James Mansion wrote:
I saw on a summary for 8.4 that there's a generic function for use as
a before row trigger that will elide null updates (ie replacement with
an identical row).
I can see that this is great - but I was wondering if it should be
more integrated and turned on in the db schema.
Trivially, doing so would mean that there is less of an issue trying
to integrate with a custom user before trigger, although that's really
no more than an inconvenience.
I understand that there is an argument for not making it the default
behaviour given that trigger execution and locking are both affected
if we do elide the update completely, but it occured to me that while
I might want the trigger and locking behaviour, I probably never want
the actual database image copy to happen. Doing so will needlessly
bloat the database file and give the vacuum procedure work to do - and
it seems interfere with the new optimisations relating to pages that
are all visible in all transactions.
Would it be possible to determine a null update cheaply and retain the
locking and trigger execution, but elide the actual row copy - and in
particular the associated impact in terms of setting status flags etc?
I guess this would need to be handled at a lower level than the
trigger approach - and would need an option that is integrated into
the schema, so we can elide the copy, and optionally the trigger
execution, and optionally the lock.
I don't follow what you're saying.
If an update is skipped by a trigger, nothing new is written to disk,
and there should be nothing to vacuum from it. That's why this trigger
can speed up certain update queries enormously.
cheers
andrew
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