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I've been reading about how Pinterest sharded their MySQL databases here.

One of the things they talk about in that post is having “unidirectional ‘mapping’” tables such as

 CREATE TABLE board_has_pins (
  board_id INT,
  pin_id INT,
  sequence INT,
  INDEX(board_id, pin_id, sequence)
) ENGINE=InnoDB;

and then an opposite pin_owned_by_board.

My question is, why would you have these “unidirectional tables” for “mapping” and not just one with a different index structure? I don't think it can stop duplicates due to the sequence being a unix timestamp, but the index does limit only being able to select by the board_id in this instance or the full index (efficiently anyway). So why not have separate indices on board_id and pin_id? Is this down to the fact that a pin could in theory live on a separate shard even though they try to keep them on the same one so it makes sense to make it “unidirectional”?

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I agree that it is a strange table.

No uniqueness constraint, so dups can occur. I would change INDEX to PRIMARY KEY.

No index the opposite direction, so unlikely to use it that way. I would add another column and INDEX(bin_id, board_id, pin_to_board sequence)

On the other hand, perhaps there is a lot of activity in the table -- such as updating all the sequence values frequently. Still, InnoDB would really prefer to have a PRIMARY KEY, and that triple seems like the 'right' PK.

I would probably work harder to avoid potential timestamp dups.

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