Fix: Improve VDiff internal query performance

[arthurschreiber]

Description

This pull request intends to improve the performance of some of the queries executed by VDiff against the internal sidecar database.

These changes should be backported as the symptoms of the bad query performance can lead to grave issues (like crashing primary vttablets) when running VDiff.

Related Issue(s)

#18578

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[arthurschreiber]

Here's an explain plan for the example query from #18578 with the new schema, showing that we're no longer doing any full table scans:

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.05"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "vd",
          "access_type": "index",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "state",
          "used_key_parts": [
            "state"
          ],
          "key_length": "67",
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "0.35",
            "data_read_per_join": "4K"
          },
          "used_columns": [
            "id"
          ]
        }
      },
      {
        "table": {
          "table_name": "vdt",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "vdiff_id"
          ],
          "key_length": "8",
          "ref": [
            "_vt.vd.id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "0.70",
            "data_read_per_join": "2K"
          },
          "used_columns": [
            "vdiff_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "vdl",
          "access_type": "ref",
          "possible_keys": [
            "vdiff_id_idx"
          ],
          "key": "vdiff_id_idx",
          "used_key_parts": [
            "vdiff_id"
          ],
          "key_length": "8",
          "ref": [
            "_vt.vd.id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "1.05",
            "data_read_per_join": "32"
          },
          "used_columns": [
            "vdiff_id"
          ]
        }
      }
    ]
  }
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.49%. Comparing base (1c5ca23) to head (718221a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18579      +/-   ##
==========================================
- Coverage   67.52%   67.49%   -0.03%     
==========================================
  Files        1607     1607              
  Lines      263324   263324              
==========================================
- Hits       177803   177741      -62     
- Misses      85521    85583      +62     

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