Re: someone working to add merge?

Tom Lane wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > This only happens because of the unique index. There's no predicate
> > locking involved. The btree code goes some lengths to make this work;
>
> That's one way to look at it; the other is to say that we have predicate
> locking for a very specific class of predicate, ie, equality of a
> unique key.
>
> In practice I think we only have a useful lock there for *primary* keys,
> because unique without NOT NULL doesn't actually constrain you to just
> one matching row ...

I agree --- an implementation that needs to use a table lock is useless,
and one with no primary key is too hard to implement and also near
useless.

I have update the TODO item to reflect this:

* Add MERGE command that does UPDATE/DELETE, or on failure, INSERT
(rules, triggers?)

To implement this cleanly requires that the table have a unique index
so duplicate checking can be easily performed.

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