Guided Notes
Writing Efficient Queries
Select only what you need, read less data, and avoid N:N joins.
Why efficiency matters
Slide 1 / 5On small tables every query is fast. On big data, an inefficient query can take minutes — and in cloud warehouses that bill by bytes scanned, it costs real money.
BigQuery shows how much data a query will process before you run it, and other databases offer EXPLAIN plans. Measure before and after instead of guessing.
Three habits make most of the difference.