foreign key indexes and parent-table locking

Hi,

I'm trying to understand the whole issue of foreign key indexes and locking. Found a note on metalink (11828.1) that seems to explain it, but either it's not clear or I'm missing something.

"Why then, does an index on the foreign key mean that the shared lock on the
parent table is not required?
"When a row in the child table is inserted, deleted or has its foreign key
updated, the corresponding index entry/entries is/are also locked. When an application attempts to delete or update the primary key of a parent row, it reads the FIRST corresponding entry in the child's foreign key index (uncommitted or otherwise) and, if locked, waits for that lock to be released."
So far so good . . . this next piece, too, seems to make sense:
"If the modified child row is NOT the first occurrence of the foreign key in
the index then the parent modification must be prevented anyway, regardless of the outcome of uncommitted transactions on other child rows with this key."
But now here's the part that leaves me hanging . . .
"Hence the error can be flagged immediately and so the transaction is not
forced to wait. This mechanism ensures the minimum reads and wait times to maintain data consistency. "

Can anyone help by either translating this last part or rephrasing it? Or explaining the issue differnetly?

Thanks

bill

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Author: Magaliff, Bill
  INET: Bill.Magaliff_at_lendware.com

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