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https://issues.apache.org/jira/browse/DERBY-5294?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13058053#comment-13058053
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Mamta A. Satoor commented on DERBY-5294:
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I appreciate all the discussion on various ways of working with pre-10.9
triggers. Kristian asked "As a side note, are there any drawbacks/risks to
"fixing triggers" unconditionally in compress table? " Following is what I
think of the compress table code changes.
My current approach is to drop and recreate *all* the triggers during compress
time rather than just the ones that would directly benefit from DERBY-1482
work(which is row level triggers using REFERENCING clause but have missing
information in system table about actual columns referenced through the
REFERENCING clause in trigger action). I think by choosing to drop and recreate
all the triggers, we will be consistent with all the triggers and compress code
will be easier to read. We do need to do the testing to make sure in
soft-upgrade mode, we do not start collecting trigger action column info if the
database is at pre-10.9 level, this is to maintain the backward compatibility.
Another thing that will be code to check is that sysdepends has correct number
of rows before and after the compress. Another ideal thing will be to have an
upgrade test fixture which can be run with low memory so we can be sure that
after compress, at hard upgrade time, we do not read the LOB columns when not
needed, otherwise with low mem configuration, we should run into OOM. I am not
sure if there is a way to run just one fixture in a suite with low memory.
My only concern is this work requires a bit of code rearranging so we don't
have duplicate code(the patch attached does have code duplication and I was
planning on removing the duplication in my next patch). This central code then
will be shared by create trigger, alter table drop column, compress table and
invalidated triggers which need recompiling. We just need to be sure that the
centralized code works fine for all the four scenarios.
> Enhance compress table to drop and recreate the triggers. This will enable
> pre-10.9 triggers (after hard upgrade to 10.9) to read only the required
> columns from the trigger table.
> -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: DERBY-5294
> URL: https://issues.apache.org/jira/browse/DERBY-5294
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 10.9.0.0
> Reporter: Mamta A. Satoor
> Attachments: DERBY5289_alltriggers_06282011_diff.txt,
> DERBY5289_alltriggers_06282011_stat.txt
>
>
> Triggers created prior to 10.9 release will continue to read all the columns
> from trigger table even after database has been upgraded to 10.9 and higher.
> Which in another words means that such triggers will not benefit from work
> done for DERBY-1482.
> With DERBY-1482 (which went in 10.9 codeline), triggers will read only the
> columns needed by the triggering sql and firing triggers. But this applies
> only to triggers created in 10.9 and higher. Any triggers created prior to
> 10.9 will not be able to take advantage of DERBY-1482 because those triggers
> do not keep the information about the trigger action columns. Currently, the
> users will have to drop and recreate the triggers which use the REFERENCING
> CLAUSE and were created prior to 10.9 to take advantage of DERBY-1482.
> The alternative to manual drop and recreate of such triggers can be explored
> as part of this jira. Couple options are
> 1)UPDATE sql should detect that the trigger does not have information about
> the trigger action columns and hence it should make the trigger collect that
> information.
> 2)At the time of upgrade, when we mark all the SPSes invalid, detect the
> triggers which do not have the information about the trigger action columns
> and make those triggers collect that information.
> 3)Enhance ALTER TABLE COMPRESS to detect the triggers which do not have the
> information about the trigger action columns and make those triggers collect
> that information. With this option, users will still have to manually do
> ALTER TABLE COMPRESS to fix the triggers but atleast they won't have to get
> the original trigger definitions and drop and recreate the triggers using
> those original trigger definitions.
> 10.9 currently does not have central place where the trigger will go and
> collect the information about trigger action columns. We do have code in
> ALTER TABLE DROP COLUMN to collect the trigger action column info but it will
> probably better to have such a code in TriggerDescriptor so it can be used by
> the approach taken to fix this jira.
> Note that without the fix for this jira, the triggers created prior to 10.9
> will work just fine after upgrade to 10.9 and higher but they will not be
> able to prevent reading of columns that are not necessary for the triggering
> sql and firing triggers
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