The following supercedes T185167, which I had closed because it was determined to be too destructive.
With T183375, undeletion no longer sets the parent IDs for the undeleted revisions to the "expected" one given by the previous revision by ID (usually, but not always, also the previous revision by timestamp), but instead to the ar_parent_id values. For example, look at the [[https://en.wikipedia.org/w/index.php?title=Ernest_Madu&action=history|history of Ernest Madu on Wikipedia]], and you'll see that [[https://en.wikipedia.org/w/index.php?title=Ernest_Madu&oldid=820864133|revision 820864133]] has parent ID 0 (with an "N" mark in the [[https://en.wikipedia.org/wiki/Special:Contributions/Andrewkazimi|user's contribs]]) instead of [[https://en.wikipedia.org/w/index.php?title=Ernest_Madu&oldid=820863993|820863993]]. But even this alone can still cause problems.
## Flaws with the current interface
Some flaws include:
* Failure to distinguish page IDs
* Broken parent revisions
* "Ancestor" revisions being scattered randomly across different titles and mixed in with unrelated histories, with no easy way to rejoin all the revisions into the history of one single title
** This is the same problem that would occur when manually changing rev_page fields (with the current behavior, deleting A, moving B to A without redirect, undeleting a single revision for A, moving A back to B without redirect, and finally undeleting the rest of the revisions for A is effectively equivalent to simply changing the rev_page field for the single revision from A's page ID to B's page ID).
* Takes forever to load for pages with thousands of deleted revisions
## Proposals 1 and 2
Moved to T206587 to keep the main task's description to a reasonable size.
## Proposal 3 (to be attempted for consensus if Proposals 1 and 2 both fail)
When undeleting, always set each undeleted revision's parent ID to the previous undeleted revision among those revisions that had the same ar_page_id field (which will usually be the same as the ar_parent_id field), or zero if there is no such revision, with the following exception: if selective undeletion had been used to effectively revert a page to an older revision and the page had not been edited since the selective undeletion, undeleting later revisions (which would imply a necessary update to the page_latest field in the page table) will leave the parent ID for the earliest undeleted revision as the page's current revision instead of changing the parent ID to zero. This can be done by making a variable that is initialized as an empty array and then changes each time a revision is being inserted. The key is the ar_page_id field and the value is the ID to be used as the parent ID. The exceptional case will not be applied if the page had been edited since the selective undeletion, in order to prevent "siblings" (two revisions with the same parent revision). Each distinct ar_page_id field would then be treated as a separate history (but nonetheless would still appear altogether in a single page's history).
## Advantages of Proposal 3
* Makes revisions that were originally from different page IDs distinguishable using rev_parent_id.
* Works better for deleted revisions that have null ar_parent_id fields.
## Disadvantage of Proposal 3
* Not idempotent—when a page is undeleted, then re-deleted, and finally undeleted again, the parent IDs might be different from what they were after the first undeletion, because Special:Undelete will no longer know that two revisions had belonged to separate page IDs.
## See also
The following is a different but related RFC.