Concepts
Databases
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.
Tablespaces
In GaussDB, a tablespace is a directory storing physical files of the databases. Multiple tablespaces can coexist. Files are physically isolated using tablespaces and managed by a file system.
Schemas
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.
Users and Roles
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. Each user can have only one role. In GaussDB, the difference between roles and users is that a role does not have the LOGIN permission by default. In GaussDB, you can put a user's role under a parent role for flexible management.
Transactions
In GaussDB, transactions are managed by multi-version concurrency control (MVCC) and two-phase locking (2PL). It enables smooth data reads and writes. In GaussDB, an Astore engine saves historical version data together with the current tuple version. The GaussDB Astore engine 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. The GaussDB Ustore engine stores historical version data to the undo rollback segments. The thread for purging undo deletes historical version data in a unified manner. 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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