Rick Hillegas created DERBY-6542:
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Summary: Improve the concurrency of identity columns by using
SYS.SYSSEQUENCES
Key: DERBY-6542
URL: https://issues.apache.org/jira/browse/DERBY-6542
Project: Derby
Issue Type: Improvement
Components: SQL
Affects Versions: 10.11.0.0
Reporter: Rick Hillegas
Assignee: Rick Hillegas
This is another attempt to improve the concurrency of identity columns. The
previous attempt was tracked by DERBY-4437.
This new attempt will try out Mike's last suggestion: use system-created
sequences managed by SYS.SYSSEQUENCES. This should reduce the contention on the
core catalogs.
I'm hopeful about this approach because of the experiments tracked by
DERBY-6533. There we are not seeing any problems related to sequence generators
but we are seeing lots of identity-related lock timeouts.
Here is the general shape of this approach:
1) When adding an identity column to a table, Derby will create a sequence
generator for the column. The sequence generator will live in the SYS schema
and its name will be the table's UUID.
2) DROP SEQUENCE will not operate on system-created sequences. System-created
sequences will be dropped/modified by DROP/ALTER TABLE commands.
3) We will add a new system function for inspecting the current, in-memory
value of an identity generator without getting a lock on SYS.SYSSEQUENCES:
SYSCS_UTIL.SYSCS_PEEK_AT_IDENTITY( tableSchemaName, tableName ).
4) Derby will create a sequence for every legacy identity column after
hard-upgrade.
5) These changes will take place only after hard-upgrade. Soft-upgrade will not
change the behavior of identity columns.
Comments on this proposal are welcome. Thanks.
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