Oracle Database In-Memory is an in-memory, column-oriented data store. It aims to improve the performance of analytic workloads without impacting the performance of transactions that continue to use Oracle's traditional row format in memory.
The product's performance comes through the in-memory, columnar compressed format and through the use of SIMD (Single Instruction processing Multiple Data values) vector processing.
Dual format architecture
Data for OLTP is maintained in the «traditional» Oracle row format while data for OLAP is stored in a purely in-memory (IM) columnar format.
This IM store does not reside in the buffer cache but rather in the SGA (in the so-called In-Memory Area). This In-Memory Area consists of two pools. (See v$inmemory_area)