On Wed, Jul 21, 2010 at 05:00:43PM -0400, Aryeh Gregor wrote:
- We have no way to efficiently get all items that are in a category
and also in a particular namespace. Particularly, we can't retrieve all subcategories without scanning all items in the category, which is inefficient when we have a few (or no) subcategories and tons of items. (This is bugs 1211, 23682, and 450.)
"Categorymembers namespace filtering is inefficient, uses ugly hack in miser mode", https://bugzilla.wikimedia.org/show_bug.cgi?id=19640 , is also very related.
I think the goal for (2) should be to allow efficient separate retrieval of subcategories, files, and other pages, but not to distinguish between namespaces otherwise.
That wouldn't work if you did want to also fix bug 19640. It is quite plausible that a bot might want to query all ns0 pages in a category, or all talk pages, or all non-talk pages, or all templates, etc.