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8.14. System Information Functions

Table 8-31 shows several functions that extract session and system information.

Table 8-31. Session Information Functions

NameReturn TypeDescription
current_database()namename of current database
current_schema()namename of current schema
current_schemas(boolean)name[]names of schemas in search path optionally including implicit schemas
current_usernameuser name of current execution context
session_usernamesession user name
usernameequivalent to current_user
version()TEXTEnterpriseDB 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, and user have 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.

Note: The search path may be altered at run time. The command is:

SET search_path TO schema [, schema, ...]

version() returns a string describing the EnterpriseDB server's version.

Table 8-32 lists functions that allow the user to query object access privileges programmatically. See Section 4.6 for more information about privileges.

Table 8-32. Access Privilege Inquiry Functions

NameReturn TypeDescription
has_table_privilege(user, table, privilege) BOOLEANdoes user have privilege for table
has_table_privilege(table, privilege) BOOLEANdoes current user have privilege for table
has_database_privilege(user, database, privilege) BOOLEANdoes user have privilege for database
has_database_privilege(database, privilege) BOOLEANdoes current user have privilege for database
has_function_privilege(user, function, privilege) BOOLEANdoes user have privilege for function
has_function_privilege(function, privilege) BOOLEANdoes current user have privilege for function
has_language_privilege(user, language, privilege) BOOLEANdoes user have privilege for language
has_language_privilege(language, privilege) BOOLEANdoes current user have privilege for language
has_schema_privilege(user, schema, privilege) BOOLEANdoes user have privilege for schema
has_schema_privilege(schema, privilege) BOOLEANdoes current user have privilege for schema
has_tablespace_privilege(user, tablespace, privilege) BOOLEANdoes user have privilege for tablespace
has_tablespace_privilege(tablespace, privilege) BOOLEANdoes current user have privilege for tablespace

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 privilege 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 privilege 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 privilege type must evaluate to EXECUTE. An example is:

SELECT has_function_privilege('joeuser', 'myfunc(int, text)', '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 privilege type must 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 privilege type must evaluate to CREATE or USAGE.

has_tablespace_privilege checks whether a user can access a tablespace in a particular way. The possibilities for its arguments are analogous to has_table_privilege. The desired access privilege type must evaluate to CREATE.

To evaluate whether a user holds a grant option on the privilege, append WITH GRANT OPTION to the privilege key word; for example 'UPDATE WITH GRANT OPTION'.

Table 8-33 lists functions that extract information from the system catalogs.

Table 8-33. System Catalog Information Functions

NameReturn TypeDescription
pg_get_viewdef(view_name)TEXTget CREATE VIEW command for view (deprecated)
pg_get_viewdef(view_name, pretty_bool)TEXTget CREATE VIEW command for view (deprecated)
pg_get_viewdef(view_oid)TEXTget CREATE VIEW command for view
pg_get_viewdef(view_oid, pretty_bool)TEXTget CREATE VIEW command for view
pg_get_ruledef(rule_oid)TEXTget CREATE RULE command for rule
pg_get_ruledef(rule_oid, pretty_bool)TEXTget CREATE RULE command for rule
pg_get_indexdef(index_oid)TEXTget CREATE INDEX command for index
pg_get_indexdef(index_oid, column_no, pretty_bool)TEXTget CREATE INDEX command for index, or definition of just one index column when column_no is not zero
pg_get_triggerdef(trigger_oid)TEXTget CREATE [ CONSTRAINT ] TRIGGER command for trigger
pg_get_constraintdef(constraint_oid)TEXTget definition of a constraint
pg_get_constraintdef(constraint_oid, pretty_bool)TEXTget definition of a constraint
pg_get_expr(expr_text, relation_oid)TEXTdecompile internal form of an expression, assuming that any Vars in it refer to the relation indicated by the second parameter
pg_get_expr(expr_text, relation_oid, pretty_bool)TEXTdecompile internal form of an expression, assuming that any Vars in it refer to the relation indicated by the second parameter
pg_get_userbyid(userid)nameget user name with given ID
pg_get_serial_sequence(table_name, column_name)TEXTget name of the sequence that a serial or bigserial column uses
pg_tablespace_databases(tablespace_oid)setof oidget set of database OIDs that have objects in the tablespace

pg_get_viewdef, pg_get_ruledef, pg_get_indexdef, pg_get_triggerdef, and pg_get_constraintdef respectively reconstruct the creating command for a view, rule, index, trigger, or constraint. (Note that this is a decompiled reconstruction, not the original text of the command.) pg_get_expr decompiles the internal form of an individual expression, such as the default value for a column. It may be useful when examining the contents of system catalogs. Most of these functions come in two variants, one of which can optionally "pretty-print" the result. The pretty-printed format is more readable, but the default format is more likely to be interpreted the same way by future versions of EnterpriseDB; avoid using pretty-printed output for dump purposes. Passing false for the pretty-print parameter yields the same result as the variant that does not have the parameter at all.

pg_get_userbyid extracts a user's name given a user ID number. pg_get_serial_sequence fetches the name of the sequence associated with a serial or bigserial column. The name is suitably formatted for passing to the sequence functions (see Section 8.9). NULL is returned if the column does not have a sequence attached.

pg_tablespace_databases allows usage examination of a tablespace. It will return a set of OIDs of databases that have objects stored in the tablespace. If this function returns any row, the tablespace is not empty and cannot be dropped. To display the specific objects populating the tablespace, you will need to connect to the databases identified by pg_tablespace_databases and query their pg_class catalogs.

The functions shown in Table 8-34 extract comments previously stored with the COMMENT command. A null value is returned if no comment could be found matching the specified parameters.

Table 8-34. Comment Information Functions

NameReturn TypeDescription
obj_description(object_oid, catalog_name)TEXTget comment for a database object
obj_description(object_oid)TEXTget comment for a database object (deprecated)
col_description(table_oid, column_number)TEXTget 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.

 
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