The concept of parent="-1" is to get parent up by 1 level. If parent="-3", is meaning up to 3 level parent channel
To illustrate how its work; here is sample channel hierarchy:
> channel level 1
> channel level 2
> channel level 3
...
Problem cause:
The issue of normal search indexing is due to many factors of environment such as networking speed, numbers of content and files to index. Since there is memory allocation, script time execution setting are also could be affected to this s...