“There is an inexorable trend towards storing and sending immutable data. We need immutability to coordinate at a distance and we can afford immutability, as storage gets cheaper.”Pat Helland · Immutability Changes Everything · 2015 · Immutability Changes Everything, CIDR 2015 — rezumat (secțiunea 2 se numește „Accountants Don't Use Erasers”)
A correction is a new row, not an eraser.
An accountant does not erase a wrong entry; they add a correcting one. The ledger stays complete, anyone can redo the arithmetic, and "the balance" is a derived value, not a fact. Helland shows that modern computing is moving the same way: transaction logs, event logs, Parquet files written once, even SSDs that do not overwrite in place. When data is immutable, whole classes of problems vanish — lost updates, inconsistent reads, "who changed this?" — and two precious things appear: audit is free and any state can be recomputed. In a data warehouse, facts are naturally immutable: a sale happened. Corrections become new rows with the opposite sign or with a validity period, and late-arriving facts are appended, not "patched" into the past. Dimensions keep their history through versions — the next lesson. The only real conflict is with the GDPR right to erasure, for which there is an answer compatible with immutability, in lesson 19.
Why it matters A warehouse corrected in place cannot answer "what did the report show last month, and why". An append-only one always can.