9.13. Miscellaneous Functions

Table 9-35 shows several functions that extract session and system 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".

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.

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

Table 9-36 shows the functions available to query and alter run-time configuration parameters.

The function current_setting yields the current value of the setting setting_name. It corresponds to the SQL command SHOW. An example:

SELECT current_setting('datestyle');

 current_setting
-----------------
 ISO, MDY
(1 row)

set_config sets the parameter setting_name to new_value. If is_local is 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. The function corresponds to the SQL command SET. An example:

SELECT set_config('show_statement_stats', 'off', false);

 set_config
------------
 off
(1 row)

Table 9-37 lists functions that allow the user to query object access privileges programmatically. See Section 5.7 for more information about privileges.

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 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 privilege 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 privilege type must evaluate to CREATE or USAGE.

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 9-38 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);

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, pg_opclass_is_visible, and pg_conversion_is_visible perform the same sort of visibility check for types (and domains), functions, operators, operator classes and conversions, 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 9-39 lists functions that extract information from the system catalogs. 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.) Most of these 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 PostgreSQL; 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_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. pg_get_userbyid extracts a user's name given a user ID number.

The function shown in Table 9-40 extract comments previously stored with the COMMENT command. A null value is returned if no comment could be found matching the specified parameters.

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.