Add support to build vector data structures greedily and perform exact search when there are no engine files

[VijayanB]

Description

Added new updatable index setting "approximate_threshold", which will be
considered when to build graph or not for native engines. This is noop for lucene.
When graph is not available, plugin will return empty results. With this change, exact search will be performed when only no engine file is available in segment.
Radial search was never included as part of exact search. This will break radial search when threshold
is added and radial search is requested. Hence, introduce exact search during radial search similar to Approx search.

Related Issues

#1942

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[VijayanB]

In this comment we will be looking at some of the options on how to expose a setting “approximate_threshold” which controls when to build vector data structure during flush and merge operation. If no value is provided during index creation, we will use default value to decide whether to build vector data structure or not during flush and merge operations.

The possible values for this parameters are:

• -1 => When users want to disable building vector data structure during indexing
• 0 => When users want to build vector data structure always ( this is the behavior till 2.17)
• 1.. INTEGER.MAX_VALUE - 2 => Will build vector data structure only when a segment contains more than this number of documents.

Proposal

1. Keep “approximate_threshold” as dynamic index setting

In this approach, users can set this value per index. They can use “Update Index Setting” API to update this value on live index at any time.

Example:

PUT my-test-knn-index/_settings
{
      "index.knn.advanced.approximate_threshold":-1
}

Pros:

1. It is easy to update setting by just passing only the property and the desired value.
2. This setting will be shared by all vector fields from same index. Users don’t have to update individual fields explicitly.
3. This is aligned with other property “index.knn.advanced.filtered_exact_search_threshold"

Cons

1. Different vector fields from same index cannot have different value.

2. Keep “approximate_threshold” as mapping parameter

In this approach, users can set this value as mapping parameter for knn vector field type. They can use “Update Mapping Setting” API to update this value.

Example:

PUT my-test-knn-index/_mappings
{
  "properties": {
    "my_vector1": {
      "type": "knn_vector",
      "space_type": "l2",
      "dimension": 2,
`      "approximate_threshold": -1,`
      "method": {
        "name": "hnsw",
        "engine": "nmslib",
        "parameters": {
          "ef_construction": 128,
          "m": 24
        }
      }
    }
  }
}

Pros

1. Different vector fields from same index can have different value. This gives more control to the users since they can tune this parameter individually at field level.
2. This works well if user is going to set it and forget during index creation step.

Cons

1. This doesn’t provide very good user experience if users want to update the value after index creation step. we would like to provide same experience as index setting, for example.

PUT my-test-knn-index/_mappings
{
  "properties": {
    "my_vector1": {
      "type": "knn_vector",
      "approximate_threshold": -1
    }
  }
}

But in order to update a mapping parameter, users has to pass other non-updatable parameters as part of update mapping api request. In above example, to update “approximate_threshold", we had to first use GET Setting API to get the non-updatable properties like type, space_type, dimension, methods etc and then use exact value in update mapping api to change ”approximate_threshold".

3. Combine 1 & 2

In this approach we will provide this as both index setting and mapping parameter, where index setting will be considered as global setting for every field, and, users can override this value for any specific field by updating as mapping parameter.

Pros

1. It is easy to update index setting by just passing only the property and the desired value.
2. The index setting will be shared by all vector fields from same index. Users don’t have to update individual fields explicitly.
3. This is aligned partially with other property “index.knn.advanced.filtered_exact_search_threshold"
4. Different vector fields from same index can have different value by overriding at field level. This gives more control to the users since they can tune this parameter individually at field level.

Cons

1. This doesn’t solve the problem of having to pass non-updatable field as part of update mapping api request. However, this will be only considered in a specific case where users had to override this value for certain fields. This significantly reduces the nuances for most of the use case where user either has an index with one vector field or they want to use same setting for every vector field.

Finalized approach

Since there is no work around to use as mapping parameter and provide better user experience at same time, we can first introduce this as “index setting”, (Proposal 1 ) and, if we receive substantial feedback on having it as mapping parameter from community, we can introduce mapping parameter in next iteration**(effectively Proposal 3).**

[VijayanB]

Link to Documentation issue opensearch-project/documentation-website#8482

[jmazanec15]

Overall, I think this looks pretty good

[VijayanB]

Test failures are not due to this PR.