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Appendix M. Glossary

   This is a list of terms and their meaning in the context of PostgreSQL
   and relational database systems in general.

   ACID
          Atomicity, Consistency, Isolation, and Durability. This set of
          properties of database transactions is intended to guarantee
          validity in concurrent operation and even in event of errors,
          power failures, etc.

   Aggregate function (routine)
          A function that combines (aggregates) multiple input values, for
          example by counting, averaging or adding, yielding a single
          output value.

          For more information, see Section 9.21.

          See Also Window function (routine).

   Access Method
          Interfaces which PostgreSQL use in order to access data in
          tables and indexes. This abstraction allows for adding support
          for new types of data storage.

          For more information, see Chapter 62 and Chapter 63.

   Analytic function
          See Window function (routine).

   Analyze (operation)
          The act of collecting statistics from data in tables and other
          relations to help the query planner to make decisions about how
          to execute queries.

          (Don't confuse this term with the ANALYZE option to the EXPLAIN
          command.)

          For more information, see ANALYZE.

   Asynchronous I/O (AIO)
          Asynchronous I/O (AIO) describes performing I/O in a
          non-blocking way (asynchronously), in contrast to synchronous
          I/O, which blocks for the entire duration of the I/O.

          With AIO, starting an I/O operation is separated from waiting
          for the result of the operation, allowing multiple I/O
          operations to be initiated concurrently, as well as performing
          CPU heavy operations concurrently with I/O. The price for that
          increased concurrency is increased complexity.

          See Also Input/Output.

   Atomic
          In reference to a datum: the fact that its value cannot be
          broken down into smaller components.

          In reference to a database transaction: see atomicity.

   Atomicity
          The property of a transaction that either all its operations
          complete as a single unit or none do. In addition, if a system
          failure occurs during the execution of a transaction, no partial
          results are visible after recovery. This is one of the ACID
          properties.

   Attribute
          An element with a certain name and data type found within a
          tuple.

   Autovacuum (process)
          A set of background processes that routinely perform vacuum and
          analyze operations. The auxiliary process that coordinates the
          work and is always present (unless autovacuum is disabled) is
          known as the autovacuum launcher, and the processes that carry
          out the tasks are known as the autovacuum workers.

          For more information, see Section 24.1.6.

   Auxiliary process
          A process within an instance that is in charge of some specific
          background task for the instance. The auxiliary processes
          consist of the autovacuum launcher (but not the autovacuum
          workers), the background writer, the checkpointer, the logger,
          the startup process, the WAL archiver, the WAL receiver (but not
          the WAL senders), the WAL summarizer, and the WAL writer.

   Backend (process)
          Process of an instance which acts on behalf of a client session
          and handles its requests.

          (Don't confuse this term with the similar terms Background
          Worker or Background Writer).

   Background worker (process)
          Process within an instance, which runs system- or user-supplied
          code. Serves as infrastructure for several features in
          PostgreSQL, such as logical replication and parallel queries. In
          addition, Extensions can add custom background worker processes.

          For more information, see Chapter 46.

   Background writer (process)
          An auxiliary process that writes dirty data pages from shared
          memory to the file system. It wakes up periodically, but works
          only for a short period in order to distribute its expensive I/O
          activity over time to avoid generating larger I/O peaks which
          could block other processes.

          For more information, see Section 19.4.4.

   Base Backup
          A binary copy of all database cluster files. It is generated by
          the tool pg_basebackup. In combination with WAL files it can be
          used as the starting point for recovery, log shipping, or
          streaming replication.

   Bloat
          Space in data pages which does not contain current row versions,
          such as unused (free) space or outdated row versions.

   Bootstrap superuser
          The first user initialized in a database cluster.

          This user owns all system catalog tables in each database. It is
          also the role from which all granted permissions originate.
          Because of these things, this role may not be dropped.

          This role also behaves as a normal database superuser, and its
          superuser status cannot be removed.

   Buffer Access Strategy
          Some operations will access a large number of pages. A Buffer
          Access Strategy helps to prevent these operations from evicting
          too many pages from shared buffers.

          A Buffer Access Strategy sets up references to a limited number
          of shared buffers and reuses them circularly. When the operation
          requires a new page, a victim buffer is chosen from the buffers
          in the strategy ring, which may require flushing the page's
          dirty data and possibly also unflushed WAL to permanent storage.

          Buffer Access Strategies are used for various operations such as
          sequential scans of large tables, VACUUM, COPY, CREATE TABLE AS
          SELECT, ALTER TABLE, CREATE DATABASE, CREATE INDEX, and CLUSTER.

   Cast
          A conversion of a datum from its current data type to another
          data type.

          For more information, see CREATE CAST.

   Catalog
          The SQL standard uses this term to indicate what is called a
          database in PostgreSQL's terminology.

          (Don't confuse this term with system catalog).

          For more information, see Section 22.1.

   Check constraint
          A type of constraint defined on a relation which restricts the
          values allowed in one or more attributes. The check constraint
          can make reference to any attribute of the same row in the
          relation, but cannot reference other rows of the same relation
          or other relations.

          For more information, see Section 5.5.

   Checkpoint
          A point in the WAL sequence at which it is guaranteed that the
          heap and index data files have been updated with all information
          from shared memory modified before that checkpoint; a checkpoint
          record is written and flushed to WAL to mark that point.

          A checkpoint is also the act of carrying out all the actions
          that are necessary to reach a checkpoint as defined above. This
          process is initiated when predefined conditions are met, such as
          a specified amount of time has passed, or a certain volume of
          records has been written; or it can be invoked by the user with
          the command CHECKPOINT.

          For more information, see Section 28.5.

   Checkpointer (process)
          An auxiliary process that is responsible for executing
          checkpoints.

   Class (archaic)
          See Relation.

   Client (process)
          Any process, possibly remote, that establishes a session by
          connecting to an instance to interact with a database.

   Cluster owner
          The operating system user that owns the data directory and under
          which the postgres process is run. It is required that this user
          exist prior to creating a new database cluster.

          On operating systems with a root user, said user is not allowed
          to be the cluster owner.

   Column
          An attribute found in a table or view.

   Commit
          The act of finalizing a transaction within the database, which
          makes it visible to other transactions and assures its
          durability.

          For more information, see COMMIT.

   Concurrency
          The concept that multiple independent operations happen within
          the database at the same time. In PostgreSQL, concurrency is
          controlled by the multiversion concurrency control mechanism.

   Connection
          An established line of communication between a client process
          and a backend process, usually over a network, supporting a
          session. This term is sometimes used as a synonym for session.

          For more information, see Section 19.3.

   Consistency
          The property that the data in the database is always in
          compliance with integrity constraints. Transactions may be
          allowed to violate some of the constraints transiently before it
          commits, but if such violations are not resolved by the time it
          commits, such a transaction is automatically rolled back. This
          is one of the ACID properties.

   Constraint
          A restriction on the values of data allowed within a table, or
          in attributes of a domain.

          For more information, see Section 5.5.

   Cumulative Statistics System
          A system which, if enabled, accumulates statistical information
          about the instance's activities.

          For more information, see Section 27.2.

   Data area
          See Data directory.

   Database
          A named collection of local SQL objects.

          For more information, see Section 22.1.

   Database cluster
          A collection of databases and global SQL objects, and their
          common static and dynamic metadata. Sometimes referred to as a
          cluster. A database cluster is created using the initdb program.

          In PostgreSQL, the term cluster is also sometimes used to refer
          to an instance. (Don't confuse this term with the SQL command
          CLUSTER.)

          See also cluster owner, the operating-system owner of a cluster,
          and bootstrap superuser, the PostgreSQL owner of a cluster.

   Database server
          See Instance.

   Database superuser
          A role having superuser status (see Section 21.2).

          Frequently referred to as superuser.

   Data directory
          The base directory on the file system of a server that contains
          all data files and subdirectories associated with a database
          cluster (with the exception of tablespaces, and optionally WAL).
          The environment variable PGDATA is commonly used to refer to the
          data directory.

          A cluster's storage space comprises the data directory plus any
          additional tablespaces.

          For more information, see Section 66.1.

   Data page
          The basic structure used to store relation data. All pages are
          of the same size. Data pages are typically stored on disk, each
          in a specific file, and can be read to shared buffers where they
          can be modified, becoming dirty. They become clean when written
          to disk. New pages, which initially exist in memory only, are
          also dirty until written.

   Datum
          The internal representation of one value of an SQL data type.

   Delete
          An SQL command which removes rows from a given table or
          relation.

          For more information, see DELETE.

   Domain
          A user-defined data type that is based on another underlying
          data type. It acts the same as the underlying type except for
          possibly restricting the set of allowed values.

          For more information, see Section 8.18.

   Durability
          The assurance that once a transaction has been committed, the
          changes remain even after a system failure or crash. This is one
          of the ACID properties.

   Epoch
          See Transaction ID.

   Extension
          A software add-on package that can be installed on an instance
          to get extra features.

          For more information, see Section 36.17.

   File segment
          A physical file which stores data for a given relation. File
          segments are limited in size by a configuration value (typically
          1 gigabyte), so if a relation exceeds that size, it is split
          into multiple segments.

          For more information, see Section 66.1.

          (Don't confuse this term with the similar term WAL segment).

   Foreign data wrapper
          A means of representing data that is not contained in the local
          database so that it appears as if were in local table(s). With a
          foreign data wrapper it is possible to define a foreign server
          and foreign tables.

          For more information, see CREATE FOREIGN DATA WRAPPER.

   Foreign key
          A type of constraint defined on one or more columns in a table
          which requires the value(s) in those columns to identify zero or
          one row in another (or, infrequently, the same) table.

   Foreign server
          A named collection of foreign tables which all use the same
          foreign data wrapper and have other configuration values in
          common.

          For more information, see CREATE SERVER.

   Foreign table (relation)
          A relation which appears to have rows and columns similar to a
          regular table, but will forward requests for data through its
          foreign data wrapper, which will return result sets structured
          according to the definition of the foreign table.

          For more information, see CREATE FOREIGN TABLE.

   Fork
          Each of the separate segmented file sets in which a relation is
          stored. The main fork is where the actual data resides. There
          also exist two secondary forks for metadata: the free space map
          and the visibility map. Unlogged relations also have an init
          fork.

   Free space map (fork)
          A storage structure that keeps metadata about each data page of
          a table's main fork. The free space map entry for each page
          stores the amount of free space that's available for future
          tuples, and is structured to be efficiently searched for
          available space for a new tuple of a given size.

          For more information, see Section 66.3.

   Function (routine)
          A type of routine that receives zero or more arguments, returns
          zero or more output values, and is constrained to run within one
          transaction. Functions are invoked as part of a query, for
          example via SELECT. Certain functions can return sets; those are
          called set-returning functions.

          Functions can also be used for triggers to invoke.

          For more information, see CREATE FUNCTION.

   GMT
          See UTC.

   Grant
          An SQL command that is used to allow a user or role to access
          specific objects within the database.

          For more information, see GRANT.

   Heap
          Contains the values of row attributes (i.e., the data) for a
          relation. The heap is realized within one or more file segments
          in the relation's main fork.

   Host
          A computer that communicates with other computers over a
          network. This is sometimes used as a synonym for server. It is
          also used to refer to a computer where client processes run.

   Index (relation)
          A relation that contains data derived from a table or
          materialized view. Its internal structure supports fast
          retrieval of and access to the original data.

          For more information, see CREATE INDEX.

   Incremental backup
          A special base backup that for some files may contain only those
          pages that were modified since a previous backup, as opposed to
          the full contents of every file. Like base backups, it is
          generated by the tool pg_basebackup.

          To restore incremental backups the tool pg_combinebackup is
          used, which combines incremental backups with a base backup.
          Afterwards, recovery can use WAL to bring the database cluster
          to a consistent state.

          For more information, see Section 25.3.3.

   Input/Output (I/O)
          Input/Output (I/O) describes the communication between a program
          and peripheral devices. In the context of database systems, I/O
          commonly, but not exclusively, refers to interaction with
          storage devices or the network.

          See Also Asynchronous I/O.

   Insert
          An SQL command used to add new data into a table.

          For more information, see INSERT.

   Instance
          A group of backend and auxiliary processes that communicate
          using a common shared memory area. One postmaster process
          manages the instance; one instance manages exactly one database
          cluster with all its databases. Many instances can run on the
          same server as long as their TCP ports do not conflict.

          The instance handles all key features of a DBMS: read and write
          access to files and shared memory, assurance of the ACID
          properties, connections to client processes, privilege
          verification, crash recovery, replication, etc.

   Isolation
          The property that the effects of a transaction are not visible
          to concurrent transactions before it commits. This is one of the
          ACID properties.

          For more information, see Section 13.2.

   Join
          An operation and SQL keyword used in queries for combining data
          from multiple relations.

   Key
          A means of identifying a row within a table or other relation by
          values contained within one or more attributes in that relation.

   Lock
          A mechanism that allows a process to limit or prevent
          simultaneous access to a resource.

   Log file
          Log files contain human-readable text lines about events.
          Examples include login failures, long-running queries, etc.

          For more information, see Section 24.3.

   Logged
          A table is considered logged if changes to it are sent to the
          WAL. By default, all regular tables are logged. A table can be
          specified as unlogged either at creation time or via the ALTER
          TABLE command.

   Logger (process)
          An auxiliary process which, if enabled, writes information about
          database events into the current log file. When reaching certain
          time- or volume-dependent criteria, a new log file is created.
          Also called syslogger.

          For more information, see Section 19.8.

   Logical replication cluster
          A set of publisher and subscriber instances with the publisher
          instance replicating changes to the subscriber instance.

   Log record
          Archaic term for a WAL record.

   Log sequence number (LSN)
          Byte offset into the WAL, increasing monotonically with each new
          WAL record.

          For more information, see pg_lsn and Section 28.6.

   LSN
          See Log sequence number.

   Master (server)
          See Primary (server).

   Materialized
          The property that some information has been pre-computed and
          stored for later use, rather than computing it on-the-fly.

          This term is used in materialized view, to mean that the data
          derived from the view's query is stored on disk separately from
          the sources of that data.

          This term is also used to refer to some multi-step queries to
          mean that the data resulting from executing a given step is
          stored in memory (with the possibility of spilling to disk), so
          that it can be read multiple times by another step.

   Materialized view (relation)
          A relation that is defined by a SELECT statement (just like a
          view), but stores data in the same way that a table does. It
          cannot be modified via INSERT, UPDATE, DELETE, or MERGE
          operations.

          For more information, see CREATE MATERIALIZED VIEW.

   Merge
          An SQL command used to conditionally add, modify, or remove rows
          in a given table, using data from a source relation.

          For more information, see MERGE.

   Multi-version concurrency control (MVCC)
          A mechanism designed to allow several transactions to be reading
          and writing the same rows without one process causing other
          processes to stall. In PostgreSQL, MVCC is implemented by
          creating copies (versions) of tuples as they are modified; after
          transactions that can see the old versions terminate, those old
          versions need to be removed.

   Null
          A concept of non-existence that is a central tenet of relational
          database theory. It represents the absence of a definite value.

   Optimizer
          See Query planner.

   Parallel query
          The ability to handle parts of executing a query to take
          advantage of parallel processes on servers with multiple CPUs.

   Partition
          One of several disjoint (not overlapping) subsets of a larger
          set.

          In reference to a partitioned table: One of the tables that each
          contain part of the data of the partitioned table, which is said
          to be the parent. The partition is itself a table, so it can
          also be queried directly; at the same time, a partition can
          sometimes be a partitioned table, allowing hierarchies to be
          created.

          In reference to a window function in a query, a partition is a
          user-defined criterion that identifies which neighboring rows of
          the query's result set can be considered by the function.

   Partitioned table (relation)
          A relation that is in semantic terms the same as a table, but
          whose storage is distributed across several partitions.

   Postmaster (process)
          The very first process of an instance. It starts and manages the
          auxiliary processes and creates backend processes on demand.

          For more information, see Section 18.3.

   Primary key
          A special case of a unique constraint defined on a table or
          other relation that also guarantees that all of the attributes
          within the primary key do not have null values. As the name
          implies, there can be only one primary key per table, though it
          is possible to have multiple unique constraints that also have
          no null-capable attributes.

   Primary (server)
          When two or more databases are linked via replication, the
          server that is considered the authoritative source of
          information is called the primary, also known as a master.

   Procedure (routine)
          A type of routine. Their distinctive qualities are that they do
          not return values, and that they are allowed to make
          transactional statements such as COMMIT and ROLLBACK. They are
          invoked via the CALL command.

          For more information, see CREATE PROCEDURE.

   Query
          A request sent by a client to a backend, usually to return
          results or to modify data on the database.

   Query planner
          The part of PostgreSQL that is devoted to determining (planning)
          the most efficient way to execute queries. Also known as query
          optimizer, optimizer, or simply planner.

   Record
          See Tuple.

   Recycling
          See WAL file.

   Referential integrity
          A means of restricting data in one relation by a foreign key so
          that it must have matching data in another relation.

   Relation
          The generic term for all objects in a database that have a name
          and a list of attributes defined in a specific order. Tables,
          sequences, views, foreign tables, materialized views, composite
          types, and indexes are all relations.

          More generically, a relation is a set of tuples; for example,
          the result of a query is also a relation.

          In PostgreSQL, Class is an archaic synonym for relation.

   Replica (server)
          A database that is paired with a primary database and is
          maintaining a copy of some or all of the primary database's
          data. The foremost reasons for doing this are to allow for
          greater access to that data, and to maintain availability of the
          data in the event that the primary becomes unavailable.

   Replication
          The act of reproducing data on one server onto another server
          called a replica. This can take the form of physical
          replication, where all file changes from one server are copied
          verbatim, or logical replication where a defined subset of data
          changes are conveyed using a higher-level representation.

   Restartpoint
          A variant of a checkpoint performed on a replica.

          For more information, see Section 28.5.

   Result set
          A relation transmitted from a backend process to a client upon
          the completion of an SQL command, usually a SELECT but it can be
          an INSERT, UPDATE, DELETE, or MERGE command if the RETURNING
          clause is specified.

          The fact that a result set is a relation means that a query can
          be used in the definition of another query, becoming a subquery.

   Revoke
          A command to prevent access to a named set of database objects
          for a named list of roles.

          For more information, see REVOKE.

   Role
          A collection of access privileges to the instance. Roles are
          themselves a privilege that can be granted to other roles. This
          is often done for convenience or to ensure completeness when
          multiple users need the same privileges.

          For more information, see CREATE ROLE.

   Rollback
          A command to undo all of the operations performed since the
          beginning of a transaction.

          For more information, see ROLLBACK.

   Routine
          A defined set of instructions stored in the database system that
          can be invoked for execution. A routine can be written in a
          variety of programming languages. Routines can be functions
          (including set-returning functions and trigger functions),
          aggregate functions, and procedures.

          Many routines are already defined within PostgreSQL itself, but
          user-defined ones can also be added.

   Row
          See Tuple.

   Savepoint
          A special mark in the sequence of steps in a transaction. Data
          modifications after this point in time may be reverted to the
          time of the savepoint.

          For more information, see SAVEPOINT.

   Schema
          A schema is a namespace for SQL objects, which all reside in the
          same database. Each SQL object must reside in exactly one
          schema.

          All system-defined SQL objects reside in schema pg_catalog.

          More generically, the term schema is used to mean all data
          descriptions (table definitions, constraints, comments, etc.)
          for a given database or subset thereof.

          For more information, see Section 5.10.

   Segment
          See File segment.

   Select
          The SQL command used to request data from a database. Normally,
          SELECT commands are not expected to modify the database in any
          way, but it is possible that functions invoked within the query
          could have side effects that do modify data.

          For more information, see SELECT.

   Sequence (relation)
          A type of relation that is used to generate values. Typically
          the generated values are sequential non-repeating numbers. They
          are commonly used to generate surrogate primary key values.

   Server
          A computer on which PostgreSQL instances run. The term server
          denotes real hardware, a container, or a virtual machine.

          This term is sometimes used to refer to an instance or to a
          host.

   Session
          A state that allows a client and a backend to interact,
          communicating over a connection.

   Shared memory
          RAM which is used by the processes common to an instance. It
          mirrors parts of database files, provides a transient area for
          WAL records, and stores additional common information. Note that
          shared memory belongs to the complete instance, not to a single
          database.

          The largest part of shared memory is known as shared buffers and
          is used to mirror part of data files, organized into pages. When
          a page is modified, it is called a dirty page until it is
          written back to the file system.

          For more information, see Section 19.4.1.

   SQL object
          Any object that can be created with a CREATE command. Most
          objects are specific to one database, and are commonly known as
          local objects.

          Most local objects reside in a specific schema in their
          containing database, such as relations (all types), routines
          (all types), data types, etc. The names of such objects of the
          same type in the same schema are enforced to be unique.

          There also exist local objects that do not reside in schemas;
          some examples are extensions, data type casts, and foreign data
          wrappers. The names of such objects of the same type are
          enforced to be unique within the database.

          Other object types, such as roles, tablespaces, replication
          origins, subscriptions for logical replication, and databases
          themselves are not local SQL objects since they exist entirely
          outside of any specific database; they are called global
          objects. The names of such objects are enforced to be unique
          within the whole database cluster.

          For more information, see Section 22.1.

   SQL standard
          A series of documents that define the SQL language.

   Standby (server)
          See Replica (server).

   Startup process
          An auxiliary process that replays WAL during crash recovery and
          in a physical replica.

          (The name is historical: the startup process was named before
          replication was implemented; the name refers to its task as it
          relates to the server startup following a crash.)

   Superuser
          As used in this documentation, it is a synonym for database
          superuser.

   System catalog
          A collection of tables which describe the structure of all SQL
          objects of the instance. The system catalog resides in the
          schema pg_catalog. These tables contain data in internal
          representation and are not typically considered useful for user
          examination; a number of user-friendlier views, also in schema
          pg_catalog, offer more convenient access to some of that
          information, while additional tables and views exist in schema
          information_schema (see Chapter 35) that expose some of the same
          and additional information as mandated by the SQL standard.

          For more information, see Section 5.10.

   Table
          A collection of tuples having a common data structure (the same
          number of attributes, in the same order, having the same name
          and type per position). A table is the most common form of
          relation in PostgreSQL.

          For more information, see CREATE TABLE.

   Tablespace
          A named location on the server file system. All SQL objects
          which require storage beyond their definition in the system
          catalog must belong to a single tablespace. Initially, a
          database cluster contains a single usable tablespace which is
          used as the default for all SQL objects, called pg_default.

          For more information, see Section 22.6.

   Temporary table
          Tables that exist either for the lifetime of a session or a
          transaction, as specified at the time of creation. The data in
          them is not visible to other sessions, and is not logged.
          Temporary tables are often used to store intermediate data for a
          multi-step operation.

          For more information, see CREATE TABLE.

   TOAST
          A mechanism by which large attributes of table rows are split
          and stored in a secondary table, called the TOAST table. Each
          relation with large attributes has its own TOAST table.

          For more information, see Section 66.2.

   Transaction
          A combination of commands that must act as a single atomic
          command: they all succeed or all fail as a single unit, and
          their effects are not visible to other sessions until the
          transaction is complete, and possibly even later, depending on
          the isolation level.

          For more information, see Section 13.2.

   Transaction ID
          The numerical, unique, sequentially-assigned identifier that
          each transaction receives when it first causes a database
          modification. Frequently abbreviated as xid. When stored on
          disk, xids are only 32-bits wide, so only approximately four
          billion write transaction IDs can be generated; to permit the
          system to run for longer than that, epochs are used, also 32
          bits wide. When the counter reaches the maximum xid value, it
          starts over at 3 (values under that are reserved) and the epoch
          value is incremented by one. In some contexts, the epoch and xid
          values are considered together as a single 64-bit value; see
          Section 67.1 for more details.

          For more information, see Section 8.19.

   Transactions per second (TPS)
          Average number of transactions that are executed per second,
          totaled across all sessions active for a measured run. This is
          used as a measure of the performance characteristics of an
          instance.

   Trigger
          A function which can be defined to execute whenever a certain
          operation (INSERT, UPDATE, DELETE, TRUNCATE) is applied to a
          relation. A trigger executes within the same transaction as the
          statement which invoked it, and if the function fails, then the
          invoking statement also fails.

          For more information, see CREATE TRIGGER.

   Tuple
          A collection of attributes in a fixed order. That order may be
          defined by the table (or other relation) where the tuple is
          contained, in which case the tuple is often called a row. It may
          also be defined by the structure of a result set, in which case
          it is sometimes called a record.

   Unique constraint
          A type of constraint defined on a relation which restricts the
          values allowed in one or a combination of columns so that each
          value or combination of values can only appear once in the
          relation — that is, no other row in the relation contains values
          that are equal to those.

          Because null values are not considered equal to each other,
          multiple rows with null values are allowed to exist without
          violating the unique constraint.

   Unlogged
          The property of certain relations that the changes to them are
          not reflected in the WAL. This disables replication and crash
          recovery for these relations.

          The primary use of unlogged tables is for storing transient work
          data that must be shared across processes.

          Temporary tables are always unlogged.

   Update
          An SQL command used to modify rows that may already exist in a
          specified table. It cannot create or remove rows.

          For more information, see UPDATE.

   User
          A role that has the login privilege (see Section 21.2).

   User mapping
          The translation of login credentials in the local database to
          credentials in a remote data system defined by a foreign data
          wrapper.

          For more information, see CREATE USER MAPPING.

   UTC
          Universal Coordinated Time, the primary global time reference,
          approximately the time prevailing at the zero meridian of
          longitude. Often but inaccurately referred to as GMT (Greenwich
          Mean Time).

   Vacuum
          The process of removing outdated tuple versions from tables or
          materialized views, and other closely related processing
          required by PostgreSQL's implementation of MVCC. This can be
          initiated through the use of the VACUUM command, but can also be
          handled automatically via autovacuum processes.

          For more information, see Section 24.1 .

   View
          A relation that is defined by a SELECT statement, but has no
          storage of its own. Any time a query references a view, the
          definition of the view is substituted into the query as if the
          user had typed it as a subquery instead of the name of the view.

          For more information, see CREATE VIEW.

   Visibility map (fork)
          A storage structure that keeps metadata about each data page of
          a table's main fork. The visibility map entry for each page
          stores two bits: the first one (all-visible) indicates that all
          tuples in the page are visible to all transactions. The second
          one (all-frozen) indicates that all tuples in the page are
          marked frozen.

   WAL
          See Write-ahead log.

   WAL archiver (process)
          An auxiliary process which, if enabled, saves copies of WAL
          files for the purpose of creating backups or keeping replicas
          current.

          For more information, see Section 25.3.

   WAL file
          Also known as WAL segment or WAL segment file. Each of the
          sequentially-numbered files that provide storage space for WAL.
          The files are all of the same predefined size and are written in
          sequential order, interspersing changes as they occur in
          multiple simultaneous sessions. If the system crashes, the files
          are read in order, and each of the changes is replayed to
          restore the system to the state it was in before the crash.

          Each WAL file can be released after a checkpoint writes all the
          changes in it to the corresponding data files. Releasing the
          file can be done either by deleting it, or by changing its name
          so that it will be used in the future, which is called
          recycling.

          For more information, see Section 28.6.

   WAL record
          A low-level description of an individual data change. It
          contains sufficient information for the data change to be
          re-executed (replayed) in case a system failure causes the
          change to be lost. WAL records use a non-printable binary
          format.

          For more information, see Section 28.6.

   WAL receiver (process)
          An auxiliary process that runs on a replica to receive WAL from
          the primary server for replay by the startup process.

          For more information, see Section 26.2.

   WAL segment
          See WAL file.

   WAL sender (process)
          A special backend process that streams WAL over a network. The
          receiving end can be a WAL receiver in a replica, pg_receivewal,
          or any other client program that speaks the replication
          protocol.

   WAL summarizer (process)
          An auxiliary process that summarizes WAL data for incremental
          backups.

          For more information, see Section 19.5.7.

   WAL writer (process)
          An auxiliary process that writes WAL records from shared memory
          to WAL files.

          For more information, see Section 19.5.

   Window function (routine)
          A type of function used in a query that applies to a partition
          of the query's result set; the function's result is based on
          values found in rows of the same partition or frame.

          All aggregate functions can be used as window functions, but
          window functions can also be used to, for example, give ranks to
          each of the rows in the partition. Also known as analytic
          functions.

          For more information, see Section 3.5.

   Write-ahead log
          The journal that keeps track of the changes in the database
          cluster as user- and system-invoked operations take place. It
          comprises many individual WAL records written sequentially to
          WAL files.