Fix #238: migration to backport prod index

[leplatrem]

Fix #238

[grahamalama]

Trying to get the timeline correct here:

• bsieber-mozilla merged this commit on May 19, 2021, which created this index

    op.create_index(
        "idx_email_primary_email_lower",
        "emails",
        [sa.text("lower(primary_email)")],
        unique=True,
    )

so functionally the same as the one we have in this PR, just a different name

• on May 20, 2021, he merged this commit, which removed the unique constraint

• jwhitlock reported the issue linked in this PR on Jun 1, 2021

I don't know how / when bsieber-mozilla's migrations were applied in prod, but maybe the first version of the migration was applied and ran, but the edit wasn't picked up 🤔. Pure speculation.

[grahamalama]

But in any case, some thoughts / questions:

• is it true that we have an index idx_email_primary_email_lower, and now we're adding this idx_email_primary_unique_email_lower? Maybe we can drop idx_email_primary_email_lower since idx_email_primary_unique_email_lower is that plus uniqueness

• if we do need this index, can we include a comment here explaining why, given the confusing history I pointed out in my previous comment?

• Can we reflect this index back onto the model?

[leplatrem]

You are perfectly right! I was only looking at diffs between schemas, and didn't spot that we also have idx_email_primary_email_lower.

Stage/Prod have this additional one idx_email_primary_unique_email_lower.

I propose that we keep the one with uniqueness, and drop the other one. Let's see how I can rework the migration for that.

Thank you for being so thorough, as usual 🙏

[grahamalama]

Nice! One topic of investigation for indexes that I though of, but otherwise looks g2g and I'll throw the approval on now.