When is an in-memory database not an in-memory database?
The “classic” definition is of a database that runs entirely
in main (or physical) memory. In other words, if you have an
“in-memory” data warehouse (DW) of 512 GB, all of your data
and indexes must fit into 512 GB, which means you may run out
of physical memory.
Many so-called “in-memory” databases don’t do this, however.
This isn’t so much a problem as a source of confusion. The
classic in-memory database is constrained by several factors, not
the least of which is the amount of physical memory available to
the system. For this reason, some “in-memory” database engines
actually use a kind of memory-optimized design. This means that
the database tries to make intelligent decisions about which data
to load into memory, how (or at what levels) it caches data, and
how it performs certain kinds of operations.
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