Is there a concise way to select the nextval for a PostgreSQL sequence multiple times in 1 query? This would be the only value being returned.
For example, I would like to do something really short and sweet like:
SELECT NEXTVAL('mytable_seq', 3) AS id;
And get:
id
-----
118
119
120
(3 rows)
select nextval('mytable_seq') from generate_series(1,3);
generate_series is a function which returns many rows with sequential numbers, configured by it's arguments.
In above example, we don't care about the value in each row, we just use generate_series as row generator. And for each row we can call nextval. In this case it returns 3 numbers (nextvals).
You can wrap this into function, but I'm not sure if it's really sensible given how short the query is.
There is a great article about this exact problem: "getting multiple values from sequences".
If performance is not an issue, for instance when using the sequence values dwarfs the time used to get them or n is small, then the SELECT nextval('seq') FROM generate_series(1,n) approach is the simplest and most appropriate.
But when preparing data for bulk loads the last approach from the article of incrementing the sequence by n from within a lock is appropriate.
CREATE OR REPLACE FUNCTION foo() RETURNS SETOF INT AS $$
DECLARE
seqval int; x int;
BEGIN
x := 0;
WHILE x < 100 LOOP
SELECT into seqval nextval('f_id_seq');
RETURN NEXT seqval;
x := x+1;
END LOOP;
RETURN;
END;
$$ LANGUAGE plpgsql STRICT;
Of course, if all you're trying to do is advance the sequence, there's setval().
You could also have the function take a parameter for how many times to loop:
CREATE OR REPLACE FUNCTION foo(loopcnt int) RETURNS SETOF INT AS $$
DECLARE
seqval int;
x int;
BEGIN
x := 0;
WHILE x < loopcnt LOOP
SELECT into seqval nextval('f_id_seq');
RETURN NEXT seqval;x := x+1;
END LOOP;
RETURN;
END;
$$ LANGUAGE plpgsql STRICT;
My current best solution is:
SELECT NEXTVAL('mytable_seq') AS id
UNION ALL
SELECT NEXTVAL('mytable_seq') AS id
UNION ALL
SELECT NEXTVAL('mytable_seq') AS id;
Which will correctly return 3 rows... but I would like something that is minimal SQL for even as much as 100 or more NEXTVAL invocations.
Unless you really want three rows returned I would set the sequence to 'INCREMENT BY 3' for each select. Then you can simple add 1 and 2 to the result have have your three sequence numbers.
I tried to add a link to the postgresql docs, but apparenty I am not allowed to post links.
来源:https://stackoverflow.com/questions/896274/select-multiple-ids-from-a-postgresql-sequence