Accept operator @@ to be used in the FullTextSearch. Find FieldIndex where columna name use _

The @@ operator was interpreted as parameter....

```c#
var query = @"select * 
                     from posts 
                     where search_vector @@ to_tsquery('english', @p0) 
                     order by ts_rank_cd(search_vector, to_tsquery('english', @p0)) desc";
var p = "postgres";
var posts = ctx.Posts.SqlQuery(query, p).ToList();
```

Look for columns name in the format with underscore like created_at, first_name, etc...