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[nova] indices on shadow-able fields


On Thu, Apr 11, 2019 at 9:27 AM Jay Pipes <jaypipes at gmail.com> wrote:
>
> On 04/08/2019 10:51 PM, Mohammed Naser wrote:
> > Hi everyone.
> >
> > In continuing with the trend of interesting database issues, I've
> > found the following interesting thing.  It looks like we don't have
> > indices for all of our tables that can be large on the deleted field.
> > I think it would be beneficial *especially* for cases when the
> > archiving code runs.
> >
> > The big ones that are huge users in terms of rows without index are
> > `instance_system_metadata` and `instance_extra`.
> > `instance_action_events` and `instance_actions` don't have any either.
> >
> > I believe without this, it results in doing a full table scan during
> > the clean up which can become pretty resource intensive.
>
> All of the queries that the archive tool performs look at the deleted
> column in various tables, yes. I suppose adding an index on the deleted
> column for the largest tables you listed above would provide some relief
> given the design of the existing archive tool.
>
> Once the archive operation is performed, however, all rows in the table
> will have a deleted column value of 0. This generally makes the deleted
> column a poor choice for placing an index, since the cardinality of the
> column will be very low. [0] However, in this case, the index would
> build up over time (between archive tool runs) and be useful during
> archiving, and less useful immediately after the archive tool runs.
>
> In short, feel free to create indexes on the deleted column for those
> tables and let us know whether they make a substantial difference in the
> performance of the archival tool. :)

Hi Jay,

Thanks for all the feedback, I appreciate your insight into SQL that's way
beyond my "it's slow, index is the fix?!" scope :)

I'll try to see if I can get some before/after numbers.

Thanks,
Mohammed

> Best,
> -jay
>
> [0] https://en.wikipedia.org/wiki/Cardinality_(SQL_statements)
>


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