Help Center> >Glossary

A

    • A
      advanced package

      Logical and functional stored procedures and functions provided by the database.

    C

      • C
        checkpoint

        A mechanism that stores data from the database memory to disks at a certain time. The database periodically stores the data of committed transactions and data of uncommitted transactions to disks. The data and redo logs can be used for database restoration if a database restarts or breaks down.

        cluster

        The smallest management unit in DWS. A cluster represents a separately running data warehouse. Users can manage the lifecycle of a cluster in DWS.

        column

        An equivalent concept of field. A database table consists of one or more columns.

        Compression Unit

        Compression Unit (CU) is the smallest storage unit in a column-storage table.

        concurrency control

        A DBMS service that ensures data integrity when multiple transactions are concurrently executed in a multi-user environment. In a multi-threaded DWS environment, concurrency control ensures that database operations are safe and all database transactions remain consistent at any given time.

        CU

        See Compression Unit

      D

        • D
          data control language

          A subset of SQL for setting or modifying database user or role rights.

          data definition language

          A subset of SQL for defining data structures and database objects.

          data dictionary

          A reserved table within a database which is used to store information about the database itself. The information includes database design information, stored procedure information, user rights, user statistics, database process information, database increase statistics, and database performance statistics.

          data manipulation language

          A subset of SQL for accessing data for database objects.

          data partitioning

          The action of dividing a table into parts (partitions) whose data does not overlap within a database instance. Tables can be partitioned by range, where the target storage location is mapped based on the range of the values in the column that is specified in the tuple.

          Data Warehouse Service

          Data Warehouse Service is an online data processing database based on the public cloud infrastructure and platform and helps you mine and analyze massive sets of data.

          database

          A collection of data that is stored together and can be accessed, managed, and updated. Data in a view in the database can be classified into the following types: numerals, full text, digits, and images.

          database instance

          A process and the database files that it controls. The cluster installs multiple database instances on one physical node. The GTM, CM, CN, and DN installed on cluster nodes are all database instances. A database instance is also called a logical node.

          DCL

          See data control language

          DDL

          See data definition language

          DML

          See data manipulation language

          DWS

          See Data Warehouse Service

        E

          • E
            ETL

            See Extract-Transform-Load

            Extract-Transform-Load

            A process of data transmission from the source to the target database.

          I

            • I
              index

              An ordered data structure in the database management system. An index accelerates querying and updating of data in database tables.

            L

              • L
                low performance

                A situation where some nodes in a cluster become unavailable, which affects the cluster performance

              M

                • M
                  Massive Parallel Processing

                  Massive Parallel Processing (MPP) refers to cluster architecture that consists of multiple machines. The architecture is also called a cluster system.

                  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.

                  MPP

                  See Massive Parallel Processing

                N

                  • N
                    node

                    A basic metering unit in DWS. It includes user-specified computing and storage resources and virtual machines (VMs) where DWS programs are deployed.

                  O

                    • O
                      OLAP

                      See Online Analytical Processing

                      Online Analytical Processing

                      Online analytical processing (OLAP) is the most important application in the database warehouse system. It is dedicated to complex analytical operations, helps decision makers and executives to make decisions, and rapidly and flexibly processes complex queries involving a great amount of data based on analysts' requirements. In addition, the OLAP provides decision makers with query results that are easy to understand, allowing them to learn the operating status of the enterprise. These decision makers can then produce informed and accurate solutions based on the query results.

                    P

                      • P
                        Page

                        Minimum memory unit for row storage in the relationship object structure. The default size of a page is 8 KB.

                      Q

                        • Q
                          Query Operator

                          An iterator or a query tree node, which is a basic unit for the execution of a query. Execution of a query can be split into one or more query operators. Common query operators include scan, join, and aggregation.

                        R

                          • R
                            read-only

                            When a data warehouse enters read-only state, it responds only to reads. The warehouse becomes read-only in many situations, for example, when you create a cluster snapshot or when 90% of the cluster's storage capacity is used.

                            redo log

                            A log that records operations on the database. Redo logs contain the information required for performing these operations again. If a database is faulty, redo logs can be used to restore the database to its pre-fault state.

                          S

                            • S
                              schema

                              A database object set that includes the logical structure, such as tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links.

                              segment

                              A segment in the database indicates a part containing one or more regions. Region is the smallest range of a database and consists of data blocks. One or more segments comprise a tablespace.

                              shared-nothing architecture

                              A distributed computing architecture, in which none of the nodes share a CPU or storage resources. This architecture has good scalability.

                              snapshot

                              A full backup of a cluster. Snapshots are stored in the storage space of Object Storage Service (OBS).

                              snapshot restoration

                              A snapshot can be used to restore a cluster to a newly created one that has the same specifications. Currently, you can restore a cluster only to a new one.

                              SQL

                              See Structure Query Language

                              statistics

                              Information that is automatically collected by databases, including table-level information (number of tuples and number of pages) and column-level information (distribution histograms of value ranges of columns). Statistics in databases are used to estimate the cost of query plans to find the plan with the lowest cost.

                              StoredProcedure

                              A group of SQL statements compiled to perform certain functions and stored in a large database system. Users can specify a name and parameters (if any) for a stored procedure to execute the procedure.

                              Structure Query Language

                              Structure Query Language (SQL) is a standard database query language. It consists of DDL, DML, and DCL.

                              system table

                              A table storing meta information about the database. The meta information includes user tables, indexes, columns, functions, and data types in a database.

                            T

                              • T
                                table

                                A set of columns and rows. The value in each column represents data of a certain type. For example, if a table contains people's names, cities, and states, it has three columns: Name, City, and State. In every row in the table, the Name column contains a name, the City column contains a city, and the State column contains a state.

                                tablespace

                                A tablespace is a logical storage structure that contains tables, indexes, large objects, and long data. A tablespace provides an abstract layer between physical data and logical data, and provides storage space for all database objects. When you create a table, you can specify which tablespace it belongs to.

                                transaction

                                A logical unit of work performed within a database management system against a database. A transaction consists of a limited database operation sequence, and must have ACID features.

                              W

                                • W
                                  WAL

                                  See Write-ahead logging

                                  Write-ahead logging

                                  Write-ahead logging (WAL) is 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.