How is database performance impacted when the mysql innodb database is very large?

I am currently making a decision on how i can reduce the s开发者_StackOverflow社区ize of the database. Why is performance slowed down when a database is slow?

Because eventually the working set of data to be queried no longer fits into memory, and must use the disk to query. As the disk is generally many orders of magnitude slower than memory, you will hit a sudden dropoff point if you're only using small datasets.

Indicies on a database are also generally O(N log N) complexity for searching (again, searching on the disk = slower). When you are not careful you can have cartesian joins which explode your data set.