feat: add --out-json flag for structured report output

[nanookclaw]

Closes #5.

Adds an optional --out-json <path> flag to pgcompare run. When omitted, behavior is unchanged. When provided, the full ReportData is written as pretty-printed JSON next to the existing HTML report.

Changes

• main.go: new --out-json cobra flag; if set, call pgcompare.GenerateJSON(data, flagOutJSON) after the existing Generate call and print the JSON path to stdout alongside the HTML path.
• internal/pgcompare/report.go: add GenerateJSON(data ReportData, outPath string) error using encoding/json with MarshalIndent (2-space indent, 0644).
• internal/pgcompare/report_test.go: add TestGenerateJSON that round-trips a small ReportData and asserts GeneratedAt, Iterations, Speedups, Before.Stats[0].QueryName, and Diffs[0].QueryName are present in the JSON.

Stats.{Min,Max,P50,P95,P99,Mean,StdDev} are time.Duration, which encoding/json serializes as int64 nanoseconds — matches the structured-automation use case described in the issue.

Example JSON output

The exact values depend on the benchmark run; duration fields are encoded as nanoseconds by Go's encoding/json for time.Duration values.

{
  "GeneratedAt": "2026-05-18T11:06:00Z",
  "Iterations": 10,
  "WarmupIterations": 1,
  "Concurrency": 1,
  "Repeats": 1,
  "Speedups": [
    1.5
  ],
  "Before": {
    "Phase": "before",
    "Stats": [
      {
        "QueryName": "q1",
        "P95": 2000000,
        "QPS": 0,
        "ErrorRate": 0,
        "Errors": null
      }
    ],
    "Plans": [
      {
        "NodeType": "Seq Scan",
        "RelationName": "",
        "IndexName": "",
        "ActualRows": 0,
        "ActualTotalTime": 0,
        "Children": null
      }
    ]
  },
  "After": {
    "Phase": "after",
    "Stats": [
      {
        "QueryName": "q1",
        "P95": 1000000,
        "QPS": 0,
        "ErrorRate": 0,
        "Errors": null
      }
    ],
    "Plans": [
      {
        "NodeType": "Index Scan",
        "IndexName": "idx_q1",
        "ActualRows": 0,
        "ActualTotalTime": 0,
        "Children": null
      }
    ]
  },
  "Diffs": [
    {
      "QueryName": "q1",
      "Before": { "NodeType": "Seq Scan" },
      "After": { "NodeType": "Index Scan", "IndexName": "idx_q1" },
      "Summary": null
    }
  ],
  "Description": null
}

Verification

go build ./...
go test ./internal/pgcompare/...   # 22 tests pass, incl. new TestGenerateJSON
gofmt -l .                          # empty
go vet ./...                        # clean

Default behavior unchanged: running pgcompare run --config ... --out report.html produces only report.html.

[sokovmd]

Could you add an example of the JSON output to the PR description?

[nanookclaw]

Added an ## Example JSON output section to the PR description. It includes the main ReportData shape plus the note that time.Duration fields are encoded as nanoseconds by Go's encoding/json.

[sokovmd]

Add example to example

[sokovmd]

Update README.md and RU.md

[sokovmd]

Update README.md and RU.md
+
and example to https://github.com/pg-tools/pgcompare/tree/main/example

[nanookclaw]

Pushed 305bd00:

• example/report.json paired with example/report.html. Same three queries (top_rated_blocked_drivers / latest_ride_status / failed_payments_by_amount), so the full ReportData layout is visible side by side.
• README.md and RU.md: a short section under "Write the report to a custom path" / equivalent showing --out-json, plus a Notes bullet. Covered: independent of --out, opt-in (no JSON written without it), document is the ReportData struct directly, time.Duration fields encode as integer nanoseconds via Go's encoding/json, and when set the JSON path is printed as the last stdout line after the HTML path.

Verified go build ./... and go test ./internal/pgcompare/... clean. JSON parses with python3 -c "import json; json.load(open('example/report.json'))" and has the expected Before/After/Diffs/Description keys.