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https://issues.apache.org/jira/browse/DERBY-5697?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13259562#comment-13259562
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Rick Hillegas commented on DERBY-5697:
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Hi Kim,
Adjusting derby.language.sequence.preallocator upward may boost your
concurrency. At some point, you will have adjusted it high enough that further
upward tweaks won't help your application. At that point, your throughput will
be constrained by other bottlenecks.
I don't see how adjusting derby.language.sequence.preallocator downward would
ever reduce your concurrency. The reason to lower it would be to leak fewer
unused preallocated values. That could be an issue if your application can't
guarantee that it will park its databases gracefully before the VM exits.
Thanks.
> Doc changes to account for correctness fixes for sequences
> ----------------------------------------------------------
>
> Key: DERBY-5697
> URL: https://issues.apache.org/jira/browse/DERBY-5697
> Project: Derby
> Issue Type: Improvement
> Components: Documentation
> Affects Versions: 10.9.0.0
> Reporter: Rick Hillegas
> Assignee: Kim Haase
>
> As a result of the correctness fixes made by DERBY-5493 and DERBY-5494, we
> should adjust the user documentation for sequences. I recommend the following
> changes:
> 1) Reference Guide
> a) Add a section for a new system function. This function lets users observe
> the instantaneous current value of a sequence generator without having to
> query SYSSEQUENCES. Querying SYSSEQUENCES didn't actually return the current
> value, it only returned an upper bound on that value, viz., the end of the
> chunk of sequence values which had been pre-allocated but not actually used.
> The new function shows you the very next value which will be returned by a
> NEXT VALUE FOR clause. Users should never directly query SYSSEQUENCES because
> that will cause sequence generator concurrency to slow down to a crawl.
> Syntax
> BIGINT SYSCS_UTIL.SYSCS_PEEK_AT_SEQUENCE(IN SCHEMANAME VARCHAR(128), IN
> SEQUENCENAME VARCHAR(128))
> As with all system functions, schema and object name arguments are
> case-sensitive.
> Execute privileges
> By default, all users have execute privileges on this function.
> b) SYSSEQUENCES system table
> The Contents description for SYSSEQUENCES.CURRENTVALUE needs to be changed to
> make a couple points: This is not the actual next value for the sequence
> generator. That value can be obtained by calling SYSCS_PEEK_AT_SEQUENCE().
> SYSSEQUENCES.CURRENTVALUE holds the end of the range of values which have
> been pre-allocated in order to boost concurrency. Users should not directly
> query SYSSEQUENCES because that will slow down the performance of sequence
> generators. Instead, users should call SYSCS_PEEK_AT_SEQUENCE().
> c) derby.language.sequence.preallocator
> The default value of this property has changed from 20 to 100.
> In addition, I think that we should point out that if the database is shut
> down gracefully, then Derby should not leak unused, preallocated values; they
> will be thrown away and the sequence generator will pick up where it left
> off, once the database re-boots. However, if the database comes down
> ungracefully, then unused, preallocated values will be leaked: when the
> database comes up again, the sequence generator will skip the unused,
> preallocated values; this will leave a gap between the last NEXT VALUE FOR
> (issued before the database came down ungracefully) and the next NEXT VALUE
> FOR (issued after the database re-boots).
> 2) Tuning Guide
> Maybe we should add a small section explaining how to boost the concurrency
> of sequences:
> a) Don't query SYSSEQUENCES directly. Instead, use SYSCS_PEEK_AT_SEQUENCE().
> b) Adjusting derby.language.sequence.preallocator may boost your concurrency.
> However, you should pay special attention to bringing down the database
> gracefully in order to not leak unused, preallocated values. See the
> Reference Manual section on derby.language.sequence.preallocator.
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