openGauss V
Published: 2023-02-22 02:20:59

U – Z

Ustore

Alias for the in-place update storage engine which solves the problems of space expansion and large tuples of the Append update storage engine. The design of efficient rollback segments is the basis of the in-place update storage engine.

Undo Record

An undo record can be inserted, queried, and organized. It connects to the ustore through the northbound interface and connects to the buffer pool through the southbound interface.

Undo Space

Manages physical resources of undo records, including adding and deleting undo files.

Undo Zone

Be bound to service threads and manages the undo logical resources of each service thread.

TransactionSlot

Records undo records by transaction granularity for transaction rollback and undo record recycling.

TIMECAPSULE

Flashback keyword. After the flashback technology is used, it takes only seconds to restore the submitted data before the database is modified. The restoration time is irrelevant to the database size.

RECYCLE BIN

After the recycle bin function is enabled, DROP TABLE can move a table and its sub-objects to the recycle bin.

PURGE

Clears objects in the recycle bin.

VACUUM

A thread that is periodically started by a database to remove junk tuples. Multiple VACUUM threads can be started concurrently by setting a parameter.

verbose

A verbose option specifies the information to be displayed.

WAL

Acronym for write-ahead logging, a standard method for logging a transaction. Corresponding logs must be written into a permanent device before a data file (carrier for a table and index) is modified.

WAL receiver

A thread created by a standby node during database replication. The thread is used to receive data and commands from the primary node and to tell the primary node that the data and commands have been acknowledged. Only one WAL receiver thread can run on one standby node.

WAL sender

A thread created on the primary node when the primary node has received a connection request from a standby node during database replication. This thread is used to send data and commands to the standby node and to receive responses from the standby node. Multiple WAL sender threads may run on one primary node. Each WAL sender thread corresponds to a connection request initiated by a standby node.

WAL writer

A thread for writing redo logs that are created when a database is started. This thread is used to write logs in the memory to a permanent device, such as a disk.

Xlog

A transaction log. A logical node can have only one .xlog file.

xDR

The x detail record is a general term that refers to call detail records (CDRs), user flow data records (UFDRs), transaction detail records (TDRs), and statistics detail records (SDRs) on the user and signaling planes.

physical node

A physical machine.

system catalog

A system catalog stores meta information about a database, including user tables, indexes, columns, functions, and data types.

pushdown

In openGauss, a primary database node can send a query plan to multiple DNs for parallel execution. This behavior is called pushdown. It achieves better query performance than extracting data to a primary database node for query.

compression

Data compression, source coding, or bit-rate reduction involves encoding information that uses fewer bits than the original representation. Compression can be either lossy or lossless. Lossless compression reduces bits by identifying and eliminating statistical redundancy. No information is lost in lossless compression. Lossy compression reduces bits by identifying and removing less noticeable information. The process of reducing the size of a data file is commonly referred as data compression, although its formal name is source coding (coding done at the source of the data, before it is stored or transmitted).

consistency

One of the ACID properties of a database transaction. Data in the database must comply with integrity constraints.

metadata

Data that provides information about other data. Metadata describes the source, size, format, or other characteristics of data. In the data field, metadata helps to explain the content of a data warehouse.

atomicity

One of the ACID properties of a database transaction. A transaction is composed of an indivisible unit of work. Operations performed in a transaction must be all finished or have not been performed. If an error occurs during transaction execution, the transaction is rolled back to the state when it was not performed.

dirty page

A page that has been modified, where the changes are not yet written to a permanent device.

incremental backup

Incremental backup only saves data changed since the last valid backup.

incremental synchronization

A data synchronization mechanism in the openGauss primary/standby solution, which is used to synchronize inconsistent data from the primary node to a standby node.

primary node

A node that allows read and write operations and works with all standby nodes in the openGauss primary/standby system. At the same time, only one node in the primary/standby system is identified as the primary node.

subject term

A standardized word or phrase that describes the subject of an article.

dump file

A specific type of trace file. A dump is typically a one-time output of diagnostic data in response to an event, whereas a trace tends to be continuous output of diagnostic data.

minimum restoration point

A method used by openGauss to ensure data consistency. During startup, openGauss checks consistency between the latest WAL logs and the minimum restoration point. If the record location of the minimum restoration point is greater than that of the latest WAL logs, the database fails to start.