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Schema

Updated at:Mar 13, 2020 GMT+08:00

Schemas allow multiple users to use the same database without interference. In this way, database objects can be organized into logical groups that are easy to manage, and third-party applications can be added to corresponding schemas without causing conflicts.

Each database has one or more schemas. Each schema contains tables and other types of objects. When a database is created, a schema named public is created by default, and all users have permissions for this schema. You can group database objects by schema. A schema is similar to an OS directory but cannot be nested.

The same database object name can be used in different schemas of the same database without causing conflicts. For example, both a_schema and b_schema can contain a table named mytable. Users with required permissions can access objects across multiple schemas of the same database.

When a user is created in the initial database postgres, the system automatically creates a schema with the same name as the user. In other databases, such a schema needs to be manually created.

Database objects are generally created in the first schema in a database search path. For details about the first schema and how to change the schema order, see Search Path.

Creating, Modifying, and Deleting Schemas

  • To create a schema, use CREATE SCHEMA. Any user can create a schema.
  • To change the name or owner of a schema, use ALTER SCHEMA. Only the schema owner can do so.
  • To delete a schema and its objects, use DROP SCHEMA. Only the schema owner can do so.
  • To create a table in a schema, use the schema_name.table_name format to specify the table. If schema_name is not specified, the table will be created in the first schema in search path.
  • To view the owner of a schema, perform the following join query on the system catalogs PG_NAMESPACE and PG_USER. Replace schema_name in the statement with the name of the schema to be queried.
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    SELECT s.nspname,u.usename AS nspowner FROM pg_namespace s, pg_user u WHERE nspname='schema_name' AND s.nspowner = u.usesysid;
    
  • To view a list of all schemas, query the system catalog PG_NAMESPACE.
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    SELECT * FROM pg_namespace;
    
  • To view a list of tables in a schema, query the system catalog PG_TABLES. For example, the following query will return a table list from PG_CATALOG in the schema.
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    SELECT distinct(tablename),schemaname from pg_tables where schemaname = 'pg_catalog';
    

Search Path

A search path is defined in the search_path parameter. The parameter value is a list of schema names separated by commas (,). If no target schema is specified during object creation, the object will be added to the first schema listed in the search path. If there are objects with the same name across different schemas and no schema is specified for an object query, the object will be returned from the first schema containing the object in the search path.

  • To view the current search path, use SHOW.
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    SHOW SEARCH_PATH;
     search_path
    ----------------
     "$user",public
    (1 row)
    

    The default value of search_path is "$user",public. $user indicates the name of the schema with the same name as the current session user. If the schema does not exist, $user will be ignored. By default, after a user connects to a database that has schemas with the same name, objects will be added to all the schemas. If there are no such schemas, objects will be added to only to the public schema.

  • To change the default schema of the current session, run the SET statement.
    Run the following command to set search_path to myschema and public (myschema will be searched first):
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    SET SEARCH_PATH TO myschema, public;
    SET
    

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