filter: sql,query

I did some tests recently on a two column primary key, one of which was an autoincrement column. It worked nicely, maintaining a separate increment for every unique value in the second column. I was ecstatic, as this was exactly what I need for some tables I'm designing.

However, today I tried to use the same idea in a three column primary key, and it seems to be acting like a lone autoincrement column, instead of how it acted with two columns.

What I want to know, is this: Is the two-column behavior an official feature, or is it an unintended effect? I personally /love/ that behavior, and think it should be a feature, but before I go further, I need to know if I can count on it to hang around. And if it is a feature, why it doesn't work in three column indexes.

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