On 1/7/15, 9:39 AM, Robert Haas wrote:
sequence.c: Is it safe to read a sequence value in a parallel worker if the
>cache_value is > 1?
No, because the sequence cache isn't synchronized between the workers.
Maybe it would be safe if cache_value == 1, but there's not much
use-case: how often are you going to have a read-only query that uses
a sequence value. At some point we can look at making sequences
parallel-safe, but worrying about it right now doesn't seem like a
good use of time.
Agreed, I was more concerned with calls to nextval(), which don't seem to be
prevented in parallel mode?
>This may be a dumb question, but for functions do we know that all pl's
>besides C and SQL use SPI? If not I think they could end up writing in a
>worker.
Well, the lower-level checks would catch that. But it is generally
true that there's no way to prevent arbitrary C code from doing things
that are unsafe in parallel mode and that we can't tell are unsafe.
As I've said before, I think that we'll need to have a method of
labeling functions as parallel-safe or not, but this patch isn't
trying to solve that part of the problem.
I was more thinking about all the add-on pl's like pl/ruby. But yeah, I don't
see that there's much we can do if they're not using SPI.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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