What counts as a breaking schema change?

🔴 Dropped tables or columns

Removing a table or column that production code still references causes immediate errors.

🔴 Type narrowing

Changing VARCHAR(255) → VARCHAR(50) or INT → SMALLINT can truncate or overflow existing data.

🟡 Unsafe NOT NULL

Adding NOT NULL without a default value breaks every row that has a NULL in that column.

🟡 Dropped indexes or constraints

Removing unique constraints or indexes can cause duplicate data and sudden query slowdowns.

🟢 Safe changes

New tables, new nullable columns, and new indexes are usually safe and green-lit by SchemaLens.

🟢 Backward-compatible expansions

Widening a VARCHAR or adding a default value is safe and does not require a deploy order dance.

How it works in your PR workflow

1️⃣ A PR touches a .sql file

SchemaLens GitHub Action or App detects the schema change and starts a diff.

2️⃣ Base vs head are compared

The schema from the PR branch is compared against the target branch to find every table, column, index, and constraint change.

3️⃣ Risk score + comment

A PR comment shows the diff summary, breaking changes, risk score, and migration SQL — exactly where reviewers already look.

Example PR comment

This is what SchemaLens posts automatically when a breaking change is detected:

⚠️ SchemaLens detected 1 breaking change (Risk: 72/100) ## Summary - 1 table modified - 1 column removed - 1 index added ## Breaking Changes | Table | Change | Severity | |-------|--------|----------| | users | DROP COLUMN phone | CRITICAL | ## Migration SQL ```sql ALTER TABLE users DROP COLUMN phone; CREATE INDEX idx_users_email ON users(email); ``` 👉 Open full diff in SchemaLens

Generate a Real PR Comment →

Pick your workflow

⚙️ GitHub Action

Add a workflow file to your repo. Full control over gates, annotations, and breaking-change policies.

View Action →

🤖 GitHub App

Zero-config install. SchemaLens comments on every PR that touches a schema file.

Install App →

🔀 Public PR Viewer

Manually review any public GitHub PR's schema changes. Great for open-source contributions and demos.

Try Free →

Never let a breaking schema change slip through again

Add SchemaLens to your PR workflow and get automatic schema diff comments on every migration.

Add GitHub Action Diff a Public PR