I try to find something... 
now I try to use digikam for tagged-photos-idea-base for knitted clothes construction team, 
much tags must be for each photo there, imho 10-50....? tags per item. 
Advanced filtering is must for us.  
usebility and workfow speed is one of first conditions.

Program is really great! Clear Interface, functionality reach, 
and most filtering features are very good, but tag-querying is not enought as is now. 
More over, IMHO  strange logic of tag-quering...
[+] sign on each tag adds to query result too much "trash" - when checked 4-5 tags, 
but revelant only 1-10% items for my case... I need get only exact tags 1+2+3
Ok, we have [-] but "not checked" and "[-] checked" is not the same?

I mean that tag-filtering/quering is for exclude
items if not true "tag1 AND tag2 AND... tagN all present"  then exclude items from result"
I try also as variant for our team piwigo, where tags work as I mean by default,
but web-based app is too slow workfow for us, but client-server for team...
So, I use digicam for prepare collections by quick tagging 
before publish at own piwigo intranet test-srv...
Some tags we have to add at piwigo, but mainly by digicam right into jpeg EXIF.
Syncronisation will be real next headache )))

IMHO, with current logic tag-filter is unusable now.
May be, on place of [+] and [-] signs for each tag - 
should be global logic AND/EXCLUDE (against OR/INCLUDE) and ruleset as  [AND], [OR], [NOT]

bad English, sorry

Reproducible: Always

Steps to Reproduce:
1. check any 3(or more) tag , for get result exclude items where not all of them presents
2. we can see all items with tags 1, 2, 3, 1+2, 1+3, 2+3  where should be only 1+2+3 
3. profit! )))))

Actual Results:  
tags 1, 2, 3, 1+2, 1+3, 2+3 

Expected Results:  
exact tags 1+2+3 all present only.
I'm can't belive that (result = any of tags 1, 2, 3, 1+2, 1+3, 2+3 ) 
is not a bug )))

Shoudn't I choose minor or feature request? 
May be, but now so good program is half-unusable for me.