DB Migration Safety Checker

This skill takes your migration file plus your actual production table sizes and returns a statement-by-statement lock analysis calibrated to your database engine and version — because lock semantics differ between PostgreSQL 14 and 16, and between MySQL 5.7 and 8.0. Where a statement would block, it rewrites the sequence using expand-contract or concurrent index builds rather than just flagging the problem. It also produces the rollback script for every destructive step, and a deploy timing recommendation based on your traffic window.

A typical input: a Rails migration that adds a NOT NULL column with a default to an orders table with 38 million rows, running on PostgreSQL 15 with live traffic. The buyer pastes the migration SQL and states the engine version and approximate row count. The skill identifies that the column add triggers a table rewrite on older Postgres but not on 15, confirms the in-place behavior, then flags that the NOT NULL constraint validation will still hold an AccessShareLock and provides the two-phase alternative: add the column nullable, backfill in batches of 5,000 rows with a 50ms sleep, then add the constraint as NOT VALID and validate separately.

Output excerpt format: RISK SUMMARY — Statement 1: ALTER TABLE orders ADD COLUMN status varchar(20) NOT NULL DEFAULT 'pending' | Lock type: AccessExclusiveLock | Estimated duration at 38M rows: ~4 min table rewrite (Postgres < 11 only; your version: safe) | Safe path: proceed as-is. Statement 2: CREATE INDEX ON orders(status) | Lock type: ShareLock (blocking) | Safe path: CREATE INDEX CONCURRENTLY — add outside transaction block. ROLLBACK SCRIPT — Step 2: DROP INDEX CONCURRENTLY IF EXISTS orders_status_idx;