SERIAL type - auto-increment grouped by other field

hi all,

I have problem with SERIAL field type (or sequence functionality).
I have table with three columns - ID, IDS, NAME.
I want auto-increment IDS grouped by ID.
Example:
1, 1, Ferdo
1, 2, John
2, 1, Martin
1, 3, Elvira
2, 2, Georgia

but...
when I use IDS - SERIAL and when I create UNIQUE (ID, IDS) result is:
1, 1, Ferdo
1, 2, John
2, 3, Martin
1, 4, Elvira
2, 5, Georgia

where i make misstake??? how can i do it??? in documentation there is
description only for one auto-increment column. I didn't find auto increment as
I described upper.

tnx a lot



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