I ntroducon to MemSQL Mul-version concurrency control summary: ● Every write creates a new version of row ● Commits are atomic ● Old versions are garbage-collected ● Reads are never blocked ● Row-level locking for writes include DELETE ● Allows for online ALTER TABLE Figure 5. MVCC control summary Disk-opmized columnstores and memory-opmized rowstore tables MemSQL supports storing and processing data using two types of data stores: a completely in-memory rowstore and a disk-backed columnstore. Rowstores and columnstores differ both in storage format (row vs. column) and in storage medium (RAM vs. disk). MemSQL allows querying rowstore and columnstore data together in the same query. The rowstore is typically used for highly concurrent online transacon processing (OLTP) and mixed OLTP/analycal workloads. The whole data set is kept in memory - providing fast writes and supporng thousands of concurrent queries. Rowstores uses a transacon log to avoid disk I/O bolenecks on writes. Transacons are commied to disk as logs and periodically compressed as snapshots of the enre database. 11