> Canonical: https://antifailure.dev/solutions/devtools > Schema changes on large tables, rehearsed at production row counts rather than fixture counts. # Schema changes on large tables. The flagship wedge, felt first by teams whose users notice p99 immediately. Measure the strongest lock held per table, how long it was held, whether another session was left waiting on it, and how the query plans moved. Start with Postgres volume, plans, and pools, then expand. lock ACCESS EXCLUSIVE · 4.2s · another session waiting Index Scan 12ms becomes Seq Scan 410ms on production-shaped volume. ## Large tables plus frequent schema change. - → Large tables . Exclusive locks and rewrites that never show up on a laptop database. - → Query plans . Plan regressions under production-shaped volume. - → Pools . Connection-pool exhaustion during migrate-and-serve. - pid 1842 ALTER subscriptions ACCESS EXCLUSIVE holds 27.4s - blocks - pid 2210 SELECT events ACCESS SHARE waiting · p99 6.9s - queued - pool migrate-and-serve pool wait connections queued 27.4s hold · events p99 820ms → 6.9s ## Exceptional Postgres instrumentation first. - → The first supported stack should be exceptional . A broad compatibility list with unreliable connectors would destroy trust. - → Postgres first . Volume, plans, and pools, then expand. - → Publish what the twin reproduced . Do not pretend unsupported components are cloned. - 01 Expand col access_tier null yes default none - 02 Backfill batch 12k / pass dual-read on pool live - 03 Contract constraint last old path kept safe not yet Publish what the twin reproduced. Do not pretend unsupported components are cloned. ## Locks, plans, and rollback feasibility before it ships. - → Lock duration . The strongest mode held per table, how long it was held, and whether another session waited on it. - → Schema coexistence . Whether old instances can still read the new schema shows up here first. - → Users notice p99 immediately . Large tables plus frequent schema change. **peak hold** 27.4s **events p99** 820ms → 6.9s **waiter** waiting Another session was left waiting. events p99 moved 820ms → 6.9s. - Migration Safety Locks, rewrites, plans, lint. Read - Load Production's own route mix against the branch. Read - All solutions Teams and jobs. Read ## Know what happens before you deploy. Create a disposable production twin for every risky change. Catch migration failures before they reach customers.