Re: [eclipselink-users] Confused on Sequence and allocationSize

["Tim Hollosy" <hollosyt@xxxxxxxxx>]

Thanks,
That makes sense, I never thought of saving needless db trips for nextval's.

./tch



On Fri, Dec 12, 2008 at 7:11 PM, Christopher Delahunt
<christopher.delahunt@xxxxxxxxxx> wrote:
> Hi Tim,
>
> Sequencing is implemented such that EclipseLink will reserve the current
> sequence number and the next X sequence numbers; X being the sequence
> allocation size.  So if NextVal returns 50, EclipseLink calculates that the
> current value was 50-x, and so it can now use numbers 50-x+1 up to 50 before
> having to go to the database to allocate more.  Can't say exactly why 50 was
> chosen as the default, though it seems to work well with table sequencing
> which is also the default.
>
> Best Regards,
> Chris
>
> -----Original Message-----
> From: eclipselink-users-bounces@xxxxxxxxxxx
> [mailto:eclipselink-users-bounces@xxxxxxxxxxx] On Behalf Of Tim Hollosy
> Sent: Friday, December 12, 2008 4:06 PM
> To: EclipseLink User Discussions
> Subject: [eclipselink-users] Confused on Sequence and allocationSize
>
> So,
> I am connecting to an oracle 9i DB.
>
> I have a simple entity with a sequence based pk.
>
> I added the annotations like so:
>
>      @Id
>        @SequenceGenerator(name="USER_PROFILE_SEQ",
> sequenceName="REDACTED_USER_PROFILE_SEQ")
>    @GeneratedValue(strategy=GenerationType.SEQUENCE,
> generator="USER_PROFILE_SEQ")
>
> Every single insert the sequence used for the Pk seemed to be some
> random number, smaller than the actual value of the sequence, even
> though I could see the sequence being selected in the logs.
>
> Then I noticed it was not random, but 50 minus the actual value of the
> sequence. I remembered a post on this list about the default value for
> allocationSize being 50, so i added an allocationSize=1 (to match my
> sequence) to my @SequenceGenerator and we were back in business.
>
> Now the question of the hour why was it subtracting 50 from the
> sequence? EL apparantly does a nextval - (allocation size) on the
> sequence (but not if the allocation size is 1?)
>
> Also, why is the default value 50 to begin with?
>
> Thanks!
> Tim
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