Sequence Question

I'm looking at ways to get batches of sequence values s faster. I don't want to set cache or increment to a large number for a variety of reasons. I need to grab id's in batches of varying numbers at various times, from 1 to several thousand at once.
Doing it 1 at a time works, but more time goes into it than I'd like. I tried setting cache_value high but the database roundtrips were eating more time that I wanted to see; I only saw a 25% improvement in average time.

SO... is the following approach safe? That is, will this be atomic, or is there a possibility that another connection could squeeze in a select nextval() between the select nextval() and the setval below?
If it's safe I'd do this and take the sequences as the new currval - #I asked for (1500 or whatever).

mydb=# select setval('my_id_seq', (select nextval('my_id_seq')+1500));

Thanks
Oscar

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