Why are joins bad when considering scalability?

Question

Why are joins bad or \'slow\'. I know i heard this more then once. I found this quote

The problem is joins are relatively slow, especially over very

Answer 1

Joins can be slower than avoiding them through de-normalisation but if used correctly (joining on columns with appropriate indexes an so on) they are not inherently slow.

De-normalisation is one of many optimisation techniques you can consider if your well designed database schema exhibits performance problems.