IBM Db2 is a relational database which focuses on transactional and warehousing workloads. In 2017, IBM Db2 was announced which is the successor name of DB2 (introduced in the 1980s). In version 9, IBM Db2 started to support non-relational structures like JSON and XML. IBM Db2 can only be run on the mainframe which is known as IBM Db2 for z/OS before the release of IBM Db2 for Linux, UNIX, and Windows (LUW). The main difference between IBM Db2 z/OS and LUW is the database administration, but basic operations are very similar. Nowadays, it can support many other platforms including i, VSE, and VM. Besides, it is kind of confusing that IBM Db2 has four different code bases. The main reason is that in the early development, IBM needs to meet the requirements of different customers and support different hardware and operating systems. It is very hard to merge them to be compatible with previous releases.
IBM Db2 dates back to early of the 1970s. At that time, IBM researcher Edgar F. Codd described relational databases theory and published the important paper “A Relational Model of Data for Large Shared Data Banks.” As IBM’s first commercial relational database, IBM Db2 came after System R and improved the SEQUEL in System R to SQL. The latest version of IBM Db2 is 11.1, which was released on April 12, 2016.
Read Uncommitted Repeatable Read Cursor Stability
IBM Db2 supports four different kinds of isolation level, which are Repeatable read, Read stability, Cursor stability and Uncommitted read. The default level is cursor stability.
IBM Db2 supports three different kinds of Stored Procedures, which are external stored procedures, external SQL procedures (execute like external stored procedures), and native SQL procedures (create by a single SQL statement). The latter two are written entirely in SQL while the first one can be written in a host language.
Two-Phase Locking (Deadlock Prevention) Optimistic Concurrency Control (OCC)
IBM Db2 supports OPTIMISTIC CONCURRENCY CONTROL (OCC) and Two-Phase Locking (2PL). OCC believes that values are unlikely to change before the update or deletion operations but will incur more retry logic in the application. IBM Db2 supports OCC since version 9.5, and the history of 2PL, which was first invented by IBM, can be dated back to 1970s.
Nested Loop Join Hash Join Sort-Merge Join
IBM Db2 can support three different kinds of join algorithms, which are nested-loop join, merge join, and hash join. Hash join has the best performance among the three algorithms if memory is large enough to ensure hash loops will not interact with disks too often. To make the hash join works better, it is recommended to estimate and then set the memory large enough but without exceeding the memory limit.
Virtual Views Materialized Views
IBM Db2 supports both views and materialized views. For materialized views, it can be specified maintaining by system clause or user clause.
Decomposition Storage Model (Columnar)
IBM Db2 stores the values of a single attribute continuously in disk, i.e. DECOMPOSITION STORAGE MODEL (DSM). DSM is suitable for OLAP workloads as queries usually perform large scan over a subset of the attributes.
IBM Db2 use disk as the primary storage location of data. Therefore, it is a disk-oriented DBMS. It is worth to mention that IBM Db2 can support multiple buffer pools, which is the connection between volatile and non-volatile storage. Multiple buffer pools have many advantages. For example, it can accelerate queries that need temporary storage by storing temporary data into separate buffer pools. Also, the data of some seldom-used applications can be stored separately to avoid flushing some hot data.
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