Add BiEngineStatistics to query job statistics

[tswast]

Some new query statistics have been added, which are present if BI engine is enabled

Suggested implementation

• Create two new classes, reflecting the API: BiEngineStatistics and BiEngineReason.
• Expose the biEngineStatistics property from JobStatistics2 on QueryJob, similar to DmlStats.

API details

Discovery representation for BiEngineStatistics:

"BiEngineStatistics": {
      "id": "BiEngineStatistics",
      "properties": {
        "biEngineMode": {
          "description": "[Output-only] Specifies which mode of BI Engine acceleration was performed (if any).",
          "type": "string",
          "default": "$(stats.bi_engine_mode)"
        },
        "biEngineReasons": {
          "type": "array",
          "items": {
            "$ref": "BiEngineReason"
          },
          "description": "In case of DISABLED or PARTIAL bi_engine_mode, these contain the explanatory reasons as to why BI Engine could not accelerate. In case the full query was accelerated, this field is not populated."
        }
      },
      "type": "object"
    },

The BiEngineReason discovery representation:

"BiEngineReason": {
      "type": "object",
      "id": "BiEngineReason",
      "properties": {
        "code": {
          "description": "[Output-only] High-level BI Engine reason for partial or disabled acceleration.",
          "default": "$(reason.code)",
          "type": "string"
        },
        "message": {
          "description": "[Output-only] Free form human-readable reason for partial or disabled acceleration.",
          "type": "string",
          "default": "$(reason.message)"
        }
      }
    },

Testing notes

Unit tests are sufficient for this work. Manually testing and/or system testing will be difficult as it requires manual steps to set-up a long-term BI Engine reservation on the project.

Related

• Go PR

[tswast]

DmlStats seems to be quite similar to this request:

python-bigquery/google/cloud/bigquery/job/query.py

Line 123 in 66b3dd9

class DmlStats(typing.NamedTuple):

Though, the sub-BiEngineReason may mean it may be easier to use our _properties implementation than using named tuples.