This is personal study note
Copyright and original reference:
https://www.youtube.com/watch?v=f_KCIBfEhNY&list=PLuHgQVnccGMDF6rHsY9qMuJMd295Yk4sa
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JOIN is computationally expensive task
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After that, you want to get "tag name", and you need to use "join"
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To remove "JOIN", tag name is inserted into topic_tag_relation table
Even if that table has duplicate, you can remove "join" for above task
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Denormalization causes "higher complicated program", "easy break down"