Every major database will load the hottest data into RAM, where the scope of "hottest" broadens to whatever amount will fit in RAM. A small percentage of them require you to confirm how much RAM it can use for this cache.
Putting the data on a ramdisk just becomes entirely redundant because it's still going to create a second memory cache that it uses.
Many operations do a local cache warming by running the common queries over the database before they it is brought online for processing. As a secondary note, people often under-estimate the size of their data because they don't account for all of the keys, indexes and relationships that also would be memory cached in an ideal situation.
Putting the data on a ramdisk just becomes entirely redundant because it's still going to create a second memory cache that it uses.
Many operations do a local cache warming by running the common queries over the database before they it is brought online for processing. As a secondary note, people often under-estimate the size of their data because they don't account for all of the keys, indexes and relationships that also would be memory cached in an ideal situation.