Multiple-tag search is broken, returns results that don't have the tag(s)

Question

On the Search Help/FAQ page, it says you can narrow your search to specific tags:

However, when I run searches with multiple tags, I frequently get extra, erroneous results.
For example see this search for [greasemonkey] [iframe] user:331508:

(Click for larger image)

The searches are returning way too many results and the top results don't even have one of the required tags!

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Note, as pointed out in the comments, this is OR behavior, contrary to what the Help/Faq implies.

• There are currently 19 questions tagged both greasemonkey and iframe.
• There are currently 412 questions matching [greasemonkey] user:331508.
• There are currently, nominally 16 questions that should be returned by the [greasemonkey] [iframe] user:331508 query? (Note that the graphic above lists 414 hits, but it's now 415 hits.)

I have tried every variation of: AND, [AND], and + that I can think of. The search still returns incorrect results.

Answer 1

This will be fixed in the next build. A shortcut is used direct against SQL for these types of searches that ends up, by nature, being inaccurate and OR-ing the tags.

It boils down to this:

And Exists(Select 1 
             From PostTags pt 
            Where p.Id = pt.PostId 
              And pt.TagId in (@tagIds1, @tagIds2, @tagIds3)) 

That means we get an effective OR in our search, and it's a very non-trivial thing to change as that's used in many code paths. Instead of going that route, we'll submit more-than-one tag searches to Lucene and exactly 1 to SQL. The 1 case is especially important performance-wise because it's what you get when clicking a tag from a user's profile...so it's quite common and worth optimizing.