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.

I

Answer 1

CREATE TABLE Tags (
    tag VARHAR(...) NOT NULL,
    bid INT ... NOT NULL,
    PRIMARY KEY(tag, bid),
    INDEX(bid, tag)
)

Notes:

• This is better than TOXI in that it does not go through an extra many:many table which makes optimization difficult.
• Sure, my approach may be slightly more bulky (than TOXI) due to the redundant tags, but that is a small percentage of the whole database, and the performance improvements may be significant.
• It is highly scalable.
• It does not have (because it does not need) a surrogate AUTO_INCREMENT PK. Hence, it is better than Scuttle.
• MySQLicious sucks because it cannot use an index (LIKE with leading wild card; false hits on substrings)
• For MySQL, be sure to use ENGINE=InnoDB in order to get 'clustering' effects.

Related discussions (for MySQL):
many:many mapping table optimization
ordered lists