Make dense_vector fields updatable to bbq_flat/bbq_hnsw

[tteofili]

This allows dense_vector fields to be updated to bbq_flat / bbq_hnsw.
Allowed update paths:

• *_flat --> bbq_flat/bbq_hnsw
• *_hnsw --> bbq_hnsw

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @tteofili, I've created a changelog YAML for you.

[john-wagster]

Other than the one comment this lgtm!

[benwtrent]

I think this is good.

One question, what happens if the index was created with dims less than our minimum BBQ dims?

Do we fail the upgrade or do we get stuck in a bad state?

[tteofili]

One question, what happens if the index was created with dims less than our minimum BBQ dims?

Do we fail the upgrade or do we get stuck in a bad state?

if you have the default (e.g., you specify nothing in index_options), the upgrade will keep the default, hence it will start failing if/when the new default becomes bbq_hnsw.
currently it doesn't because if you change nothing it will keep the default (int8_hnsw).
but if, during upgrade, you also update the mappings and specify index_options with bbq, it will fail.

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	8.19	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 128291

[tteofili]

💚 All backports created successfully

Status	Branch	Result
✅	8.19

Questions ?

Please refer to the Backport tool documentation