What does this mean: Outer Join (+) on a Value?

I thought the (+) syntax was only used to outer-join two tables, like this:

WHERE invoice.customer_num (+) = customer.customer_num

Today I came across some code that looks like this:

WHERE invoice.customer_num (+)  = customer.customer_num
  AND invoice.begin_date   (+) <= trunc(sysdate)
  AND invoice.end_date     (+) >= trunc(sysdate)

Aren't the second and third occurances of (+) unnecessary (or even meaningless)? I can find no mention of this usage anywhere. Is this an idiom left-over from earlier versions of Oracle (I'm at 9i)? Or maybe from another database vendor's SQL implementation?

(Interestingly, although the rows returned are the same with or without the extra (+)'s, the explain plans are different - having the extra (+)'s makes the query much faster... maybe that's why they did it...) Received on Fri Jan 09 2004 - 12:11:22 CST