I have a Myisam table with composite unique key of 2 columns and 90 million data. Now we are facing memory and load issues and after going through the web I am planning to include partitioning and changing this table to Innodb for better performance. But I have following concerns:

1. Changing to innodb will have a huge downtime, Is it possible to minimize the downtime?
2. Most of the select query are on a particular column of the key on which I am planning to have the hash partitioning, how much it will effect the query on another key column?

Will these changes improve the performance to the extent mentioned theoretically? Is there any better solution for such cases. Any suggestion or experience can be helpful. 

My queries are simple like

`Select * from Table where Col1= "Value"`  

`Select * from Table where Col1="Value" and Col2 IN (V1,V2,V3)`

Inserts are very frequently.