Smaller is better - use the smallest acceptable type of column
Avoid using NULL values
Chart is better than varchar in the following cases:
All values have almost the same length (when maximum column lenght is almost the same as average length)
For short strings. Because varchar allocates 1 or 2 extra bytes to record the values length.
When values frequently changing. Because to change varchar you often need to allocate new memory
Varchar columns should have only necessary length. Because MySQL often allocates fixed-size chunks of memory to hold values internally. This is especially bad for sorting or operations that use in-memory temporary tables.
In most cases it is better to use timestamp instead of datetime. It’s two times more space efficient
For boolean types you should use BIT(1).
For ID its better to use integer column
Server Profiling
Best way to find the slow queries it’s to analize slow query log. At first you need to turn this options on, then confuge the logs destination, then restart sql server.
You can use pt-query-digest to analyze slow query logs
Indexis
MySQL will use index only in case of isolated column. In the following example there is the full table scan:
SELECTactor_idFROMsakila.actorWHEREactor_id+1=5;
If you need to index long string column, index only some prefix of it column.