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All 5 Flashcards — Data warehouses
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Question
What is a data warehouse for?
Answer
**Analysing history** — few enormous reads — as opposed to an operational database recording many small transactions.
Question
What does ETL stand for?
Answer
**Extract** from each source, **Transform** into one consistent shape, **Load** into the warehouse. Transform is where most of the work is.
Question
Why is a warehouse denormalised?
Answer
So analytical queries **avoid joins**. It is safe because the warehouse is rebuilt in bulk and never edited, so update anomalies cannot arise.
Question
How current is a data warehouse?
Answer
Only as current as the **last load** — often nightly. It never answers "what is happening right now".
Question
Is a data warehouse a backup?
Answer
**No.** It holds **transformed** data, not the originals, so the business could not be restored from it.
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