SQL

Pagination looks simple until a table becomes large or new rows are inserted while a client is paging through results. LIMIT and OFFSET are easy to understand, but deep offsets can become expensive and changing data can make rows appear twice or disappear between requests. Keyset pagination, also called seek pagination, avoids those problems by asking for rows after a known position instead of asking the database to skip a number of rows.

A schema migration can be syntactically correct and still cause an outage. Production deployments often run old and new application versions at the same time, background workers may lag behind, and a large table can turn a simple-looking DDL statement into a long lock. The expand-and-contract pattern reduces those risks by splitting an incompatible change into compatible stages. Why one-step schema changes are risky Suppose an application wants to rename users.full_name to users.display_name.