Re: using "alter index xxx unusable"

"Michael Peel" <peelm_at_NOSPAMaccuread.com> wrote in message news:Xns972B925A72A1BpeelmNOSPAMaccureadc_at_194.6.79.66...
> Hi all,
>
> First time caller here, so a bit nervous.
>
> Oracle 10.1.0.4 (HPUX)
>
> I'm building a consolidated table of about 30 million rows for a
> reporting type instance.
>
> I'm using the following steps.
>
> Mark indexes on consol table unused.
> Setting skip_unusable_indexes = true for the session
> Truncating the consol table.
> Insert using append.
> Rebuild indexes.
>
> What I've found is that after the truncate table statement the indexes on
> the table become valid. I therefore have to re-issue the alter index
> unusable statement, before the insert.
>
> Not that it's a major issue, just wondered if this was normal....
>
> Cheers,
>
> Michael Peel
> D.B.A
> I.S. Services
> AccuRead LTD
>

This behaviour is a little surprising, but totally normal. As you truncate a table, you automatically truncate all the indexes . So the indexes are suddenly self-consistent again which makes the indexes usable again. (Oracle is not aware of the fact that you might really want them to be unusable - normally it's an unfortunate accident).

However, you probably don't need to set the indexes unusable to benefit from the insert /*+ append */, as the indexes are maintained with an array insert after the insert is complete. (There are some funny side-issues with enabled foreign key constraints, though, but too many if/then/buts to put into a short note).

-- 
Regards

Jonathan Lewis

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