All memory-optimized tables must have at least one index. The indexes on a memory-optimized table store the connections between the rows. It is also important to remember that, as the name suggests, In-Memory OLTP indexes reside inside non-permanent memory (RAM) and are not written to disk. The only portions written to disk are the data rows of the base memory-optimized table and data changes, which are written to the transaction log (which is a requirement to fulfill the Durability part of the ACID concept).

There are two types of index available for memory-optimized tables:

• Non-clustered index 
• Hash index