Remove on-heap fielddata.

[jtibshirani]

Fielddata is a collection of on-heap data structures that support aggregating and sorting on certain field types. Since fielddata is expensive to build, it's held in a dedicated cache. The cache can consume significant heap memory and must be limited by a circuit breaker. Fielddata is a source of complexity and can make Elasticsearch harder to manage.

We'd like to explore removing the concept of on-heap fielddata entirely. This would include the following:

• Remove the need for on-heap global ordinals. This could involve moving global ordinals off-heap, or reducing the need for ordinals in aggregations. (needs research)
• Remove support for fielddata on _id. (Remove support for fielddata loading on _id. #64511)
• Remove support for fielddata on text fields. (needs research)

This meta issue is meant to clarify the plans around fielddata. Some individual changes require more research -- we'll link to sub-issues when there are more details.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Aggregations)

[nik9000]

* reducing the need for ordinals in aggregations

We keep bumping into more and more cases where it is useful to have global ordinals, sadly. I think we're stuck getting them off the heap.

[javanna]

We have no concrete plans to tackle this issue, hence I am closing it.