99% is a strong claim. Maybe 70%? If you have a query that needs to look at over about 5K rows and you need responses under 100ms you need to know what its doing.
For each query I had to optimize, I wrote dozens, if not hundreds, that worked without any intervention. 99% is just a rough guess.
> If you have a query that needs to look at over about 5K rows and you need responses under 100ms you need to know what its doing.
Thats very unspecific, but getting 5000 rows from a table should take a few milliseconds on modern hardware.
Because 99% of the time you don't notice that it just works. 99% of the time you don't have to bend your query or schema to make it work.