Regression: index dropped in migration and not recreated

[sparrowt]

Problem

In the following cases (possibly more) any indexes on the column are dropped, but aren't recreated afterwards:
(a) when a column is made null-able (i.e. adding null=True)
(b) when a column is renamed
(c) when a table is renamed

This is quite a major issue because it is silently leaving the database in the wrong state - the migration doesn't fail, but now certain columns with db_index=True won't actually have an index which could cause very slow queries.

Cause

As far as I can see this is a triple-regression introduced by 2e60754 (for #24).

That commit added new calls to _delete_indexes in 2 places causing the first two cases listed above:

(a) 2e6075456a#diff-e4c1520d8b49ccbf46382bd2eed4e740R400
(b) 2e6075456a#diff-e4c1520d8b49ccbf46382bd2eed4e740R334-R335

and an explicit index deletion here before renaming a table causing the third case:

(c) 2e6075456a#diff-e4c1520d8b49ccbf46382bd2eed4e740R222-R225

but in none of those cases is the index re-instated afterwards, even if the field still has db_index=True. Index restoration only happens in certain specific cases (e.g. type change / removal of nullability) which doesn't include the above 3 cases.

Reproduction

See the 3 tests I added on this branch:
master...sparrowt:issue58-index-reinstatement
which fail due to the bugs described above, but pass if run on django-mssql-backend v2.4.2 (before #24)

[sparrowt]

Considering how to solve this, one approach would be to expand the condition under # Restore an index, or to start explicitly keeping track of which indexes have been dropped and need re-creating.

However before that we should ask: is it actually necessary for all indexes to be dropped in these 3 cases?

If it can be avoided then it will save wasting time during migration of a large table. MS SQL Server seems to allow those 3 operations to occur while the index is there (it didn't explode in those cases using v2.4.2).

@OskarPersson do you know what the reasons were for adding all these "delete index before doing X" in 2e60754?

[sparrowt]

This issue still remains in the new Microsoft-supported fork of this project so I have filed it there 👆

[sparrowt]

Now fixed in microsoft/mssql-django#14