CREATE INDEX blocks all writes during build

Non-concurrent index creation acquires ACCESS EXCLUSIVE lock on the table, blocking all INSERT, UPDATE, DELETE operations until index build completes, potentially causing write outages on large tables

Use CREATE INDEX CONCURRENTLY instead of CREATE INDEX on production tables. This allows writes to continue during index creation. Note: CONCURRENTLY takes longer, cannot run inside a transaction, and may fail leaving an invalid index (check with \d tablename).