PostgreSQL ORDBMS

Server Hierarchy

Managing Databases create database name; createdb name At the time of initialisation of Data Storage area by initdb command following database are created postgres template1 template0 datistemplate Datisallowconn Select * from pg_database drop database name; dropdb name

tablespaces Locations in the file system for database objects –Initial volume (where the database clustered was initially put) may run out of space –Performance (heavily used index may be placed on fast disk) create tablespace fastspace location ‘/mnt/sda1/postgresql/data’ (location must be an empty directory owned by postgres) Grant Create Privilege to others for creating objects(tables, indexes or even a database) set default_tablespace = fastspace;

tablespaces tablespaces associated with a database store –System catalogs –Temporary files created by the server –Tables and indexes create without tablespace clause initdb command creates following tablespaces –pg_global (used for shared system catalogs) –pg_default (default tablespace for template1 and template0 and hence the default tablespace for other databases also, unless overridden by tablespace clause in create database command drop tablespace name; (from an empty tablespace)

Schema a database contains, one or more named schemas, which in turn may contain, one or more named database objects like tables, functions, types etc. –server.database.schema.table –To allow many users to connect without interfering with each other –To organize database objects into logical groups to make them more meaningful –Third party applications may be put in different schemas so that they do not collide with the names of other objects. –Schemas are analogous to directories at OS level, except Schemas can not be nested.

Schemas contd… create schema name; create table schemaname.tablename (..) drop schema name; (for an empty schema) drop schema name cascade; create schema schemaname authorization username;

Public Schema Every new database contains public schema Tables created without specifying the schema name are put into public schema

Schema Search Path unqualified object names (e.g. tablename only) are determined by following a search path, which is a list of schemas to look in The first matching table in the search path is taken to be the one which is wanted. If no match found in the search path an error is raised, even if the matching name exists in some other schema The first schema named in the search path is called the current schema, where new tables will be created (if schema name is not specified in the create table statement) show search_path will show “$user”,public

Schemas and Privileges By default users can not access objects in schemas. To allow owner can give usage privilege on the schema. To allow users to make use of the objects in the schema, additional privilege may need to be granted, as appropriate for the object create privilege is required for creating objects in that schema. By default every user has create and usage privileges on public schema.

System Catalog Schema In addition to public and user-created schemas, each database contains a pg_catalog schema, which contains the system tables and all the built-in data types, functions, and operators. pg_catalog is always effectively part of the search path. This ensures all the built-in names will be found.

Usage patterns If no schemas exists then all the users will be working in public schema implicitly. This setup is mainly recommended when there is only a single user or a few cooperating users in a database. Create schema for each user with the same name as that user. Default search path starts with $user, which resolves to the user name. Therefore, if each user has a separate schema, they access their own schemas by default. For this setup, revoke access to the public schema (or drop it altogether), so users are truly constrained to their own schemas.

Usage patterns contd… To install shared applications (tables to be used by everyone, additional functions provided by third parties, etc.), put them into separate schemas. Remember to grant appropriate privileges to allow the other users to access them. Users can then refer to these additional objects by qualifying the names with a schema name, or they can put the additional schemas into their search path, as they choose.

PostgreSQL is extensible PostgreSQL is extensible because its operation is catalog-driven One key difference between PostgreSQL and standard relational database systems is that PostgreSQL stores much more information in its catalogs: not only information about tables and columns, but also information about data types, functions, access methods These tables can be modified by the user, and since PostgreSQL bases its operation on these tables, this means that PostgreSQL can be extended by users.

PostgreSQL is extensible By comparison, conventional database systems can only be extended by changing hardcoded procedures in the source code or by loading modules specially written by the DBMS vendor. The PostgreSQL server can moreover incorporate user-written code into itself through dynamic loading. That is, the user can specify an object code file (e.g., a shared library) that implements a new type or function, and PostgreSQL will load it as required. This ability to modify its operation “on the fly” makes PostgreSQL uniquely suited for rapid prototyping of new applications and storage structures.

Numeric DataTypes

Character Data Types

Date/Time Data Types

Geometric Data Types

Network Address Data Types

Complex Types A composite type describes the structure of a row or record; it is in essence just a list of field names and their data types. PostgreSQL allows values of composite types to be used in many of the same ways that simple types can be used. For example, a column of a table can be declared to be of a composite type.

Complex Types create type inventory_item as ( name text, supplier_id integer, price numeric); create table on_hand (item inventory_item, count int); insert into on_hand values ( row('fuzzy dice',42,1.92),10000); create function price_extension(inventory_item, integer) returns numeric as 'select $1.price * $2' language 'SQL'; select * from on_hand; select price_extension(item, 10) from on_hand;

Inheritance Inheritance allows a table to inherit some of its column attributes from one or more other tables, creating a parent-child relationship. This causes the child table to have each of the same columns and constraints as its inherited table (or tables), as well as its own defined columns. create table childtable definition inherits ( parenttable [,...] )

Inheritance booktown=# CREATE TABLE distinguished_authors (award text) booktown-# INHERITS (authors); CREATE booktown=# \d distinguished_authors Table "distinguished_authors" Attribute | Type | Modifier id | integer| not null last_name| text| first_name| text | award |text |

Inheritance booktown=# INSERT INTO distinguished_authors booktown-# VALUES (nextval('author_ids'), booktown(# 'Simon', 'Neil', 'Pulitzer Prize'); INSERT booktown=# SELECT * FROM distinguished_authors booktown-# WHERE last_name = 'Simon'; id | last_name | first_name | award | Simon | Neil | Pulitzer Prize (1 row) booktown=# SELECT * FROM authors WHERE last_name = 'Simon'; id | last_name | first_name | Simon | Neil(1 row) booktown=# SELECT * FROM ONLY authors WHERE last_name = 'Simon'; id | last_name | first_name (0 rows)

Arrays PostgreSQL supports non-atomic values in individual table columns through data constructs called arrays. An array itself is not a data type, but an extension of any PostgreSQL data type.

Arrays create table favorite_books (employee_id integer, books text[]); insert into favorite_books values (102, '{"The Hitchhiker\'s Guide to the Galaxy"}'); insert into favorite_books values (103, '{"The Hobbit", "Kitten, Squared"}'); select books from favorite_books; books {"The Hitchhiker's Guide to the Galaxy"} {"The Hobbit","Kitten, Squared"} (2 rows) select books[1] from favorite_books; books The Hitchhiker's Guide to the Galaxy The Hobbit (2 rows) select books[1:2] from favorite_books;

Arrays create table favorite_authors (employee_id integer, authors_and_titles text[][]); insert into favorite_authors values (102, '{{"Sidney Sheildon","Otherside of Midnight"}, {"Charles Dickens","Great Expectations" }, {"Prem Chand","Gaban"}}'); select authors_and_titles[1][1] AS author, authors_and_titles[1][2] as title from favorite_authors; author | title J.R.R. Tolkien | The Silmarillion (1 row)

Function Overloading More than one function may be defined with the same SQL name, so long as the arguments they take are different. In other words, function names can be overloaded. When a query is executed, the server will determine which function to call from the data types and the number of the provided arguments.

Operator Overloading Operators may become overloaded in much the same way as functions. This means that an operator is created with the same name as an existing operator, but affects a different set of defined types. More than one operator may have the same name, although two operators may not share the same name if they accept the same argument definitions. As long as a function exists to accept the number and type of arguments implied by the type of operator defined, though, the operator may be overloaded.

Access to PostgreSQL PGAdmin III – GUI based client Psql - command based client