schema-diff

name	schema-diff
description	Preview a SQLite schema change and its impact before applying it to a production database

/schema-diff — Preview a schema change

When to use

User wants to change src/db/database.js — adding a table, adding a column, changing a constraint — and wants to see the impact before applying it.

Steps

1. Read the current schema from src/db/database.js.

2. Read the production database schema (if accessible):

   sqlite3 data/accelerate.db .schema
   

3. Compare the two. Differences mean a past change was made in code that hasn't been applied to the production DB yet.

4. Read the proposed change the user wants to make.

5. Print three things:

   • Diff summary — which tables and columns change
   • Rows affected — estimate from a SELECT COUNT(*) on affected tables
   • Risk level — LOW, MEDIUM, HIGH with explanation

   Risk heuristics:

   • LOW: new table with no foreign keys, new nullable column
   • MEDIUM: new column with DEFAULT, adding index, new CHECK constraint
   • HIGH: DROP COLUMN, NOT NULL without DEFAULT, RENAME, foreign key changes, data type changes

6. Generate the migration SQL:

   • For new tables: the CREATE TABLE statement
   • For new columns: the ALTER TABLE ... ADD COLUMN statement
   • For destructive changes: the multi-step copy-rename dance SQLite requires

7. Remind the user of the backup runbook: docs/50-operations/runbooks/backup-database.md

8. Do not apply the change. This skill is read-only. The user makes the decision.

Output format

Schema Change Preview
---------------------
Current: src/db/database.js (N tables)
Proposed: + adds column `foo` to `inquiries`

Diff:
  inquiries:
    + foo TEXT DEFAULT NULL

Rows affected:
  inquiries: 142 rows (will get NULL foo)

Risk: LOW
  New nullable column, no constraints, no data transformation.

Migration SQL:
  ALTER TABLE inquiries ADD COLUMN foo TEXT;

Reminder: back up the production DB before applying.
  See docs/50-operations/runbooks/backup-database.md