array vs flat tables performance

I'm about to add another table to the project I'm working on and the
suggested plan of action is to use arrays. Since performance is an issue,
I'm wondering, what exactly is the difference?

in other words, I could have a flat table such as:

UID1 - ITEM1
UID1 - ITEM2
UID1 - ITEM3
UID7 - ITEM12

or I could use arrays:

UID1 - {ITEM1,ITEM2,ITEM3}
UID7 - {ITEM12}

is the latter simply syntactic sugar for the former?

thanks
Oren

--
Oren Mazor // Developer, Sysadmin, Explorer
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"Ut sementem feceris, ita metes"