gdb 循环观察 expr

Question

代码片段：

foreach(lc, vars)
{
    if (IsA(lfirst(lc), Var))
    {
        Var *var = (Var *) lfirst(lc);
        /* if system column, return error */
        if (var->varattno < 0)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                        errmsg("system column is not supported on incrementally maintainable materialized view")));
    }
}

我尝试过的：

(gdb) watch var->varattno
Attempt to extract a component of a value that is not a structure pointer.

foreach 将循环遍历。我想看看var->varattno什么时候循环。