Shadow-DB preview

db-x preview --shadow answers the question every DBA quietly worries about: how long will this migration lock the table?

Roadmap

Ships in DB-X v0.2 — see roadmap.

How it works

1. Spin an ephemeral Postgres container or fork a managed-snapshot-based copy.
2. Apply the planned DDL against the copy.
3. Capture timings, lock waits, and any errors.
4. Tear the copy down.
5. Emit a structured JSON report.

The report shape

{
  "schema": "db-x.shadow-preview/v1",
  "target": "postgres://...",
  "snapshot": "snapshot-abc",
  "actions": [
    {
      "id": "todos.priority",
      "ddl": "ALTER TABLE todos ADD COLUMN priority int DEFAULT 0 NOT NULL",
      "durationMs": 4823,
      "locks": [
        { "mode": "AccessExclusiveLock", "object": "todos", "waitMs": 0 }
      ],
      "rowsAffected": 1500000,
      "error": null
    }
  ]
}

Why it matters

• The report is structured JSON — pipe it to a CI bot, a Slack hook, or an MCP tool. No screen-scraping.
• The Shadow-DB run happens before anyone touches production. By the time the real db-x apply runs, the timings are known quantities.
• Combined with AI review, a Claude / Cursor session can read the report and explain to the human in plain English why a particular step is risky.