src/main/java/com/alibaba/druid/sql/dialect/postgresql/visitor/PGEvalVisitor.java

@@ -135,6 +135,11 @@ public class PGEvalVisitor extends PGASTVisitorAdapter implements SQLEvalVisitor
     public void registerFunction(String funcName, Function function) {
         functions.put(funcName, function);
     }
+    
+    @Override
+    public void unregisterFunction(String funcName) {
+        functions.remove(funcName);
+    }
 
     public boolean visit(SQLIdentifierExpr x) {
         return SQLEvalVisitorUtils.visit(this, x);