PostgreSQL 7.4beta4 Documentation
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# 9.13. Miscellaneous Functions

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

Table 9-35. 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`, 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 PostgreSQL server's version.

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

Table 9-36. Configuration Settings Functions

Name | Return Type | Description
--- | --- | ---
`current_setting`(setting_name) | `text` | current value of setting
`set_config(setting_name, new_value, is_local)` | `text` | set parameter and return new value

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.

Table 9-37. Access Privilege Inquiry Functions

Name | Return Type | Description
--- | --- | ---
`has_table_privilege`(user, table, privilege) | `boolean` | does user have privilege for table
`has_table_privilege`(table, privilege) | `boolean` | does current user have privilege for table
`has_database_privilege`(user, database, privilege) | `boolean` | does user have privilege for database
`has_database_privilege`(database, privilege) | `boolean` | does current user have privilege for database
`has_function_privilege`(user, function, privilege) | `boolean` | does user have privilege for function
`has_function_privilege`(function, privilege) | `boolean` | does current user have privilege for function
`has_language_privilege`(user, language, privilege) | `boolean` | does user have privilege for language
`has_language_privilege`(language, privilege) | `boolean` | does current user have privilege for language
`has_schema_privilege`(user, schema, privilege) | `boolean` | does user have privilege for schema
`has_schema_privilege`(schema, privilege) | `boolean` | does current user have privilege for 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 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);

Table 9-38. Schema Visibility Inquiry Functions

Name | Return Type | Description
--- | --- | ---
`pg_table_is_visible`(table_oid) | `boolean` | is table visible in search path
`pg_type_is_visible`(type_oid) | `boolean` | is type (or domain) visible in search path
`pg_function_is_visible`(function_oid) | `boolean` | is function visible in search path
`pg_operator_is_visible`(operator_oid) | `boolean` | is operator visible in search path
`pg_opclass_is_visible`(opclass_oid) | `boolean` | is operator class visible in search path
`pg_conversion_is_visible`(conversion_oid) | `boolean` | is conversion 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`, `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.

Table 9-39. System Catalog Information Functions

Name | Return Type | Description
--- | --- | ---
`pg_get_viewdef`(view_name) | `text` | get `CREATE VIEW` command for view (deprecated)
`pg_get_viewdef`(view_name, pretty_bool) | `text` | get `CREATE VIEW` command for view (deprecated)
`pg_get_viewdef`(view_oid) | `text` | get `CREATE VIEW` command for view
`pg_get_viewdef`(view_oid, pretty_bool) | `text` | get `CREATE VIEW` command for view
`pg_get_ruledef`(rule_oid) | `text` | get `CREATE RULE` command for rule
`pg_get_ruledef`(rule_oid, pretty_bool) | `text` | get `CREATE RULE` command for rule
`pg_get_indexdef`(index_oid) | `text` | get `CREATE INDEX` command for index
`pg_get_indexdef`(index_oid, column_no, pretty_bool) | `text` | get `CREATE INDEX` command for index, or definition of just one index column when column_no is not zero
`pg_get_triggerdef`(trigger_oid) | `text` | get `CREATE [ CONSTRAINT ] TRIGGER` command for trigger
`pg_get_constraintdef`(constraint_oid) | `text` | get definition of a constraint
`pg_get_constraintdef`(constraint_oid, pretty_bool) | `text` | get definition of a constraint
`pg_get_expr`(expr_text, relation_oid) | `text` | decompile 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) | `text` | decompile internal form of an expression, assuming that any Vars in it refer to the relation indicated by the second parameter
`pg_get_userbyid`(userid) | `name` | get user name with given ID

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.

Table 9-40. Comment Information Functions

Name | Return Type | Description
--- | --- | ---
`obj_description`(object_oid, catalog_name) | `text` | get comment for a database object
`obj_description`(object_oid) | `text` | get comment for a database object (deprecated)
`col_description`(table_oid, column_number) | `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.

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