Analyze a Slow Query Plan with EXPLAIN
Run EXPLAIN ANALYZE on the slow query so the plan shows whether it is a sequential scan, a bad join, or missing index.
Task
Apply EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; so the schema change is safe, reversible, and does not lock the table during peak traffic.
When to use this
A migration deploys EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; during peak hours, takes a 40-second lock, and causes a connection pool exhaustion.
Prerequisites
- A database with migration tooling configured
- A staging environment mirroring production schema
Steps
1. Configure EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; in the project configuration file with the recommended defaults.
EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42;
Expected output: The configuration is accepted and EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; runs without errors on the first invocation.
2. Run the check locally to verify EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; produces the expected result before pushing.
pnpm test
Expected output: Local run matches CI behavior; no surprises after push.
3. Add the gate to the CI pipeline so EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; blocks merge on failure.
- name: Run add query plan analyze
run: pnpm test
Expected output: Pull requests with failing EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; are blocked from merging.
Troubleshooting
Problem
The check passes locally but fails in CI
Solution
Pin dependency versions and the runtime to match CI exactly; reproduce with the same Docker image or act.
Problem
Configuration is rejected with a schema error
Solution
Check the tool version matches the config schema; older versions may not support newer fields.
Limitations
- The gate only runs on events that trigger the workflow; locally pushed commits bypass it.
- Configuration drift is possible if someone edits settings outside version control.
Make migrations safe
Start by measuring the problem. Run EXPLAIN ANALYZE on the slow query so the plan shows whether it is a sequential scan, a bad join, or missing index. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; against the current codebase and inspect the output before wiring it into CI.
EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42;Verify and roll back
Once the gate is in place, make failures actionable. A failing EXPLAIN (ANALYZE, BUFFERS, FORMAT JSON) SELECT * FROM orders WHERE user_id = 42; should point to the exact file and line, not a generic error. Pin the tool version in the lockfile so a upstream release does not silently change behavior. Review the gate output in every pull request just like a test result.