Greetings, As mentioned in the PostgreSQL Weekly News, SQL2003 has been approved. Looking at one of the recent drafts it appears that MERGE has been added to the spec. MERGE is described as "Conditionally update rows of a table, or insert new rows into a table, or both." Support for this has been asked for in the past and if anything demand for this has increased. Is anyone working on this? Is there any status on it?
Additionally, from the draft I'm reading the SQL2003 standard makes
MERGE very capable but without obvious defaults for the simple case.
While I believe support for the standard should be paramount it would
make a great deal of sense to allow simple operations without
unnecessary complexity.
For example: Table T1 exists with columns a, b, c where a, b is the
primary key. T1 contains a single tuple '1, 2, 3'. I would now like
to either update or insert the primary key '1, 2' with the value '4'.
From what I understand of the specification this would be done by:
a)
insert into T2 values (1,2,4);
merge into T1
USING T2
ON T1.a=T2.a and T1.b=T2.b
WHEN MATCHED THEN
UPDATE SET T1.c = T2.c
ON T1.a=T2.a and T1.b=T2.b
WHEN NOT MATCHED THEN
INSERT (a,b,c) VALUES (T2.a,T2.b,T2.c);
This requires an addtional table (T2). There may be a better way
around this but I know that PostgreSQL already allows select
statements without a from clause, so this could be simplified to:
b)
merge into T1
USING (select 1 as a, 2 as b, 4 as c) as T2
ON T1.a=T2.a and T1.b=T2.b
WHEN MATCHED THEN
UPDATE SET T1.c = T2.c
ON T1.a=T2.a and T1.b=T2.b
WHEN NOT MATCHED THEN
INSERT (a,b,c) VALUES (T2.a,T2.b,T2.c);
Still pretty long-winded for what most would consider a relatively
simple request. My goal would be the ability to have implied ON/WHEN
clauses and USING VALUES, like so:
c)
merge into T1 USING VALUES (1,2,4);
The ON clauses are implied primary key matches. The WHEN clause for
MATCHED is then to UPDATE SET all columns which are not part of the
primary key. The WHEN clause for NOT MATCHED is to INSERT the row.
Clearly this last usage is what I would prefer for this case. It also
parallels the 'replace into' which MySQL has which would make
migration from MySQL to PostgreSQL much easier for programs and users.
Please let me know if there's something I'm missing in the
specification that would allow for a simple case similar to what I've
illustrated, either with MERGE or without. Of course, the expectation
is that MERGE wouldn't be able to fail because of another instance
adding a row with the same primary key.
I plan to forward this suggestion on to the SQL committee as well,
though I don't know what kind of response, if any, I'll get. Feel
free to address the standard MERGE support seperately from my
suggestion. I have need for both and so both are of interest and do
not have to come at the same time.
Many thanks,
Stephen
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