At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log records every change made to the database's data files. This log exists primarily for crash-safety purposes: if the system crashes, the database can be restored to consistency by "replaying" the log entries made since the last checkpoint. However, the existence of the log makes it possible to use a third strategy for backing up databases: we can combine a file-system-

To retrieve data from a table, the table is queried. An SQL SELECT statement is used to do this. The statement is divided into a select list (the part that lists the columns to be returned), a table list (the part that lists the tables from which to retrieve the data), and an optional qualification (the part that specifies any restrictions). For example, to retrieve all the rows of table weather, type: SELECT * FROM weather; Here * is a shorthand for "all columns". [1] So the same result woul

NameCREATE DOMAIN -- define a new domain Synopsis CREATE DOMAIN name [ AS ] data_type [ COLLATE collation ] [ DEFAULT expression ] [ constraint [ ... ] ] where constraint is: [ CONSTRAINT constraint_name ] { NOT NULL | NULL | CHECK (expression) } Description CREATE DOMAIN creates a new domain. A domain is essentially a data type with optional constraints (restrictions on the allowed set of values). The user who defines a domain becomes its owner. If a schema name is give

When using an external authentication system such as Ident or GSSAPI, the name of the operating system user that initiated the connection might not be the same as the database user that is to be connect as. In this case, a user name map can be applied to map the operating system user name to a database user. To use user name mapping, specify map=map-name in the options field in pg_hba.conf. This option is supported for all authentication methods that receive external user names. Since different

The genetic algorithm (GA) is a heuristic optimization method which operates through randomized search. The set of possible solutions for the optimization problem is considered as a population of individuals. The degree of adaptation of an individual to its environment is specified by its fitness. The coordinates of an individual in the search space are represented by chromosomes, in essence a set of character strings. A gene is a subsection of a chromosome which encodes the value of a single p

NameCREATE SEQUENCE -- define a new sequence generator Synopsis CREATE [ TEMPORARY | TEMP ] SEQUENCE [ IF NOT EXISTS ] name [ INCREMENT [ BY ] increment ] [ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ] [ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ] [ OWNED BY { table_name.column_name | NONE } ] Description CREATE SEQUENCE creates a new sequence number generator. This involves creating and initializing a new special single-row table with

The specific function that is referenced by a function call is determined using the following procedure. Function Type Resolution Select the functions to be considered from the pg_proc system catalog. If a non-schema-qualified function name was used, the functions considered are those with the matching name and argument count that are visible in the current search path (see Section 5.8.3). If a qualified function name was given, only functions in the specified schema are considered. If the s

Namepg_recvlogical -- control PostgreSQL logical decoding streams Synopsis pg_recvlogical [option...] Description pg_recvlogical controls logical decoding replication slots and streams data from such replication slots. It creates a replication-mode connection, so it is subject to the same constraints as pg_receivexlog, plus those for logical replication (see Chapter 47). Options At least one of the following options must be specified to select an action: --create-slot Create a new

Each table has a primary heap disk file where most of the data is stored. If the table has any columns with potentially-wide values, there also might be a TOAST file associated with the table, which is used to store values too wide to fit comfortably in the main table (see Section 65.2). There will be one valid index on the TOAST table, if present. There also might be indexes associated with the base table. Each table and index is stored in a separate disk file — possibly more than one file, if

A table in a relational database is much like a table on paper: It consists of rows and columns. The number and order of the columns is fixed, and each column has a name. The number of rows is variable — it reflects how much data is stored at a given moment. SQL does not make any guarantees about the order of the rows in a table. When a table is read, the rows will appear in an unspecified order, unless sorting is explicitly requested. This is covered in Chapter 7. Furthermore, SQL does not ass