Revisited: Transactions, insert unique.

Hi,

Previously I wanted to ensure that I am inserting something unique into a
table, the answer was to create a unique index on the relevant columns.

But what if I don't want to get an error which would force a rollback? Say
I want to insert something if it doesn't already exist, but update it if it
does.

Do I have to lock the whole table?

Would it be a good idea to be able to request a lock on an arbitrary string
like in MySQL? Then I could perhaps do something like

LOCK HANDLE('max255charstring',TimeoutInSeconds)
e.g.
LOCK HANDLE('mytable,field1=x,field2=y',10)

Then I could control access to a row that may not even exist, or do other
snazzy transaction stuff.

Cheerio,
Link.