How to implement tag system

Question

I was wondering what the best way is to implement a tag system, like the one used on SO. I was thinking of this but I can\'t come up with a good scalable solution.

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Answer 1

If your database supports indexable arrays (like PostgreSQL, for example), I would recommend an entirely denormalized solution - store tags as an array of strings on the same table. If not, a secondary table mapping objects to tags is the best solution. If you need to store extra information against tags, you can use a separate tags table, but there's no point in introducing a second join for every tag lookup.