Get DataObject to automatically add the recommended index

[sminnee]

The docs suggest that you add this index:

ALTER TABLE `QueuedJobDescriptor` ADD INDEX ( `JobStatus` , `JobType` )

DataObject can be configured to add this index automatically with the $indexes static property. That would ensure that all installs receive this performance enhancement.

[mfendeksilverstripe]

There is already index on JobStatus. I'm not sure if adding index on JobType will give much performance gain as cardinality of this index is really low. Note that JobType is actually Queue type. What we typically refer to as type of job is the Implementation. Adding index on this field would probably make more sense but I still think the cardinality of this index is too low as well.

[sminnee]

Right. I assumed that whoever wrote the doc (probably the original developer, @nyeholt) expected this index to be in place to the application to work well, but didn't investigate fully.

For my part I know that installations that don't have the job cleanup activated (and therefore get lots of completed jobs left in the system but hidden from the UI) end up with a very slow admin UI. I wondered if the absence of this index was the root cause.