Why is SELECT * considered harmful?

Question

Why is SELECT * bad practice? Wouldn\'t it mean less code to change if you added a new column you wanted?

I understand that SELECT COUNT(*)

Answer 1

There is also more pragmatic reason: money. When you use cloud database and you have to pay for data processed there is no explanation to read data that you will immediately discard.

For example: BigQuery:

Query pricing

Query pricing refers to the cost of running your SQL commands and user-defined functions. BigQuery charges for queries by using one metric: the number of bytes processed.

and Control projection - Avoid SELECT *:

Best practice: Control projection - Query only the columns that you need.

Projection refers to the number of columns that are read by your query. Projecting excess columns incurs additional (wasted) I/O and materialization (writing results).

Using SELECT * is the most expensive way to query data. When you use SELECT *, BigQuery does a full scan of every column in the table.