Table 6-27 shows several functions that extract session and system information.
Table 6-27. Session Information Functions
| Name | Return Type | Description | 
|---|---|---|
| current_database() | name | name of current database | 
| current_schema() | name | name of current schema | 
| current_schemas(boolean) | name[] | names of schemas in search path optionally including implicit schemas | 
| current_user | name | user name of current execution context | 
| session_user | name | session user name | 
| user | name | equivalent to current_user | 
| version() | text | PostgreSQL version information | 
    The session_user is the user that initiated a
    database connection; it is fixed for the duration of that
    connection. The current_user is the user identifier
    that is applicable for permission checking. Normally, it is equal
    to the session user, but it changes during the execution of
    functions with the attribute SECURITY DEFINER.
    In Unix parlance, the session user is the "real user" and
    the current user is the "effective user".
   
Note:
current_user,session_user, anduserhave special syntactic status in SQL: they must be called without trailing parentheses.
    current_schema returns the name of the schema that is
    at the front of the search path (or a null value if the search path is
    empty).  This is the schema that will be used for any tables or
    other named objects that are created without specifying a target schema.
    current_schemas(boolean) returns an array of the names of all
    schemas presently in the search path.  The boolean option determines whether or not
    implicitly included system schemas such as pg_catalog are included in the search 
    path returned.
   
    
    The search path may be altered by a run-time setting.  The
    command to use is      SET SEARCH_PATH 'schema'[,'schema']...
    
   
    version() returns a string describing the
    PostgreSQL server's version.
   
Table 6-28 shows the functions available to query and alter run-time configuration parameters.
Table 6-28. Configuration Settings Information Functions
| Name | Return Type | Description | 
|---|---|---|
| current_setting(setting_name) | text | value of current setting | 
| set_config( | text | new value of current setting | 
    The current_setting is used to obtain the current
    value of the setting_name setting, as a query
    result. It is the equivalent to the SQL SHOW command.
    For example:
select current_setting('DateStyle');
            current_setting
---------------------------------------
 ISO with US (NonEuropean) conventions
(1 row)
    set_config allows the setting_name
     setting to be changed to new_value.
    If is_local is set to true,
    the new value will only apply to the current transaction. If you want
    the new value to apply for the current session, use
    false instead. It is the equivalent to the SQL
    SET command. For example:
select set_config('show_statement_stats','off','f');
 set_config
------------
 off
(1 row)
Table 6-29 lists functions that allow the user to query object access privileges programmatically. See Section 2.7 for more information about privileges.
Table 6-29. Access Privilege Inquiry Functions
| Name | Return Type | Description | 
|---|---|---|
| has_table_privilege(user,table,access) | boolean | does user have access to table | 
| has_table_privilege(table,access) | boolean | does current user have access to table | 
| has_database_privilege(user,database,access) | boolean | does user have access to database | 
| has_database_privilege(database,access) | boolean | does current user have access to database | 
| has_function_privilege(user,function,access) | boolean | does user have access to function | 
| has_function_privilege(function,access) | boolean | does current user have access to function | 
| has_language_privilege(user,language,access) | boolean | does user have access to language | 
| has_language_privilege(language,access) | boolean | does current user have access to language | 
| has_schema_privilege(user,schema,access) | boolean | does user have access to schema | 
| has_schema_privilege(schema,access) | boolean | does current user have access to schema | 
    has_table_privilege checks whether a user
    can access a table in a particular way.  The user can be
    specified by name or by ID
    (pg_user.usesysid), or if the argument is
    omitted
    current_user is assumed.  The table can be specified
    by name or by OID.  (Thus, there are actually six variants of
    has_table_privilege, which can be distinguished by
    the number and types of their arguments.)  When specifying by name,
    the name can be schema-qualified if necessary.
    The desired access type
    is specified by a text string, which must evaluate to one of the
    values SELECT, INSERT, UPDATE,
    DELETE, RULE, REFERENCES, or
    TRIGGER.  (Case of the string is not significant, however.)
    An example is:
SELECT has_table_privilege('myschema.mytable', 'select');
    has_database_privilege checks whether a user
    can access a database in a particular way.  The possibilities for its
    arguments are analogous to has_table_privilege.
    The desired access type must evaluate to
    CREATE,
    TEMPORARY, or
    TEMP (which is equivalent to
    TEMPORARY).
   
    has_function_privilege checks whether a user
    can access a function in a particular way.  The possibilities for its
    arguments are analogous to has_table_privilege.
    When specifying a function by a text string rather than by OID,
    the allowed input is the same as for the regprocedure data type.
    The desired access type must currently evaluate to
    EXECUTE.
   
    has_language_privilege checks whether a user
    can access a procedural language in a particular way.  The possibilities
    for its arguments are analogous to has_table_privilege.
    The desired access type must currently evaluate to
    USAGE.
   
    has_schema_privilege checks whether a user
    can access a schema in a particular way.  The possibilities for its
    arguments are analogous to has_table_privilege.
    The desired access type must evaluate to
    CREATE or
    USAGE.
   
Table 6-30 shows functions that determine whether a certain object is visible in the current schema search path. A table is said to be visible if its containing schema is in the search path and no table of the same name appears earlier in the search path. This is equivalent to the statement that the table can be referenced by name without explicit schema qualification. For example, to list the names of all visible tables:
SELECT relname FROM pg_class WHERE pg_table_is_visible(oid);
Table 6-30. Schema Visibility Inquiry Functions
| Name | Return Type | Description | 
|---|---|---|
| pg_table_is_visible(tableOID) | boolean | is table visible in search path | 
| pg_type_is_visible(typeOID) | boolean | is type visible in search path | 
| pg_function_is_visible(functionOID) | boolean | is function visible in search path | 
| pg_operator_is_visible(operatorOID) | boolean | is operator visible in search path | 
| pg_opclass_is_visible(opclassOID) | boolean | is operator class visible in search path | 
   pg_table_is_visible performs the check for
   tables (or views, or any other kind of pg_class entry).
   pg_type_is_visible,
   pg_function_is_visible,
   pg_operator_is_visible, and
   pg_opclass_is_visible perform the same sort of
   visibility check for types, functions, operators, and operator
   classes, respectively.  For functions and operators, an object in
   the search path is visible if there is no object of the same name
   and argument data type(s) earlier in the path.  For
   operator classes, both name and associated index access method are
   considered.
   
All these functions require object OIDs to identify the object to be checked. If you want to test an object by name, it is convenient to use the OID alias types (regclass, regtype, regprocedure, or regoperator), for example
SELECT pg_type_is_visible('myschema.widget'::regtype);Note that it would not make much sense to test an unqualified name in this way --- if the name can be recognized at all, it must be visible.
   Table 6-31 lists functions that
   extract information from the system catalogs.
   pg_get_viewdef(),
   pg_get_ruledef(),
   pg_get_indexdef(), and
   pg_get_constraintdef() respectively
   reconstruct the creating command for a view, rule, index, or
   constraint.  (Note that this is a decompiled reconstruction, not
   the verbatim text of the command.)  At present
   pg_get_constraintdef() only works for
   foreign-key constraints.  pg_get_userbyid()
   extracts a user's name given a usesysid
   value.
  
Table 6-31. Catalog Information Functions
| Name | Return Type | Description | 
|---|---|---|
| pg_get_viewdef(viewname) | text | Get CREATE VIEW command for view (deprecated) | 
| pg_get_viewdef(viewOID) | text | Get CREATE VIEW command for view | 
| pg_get_ruledef(ruleOID) | text | Get CREATE RULE command for rule | 
| pg_get_indexdef(indexOID) | text | Get CREATE INDEX command for index | 
| pg_get_constraintdef(constraintOID) | text | Get definition of a constraint | 
| pg_get_userbyid(userid) | name | Get user name with given ID | 
The function shown in Table 6-32 extract comments previously stored with the COMMENT command. A null value is returned if no comment can be found matching the specified parameters.
Table 6-32. Comment Information Functions
| Name | Return Type | Description | 
|---|---|---|
| obj_description(objectOID,tablename) | text | Get comment for a database object | 
| obj_description(objectOID) | text | Get comment for a database object (deprecated) | 
| col_description(tableOID,columnnumber) | text | Get comment for a table column | 
    The two-parameter form of obj_description() returns the
    comment for a database object specified by its OID and the name of the
    containing system catalog.  For example,
    obj_description(123456,'pg_class')
    would retrieve the comment for a table with OID 123456.
    The one-parameter form of obj_description() requires only
    the object OID.  It is now deprecated since there is no guarantee that
    OIDs are unique across different system catalogs; therefore, the wrong
    comment could be returned.
   
    col_description() returns the comment for a table column,
    which is specified by the OID of its table and its column number.
    obj_description() cannot be used for table columns since
    columns do not have OIDs of their own.