@Background Pony #FDCD
Yes. In brief, indexes allow very rapid access to frequently required information in a database table (in this case, the tag). Sometimes things we do can screw up the accuracy of the index. Like - aliasing tags does that sometimes. Reindexing it means we toss out the old index and rebuild the index from scratch.
Typically things like “Social Security Number” is used as an index, because of the belief that SSNs are unique (although this is not actually true and can result in indexing problems if handled badly). Wow. I just had a flashback to the datawarehousing I did for some insurance companies. THAT was unexpected.
On this site, things like “How many times is a tag used”(the number following the tag) can get screwed up if the index for the tag has been messed with as the result of some of the mass tagging things we do.
BTW, Philomena is much better with indexes. When we were BooruOnRails if there was a competing processes like someone adding the tag at the precise moment someone else deleted it the index would end up in lala land. And trying to fix it was a crap shoot that could sometimes take days for jobs to complete.
So, on Philomena it’s much rare that these get out of order, but those are mostly limited to staff operations now, and when it does happen fixing it is super reliable and fast.
[edit: much rare]