"Where used" not returning all the "using" items

Question

I am using the core service (Tridion 2013 (no SP)) to get some stats and I am experiencing a weird behavior. In the system there are multiple pages (494) using the same page template. Using the core service I run this code:

string id = "tcm:73-18028-128";

var wuFilter = new UsingItemsFilterData();

//wuFilter.IncludeLocalCopies = true;
//wuFilter.IncludeVersionsColumn = true;
wuFilter.IncludedVersions = VersionCondition.OnlyLatestAndCheckedOutVersions;
//wuFilter.BaseColumns = ListBaseColumns.Extended;

wuFilter.ItemTypes = new ItemType[]{ItemType.Page};

var xml = Tridion.GetListXml(id, wuFilter);
XNamespace ns = xml.Name.Namespace;
Debug.WriteLine("Pages Using Template "+id+":" + xml.Descendants(ns+"Item").Count());

That gives me 87 pages.

When I select such page template in the CME and do a "Where used", I also get 87.

Now, If I get all the pages in that publication and count those using the previous PT, I get 494. I used this code for this:

var filter = new RepositoryItemsFilterData ();
filter.Recursive = true;
filter.ItemTypes = new ItemType[]{ItemType.Page};

var otherXml = Tridion.GetListXml("tcm:0-73-1", filter);
XNamespace ns2 = otherXml.Name.Namespace;
int counter = 0;
foreach (XElement node in otherXml.Descendants(ns2 + "Item"))
{
    string id2 = node.Attributes("ID").FirstOrDefault().Value;
    PageData page = Tridion.Read(id2, new ReadOptions()) as PageData;
    if(page.PageTemplate.IdRef == id){
        counter++;
    }
}
Debug.WriteLine("Pages Using Template "+id+":" + counter);

Anybody experienced weird behavior with "Where Used"?

Answer 1

Moving some of the comments into an answer - since nobody reads comments anyway...

What you are describing is actually how the system works. The reference between the Page Template and the Structure Group is what "Where Used" will track, and not the individual pages within the Structure Group.

So, technically, what you are seeing is the correct behavior - the Page Template is used by the Structure Group and the pages - when rendered - will use this information to find the template to use. If you need to calculate which pages are currently published with a given template (which probably matches closer to your definition of "being used") then it might be easier to get this data from the Content Delivery DB if this is an option.

As to whether the current behavior is the correct one... that's a complex question, and as I mention in a comment, it challenges the current definition of "being used" altogether. If I set a Page Template in a Structure Group and never publish the pages within it - is this Page Template being used by the pages? Is it "at risk" of being used? What if I published the pages, then change the information in the Structure Group and then delete the page template? Which template is in use now by the pages?

Interesting discussion for which there is probably a valid answer - I would prefer to keep the system as is than interfering with the definition of "being used" and risk making wrong assumptions that would make it even harder to determine what templates are used where.

Answer 2

If you are interested in knowing how many pages will be affected you should use the "show which items will be published" option on the publish dialog (which you will reach by publishing the page template):