The Foo bar
Fancy algorithms are slow when n is small, and n is usually small.
Rob Pike
There are only two hard things in Computer Science: cache invalidation and naming things.
Phil Karlton
XML is like violence — if it doesn’t solve your problems, you are not using enough of it.
Nokogiri README
If you don’t know how compilers work, then you don’t know how computers work.
Steve Yegge
Software is easy to make, except when you want it to do something new.
Rosenberg’s Law
When faced with a problem you do not understand, do any part of it you do understand, then look at it again
Grady Booch (citing Heinlein)
An underlying problem with artificial intelligence that I have personally experience in my forty years in this area is that as soon as an AI technique works, it’s no longer considered AI and is spun off as its own field (for example, character recognition, speech recognition, machine vision, robotics, data mining, medical informatics, automated investing.)
Ray Kurzweil
If it’s your decision, it’s design; if not, it’s a requirement.
Alastair Cockburn
Inside every large, complex program is a small, elegant program that does the same thing, correctly.
John Graham-Cumming in Some things I’ve learnt about programming
On database indices efficiency

While researching why SELECT COUNT(*) is slow on PostgreSQL, i found this nice rule-of-thumb for B-tree (the most common type of database index) efficiency:

10% selectivity is the minimum selectivity necessary for a b-tree index to be helpful.

Where selectivity = unique index values / total number records