Appendix M. Glossary

Appendix M. Glossary
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+ 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).

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. +

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 25.1.6. +

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), + 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 48. +

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 20.4.5. +

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. +

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 23.1. +

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 5.4. +

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 30.5. +

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. +

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 20.3. +

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 5.4. +

Cumulative Statistics System

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

+ For more information, see + Section 28.2. +

Data area

See Data directory.

Database

+ A named collection of + local SQL objects. +

+ For more information, see + Section 23.1. +

Database cluster

+ A collection of databases and global SQL objects, + and their common static and dynamic metadata. + Sometimes referred to as a + cluster. +

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

Database server

See Instance.

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 73.1. +

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 38.17. +

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 73.1. +

+ (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 73.3. +

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. +

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. +

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 25.3. +

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 20.8. +

Log record

+ Archaic term for a WAL record. +

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, or + DELETE operations. +

+ For more information, see + CREATE MATERIALIZED VIEW. +

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 19.3. +

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. +

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, or + DELETE 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 5.9. +

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 20.4.1. +

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 23.1. +

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.) +

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 37) that expose some + of the same and additional information as mandated by the + SQL standard. +

+ For more information, see + Section 5.9. +

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 23.6. +

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 73.2. +

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. +

+ 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. +

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. +

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 25.1 . +

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 26.3. +

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 30.6. +

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 30.6. +

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 27.2. +

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 writer (process)

+ A process that writes WAL records + from shared memory to + WAL files. +

+ For more information, see + Section 20.5. +

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. +