Efficient Datastructure for tags?

Imagine you wanted to serialize and deserialize stackoverflow posts including their tags as space efficiently as possible (in binary), but also for performance when doing tag lookups. Is there a good datastructure for that kind of scenario?

Stackoverflow has about 28532 different tags, you could create a table with all tags and assign them an integer, Furthermore you could sort them by frequency so that the most common tags have the lowest numbers. Still storing them simply like a string in the format "1 32 45" seems a bit inefficent borth from a searching and storing perspective

Another idea would be to save tags as a variable bitarray which is attractive from a lookup and serializing perspective. Since the most common tags are first you potentially could fit tags into a small amount of memory.

The problem would be of course that uncommon tags would yield huge bitarrays. Is there any standard for "compressing" bitarrays for large spans of 0's? Or should one use some other structure completely?

EDIT

I'm not looking for a DB solution or a solution where I need to keep entire tables in memory, but a structure for filtering individual items