Oracle Database supports In-memory scans when not all columns in a table have been populated into the In-Memory Column Store (IM column store).
This situation can occur when columns have been specified as
NO INMEMORY to save space. In-memory hybrid scans can access some data from the IM column store, and some data from the row store, improving performance by orders of magnitude over pure row store queries.
This page provides more detailed information about queries referencing both
NO INMEMORY columns behaving differently in Oracle Database 21c.
This practice shows how queries referencing both
NO INMEMORY columns can access columnar data. This optimizer access method called IM hybrid scan can improve performance by orders of magnitude. If the optimizer chooses a table scan, the storage engine automatically determines whether an IM hybrid scan performs better than a regular row store scan from the buffer cache.