Question

original simple table A
------------------------
rowid   id  name
123      1    A
124      4    G
125      2    R
126      3    P

index on A.id
-------------
id  rowid
1   123
2   125
3   126
4   124

updated simple table A
----------------------

rowid   id  name
123      1    A
124      5    G
125      2    R
126      7    P

Assuming that the index did not update itself when the table was being updated, how will the index update happen now?

Will it be rebuild from scratch? Or will it somehow use the old index?

Answer 1

The index would have to be marked unusable / disabled to not update when the row was updated, when it is re-enabled / marked usable, it would be built from scratch since at that point the contents of the index are useless, but the schema of the index can be re-used.

This is a different scenario to your other question, which in the other situation it was rebuilding an index that was still enabled.