Concepts
Database
Databases manage various data objects and are isolated from each other. While creating a database, you can specify a tablespace. If you do not specify it, the object will be saved to the PG_DEFAULT tablespace by default. Objects managed by a database can be distributed to multiple tablespaces.
Tablespace
In GaussDB, a tablespace is a directory storing physical files of the databases the tablespace contains. Multiple tablespaces can coexist. Files are physically isolated using tablespaces and managed by a file system.
Schema
GaussDB schemas logically separate databases. All database objects are created under certain schemas. In GaussDB, schemas and users are loosely bound. When you create a user, a schema with the same name as the user will be created automatically. You can also create a schema or specify another schema.
User and Role
GaussDB uses users and roles to control the access to databases. A role can be a database user or a group of database users, depending on role settings. In GaussDB, the difference between roles and users is that a role does not have the LOGIN permission by default. In GaussDB, one user can have only one role, but you can put a user's role under a parent role to grant multiple permissions to the user.
Transaction
In GaussDB, transactions are managed by multi-version concurrency control (MVCC) and two-phase locking (2PL). It enables smooth data reads and writes. GaussDB MVCC saves historical version data together with the current tuple version. GaussDB uses a VACUUM thread instead of rollback segments to periodically delete historical version data. Generally, you do not need to pay special attention to the VACUUM thread unless you need to optimize the performance. In addition, GaussDB automatically commits transactions for single-statement queries (without using statements such as BEGIN to explicitly start a transaction block).
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