As discovered in MDL-35073, there is different results between oracle and the rest of the drivers when it comes to defining indexes which have already been defined. On oracle this breaks install, the other drivers seem to ignore it.

Eloy Said:
"All the Moodle DB drivers do create the indexes for all PKs/FKs automatically. Else, it's a bug in the Moodle DB driver.
In the other side, it should behave the same under all DBs (the failure) and it seems different drivers are not breaking under the same circumstances. Surely that should be considered a bug too."