Should tags reflect questions exclusively or the entire question-answer post

Question

This question has been discussed on other stackexchange sites as well but I feel this is a good time to ask it and get a basic understanding of where we are at. So here we go. When asking questions the OP sets tags. These tags then reflects the question as the Original Post sees it. We can edit the tags list if there is an obvious omission. When answers arrive it may become evident that the question may not completely reflect what the answers reveal and so when reading the Q-A combination new tags may be appropriate.

As I see it, there is an advantage to have tags reflect the entire Q-A post rather than just the question. This is because when looking for similar information by using a tag, the information may not necessarily be in the question but rather in the answer. A good example from TeX.sx is when people ask how to solve a graphics problem. Usually there are two basically incompatible avenues to go down (pgf-tikz and pstricks) and the question may not ask for either of them. Hence answers concerning these solutions may go untagged with respect to these major directions.

Some say you can simply do a free form search. This is of course true but then we would not need the tags in the first place. So the question from me is tags for question content only or tags for combined Q-A- content. I look forward to see where this leads.

Answer 1

I would say to start out with the tags for your question itself - it should be pretty easy to figure out.

As answers come along, they may introduce you to new terminology, etc. that is related to the question. As tags are primarily for sorting, it would be best for the question to be sorted according to others like it.

For example, if I ask a question about a planet outside of the solar system, and someone answers with something like,

Planets outside the solar system are commonly referred to as exoplanets...

Then you might consider tagging your question with exoplanet, as it is another term useful for sorting the question.

However, if you ask the same question and the reply is something like:

The reason that certain planets are like that is because they have irregular rotation.

Then you should not retag your question with rotation, because it does not help sort the question. The question does not really involve rotation beyond the fact that it is key to the answer. In other words, someone searching questions would not know to search for rotation unless they already knew the answer, so it's not a useful category for that question.

It's a little complicated, but it's also fairly common sense.