Data engineering·Files, formats and table layers
you replaced select star with the six columns you needed and the bytes scanned fell by ninety percent.
Projection pushdown
Also calledcolumn pruning
Reading only the columns a query actually references, which columnar storage makes possible and which select-star defeats. It is the cheapest optimisation available in an analytical stack, and it is invisible in the query plan of an engineer who has never had to pay per byte. It also makes wide tables affordable, which is the reason one-big-table designs are viable at all.