PostgreSQL 8.3.1 Documentation
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# 9.22. System Information Functions

Table 9-44 shows several functions that extract session and system information.

In addition to the functions listed in this section, there are a number of functions related to the statistics system that also provide system information. See Section 26.2.2 for more information.

Table 9-44. 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
``inet_client_addr`()` | `inet` | address of the remote connection
``inet_client_port`()` | `int` | port of the remote connection
``inet_server_addr`()` | `inet` | address of the local connection
``inet_server_port`()` | `int` | port of the local connection
``pg_my_temp_schema`()` | `oid` | OID of session's temporary schema, or 0 if none
``pg_is_other_temp_schema`(`oid`)` | `boolean` | is schema another session's temporary schema?
``pg_postmaster_start_time`()` | `timestamp with time zone` | server start time
``session_user`` | `name` | session user name
``user`` | `name` | equivalent to `current_user`
``version`()` | `text` | PostgreSQL version information

The `session_user` is normally the user who initiated the current database connection; but superusers can change this setting with SET SESSION AUTHORIZATION. The `current_user` is the user identifier that is applicable for permission checking. Normally, it is equal to the session user, but it can be changed with SET ROLE. It also 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 can be altered at run time. The command is:

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

`inet_client_addr` returns the IP address of the current client, and `inet_client_port` returns the port number. `inet_server_addr` returns the IP address on which the server accepted the current connection, and `inet_server_port` returns the port number. All these functions return NULL if the current connection is via a Unix-domain socket.

`pg_my_temp_schema` returns the OID of the current session's temporary schema, or 0 if it has none (because it has not created any temporary tables). `pg_is_other_temp_schema` returns true if the given OID is the OID of any other session's temporary schema. (This can be useful, for example, to exclude other sessions' temporary tables from a catalog display.)

`pg_postmaster_start_time` returns the `timestamp with time zone` when the server started.

`version` returns a string describing the PostgreSQL server's version.

Table 9-45 lists functions that allow the user to query object access privileges programmatically. See Section 5.6 for more information about privileges.

Table 9-45. Access Privilege Inquiry Functions

Name | Return Type | Description
--- | --- | ---
``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`(`user`, `table`, `privilege`)` | `boolean` | does user have privilege for table
``has_table_privilege`(`table`, `privilege`)` | `boolean` | does current user have privilege for table
``has_tablespace_privilege`(`user`, `tablespace`, `privilege`)` | `boolean` | does user have privilege for tablespace
``has_tablespace_privilege`(`tablespace`, `privilege`)` | `boolean` | does current user have privilege for tablespace
``pg_has_role`(`user`, `role`, `privilege`)` | `boolean` | does user have privilege for role
``pg_has_role`(`role`, `privilege`)` | `boolean` | does current user have privilege for role

`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`, `CONNECT`, `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 (see Section 8.16). 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_table_privilege` checks whether a user can access a table in a particular way. The user can be specified by name or by OID (`pg_authid.oid`), 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`, `REFERENCES`, or `TRIGGER`. (Case of the string is not significant, however.) An example is:

    SELECT has_table_privilege('myschema.mytable', 'select');

`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`.

`pg_has_role` checks whether a user can access a role in a particular way. The possibilities for its arguments are analogous to `has_table_privilege`. The desired access privilege type must evaluate to `MEMBER` or `USAGE`. `MEMBER` denotes direct or indirect membership in the role (that is, the right to do `SET ROLE`), while `USAGE` denotes whether the privileges of the role are immediately available without doing `SET ROLE`.

To test 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 9-46 shows functions that determine whether a certain object is visible in the current schema search path. For example, 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. To list the names of all visible tables:

    SELECT relname FROM pg_class WHERE pg_table_is_visible(oid);

Table 9-46. Schema Visibility Inquiry Functions

Name | Return Type | Description
--- | --- | ---
``pg_conversion_is_visible`(`conversion_oid`)` | `boolean` | is conversion 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_table_is_visible`(`table_oid`)` | `boolean` | is table visible in search path
``pg_ts_config_is_visible`(`config_oid`)` | `boolean` | is text search configuration visible in search path
``pg_ts_dict_is_visible`(`dict_oid`)` | `boolean` | is text search dictionary visible in search path
``pg_ts_parser_is_visible`(`parser_oid`)` | `boolean` | is text search parser visible in search path
``pg_ts_template_is_visible`(`template_oid`)` | `boolean` | is text search template visible in search path
``pg_type_is_visible`(`type_oid`)` | `boolean` | is type (or domain) visible in search path

Each function performs the visibility check for one type of database object. Note that `pg_table_is_visible` can also be used with views, indexes and sequences; `pg_type_is_visible` can also be used with domains. 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`, `regoperator`, `regconfig`, or `regdictionary`), 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 9-47 lists functions that extract information from the system catalogs.

Table 9-47. System Catalog Information Functions

Name | Return Type | Description
--- | --- | ---
``format_type`(`type_oid`, `typemod`)` | `text` | get SQL name of a data type
``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_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_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_serial_sequence`(`table_name`, `column_name`)` | `text` | get name of the sequence that a `serial` or `bigserial` column uses
`pg_get_triggerdef`(`trigger_oid`) | `text` | get `CREATE [ CONSTRAINT ] TRIGGER` command for trigger
``pg_get_userbyid`(`roleid`)` | `name` | get role name with given ID
``pg_get_viewdef`(`view_name`)` | `text` | get underlying `SELECT` command for view (deprecated)
``pg_get_viewdef`(`view_name`, `pretty_bool`)` | `text` | get underlying `SELECT` command for view (deprecated)
``pg_get_viewdef`(`view_oid`)` | `text` | get underlying `SELECT` command for view
``pg_get_viewdef`(`view_oid`, `pretty_bool`)` | `text` | get underlying `SELECT` command for view
``pg_tablespace_databases`(`tablespace_oid`)` | `setof oid` | get the set of database OIDs that have objects in the tablespace

`format_type` returns the SQL name of a data type that is identified by its type OID and possibly a type modifier. Pass NULL for the type modifier if no specific modifier is known.

`pg_get_constraintdef`, `pg_get_indexdef`, `pg_get_ruledef`, and `pg_get_triggerdef`, respectively reconstruct the creating command for a constraint, index, rule, or trigger. (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 can be useful when examining the contents of system catalogs. `pg_get_viewdef` reconstructs the `SELECT` query that defines a view. 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 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_serial_sequence` returns the name of the sequence associated with a column, or NULL if no sequence is associated with the column. The first input parameter is a table name with optional schema, and the second parameter is a column name. Because the first parameter is potentially a schema and table, it is not treated as a double-quoted identifier, meaning it is lowercased by default, while the second parameter, being just a column name, is treated as double-quoted and has its case preserved. The function returns a value suitably formatted for passing to the sequence functions (see Section 9.15). This association can be modified or removed with `ALTER SEQUENCE OWNED BY`. (The function probably should have been called `pg_get_owned_sequence`; its name reflects the fact that it's typically used with `serial` or `bigserial` columns.)

`pg_get_userbyid` extracts a role's name given its OID.

`pg_tablespace_databases` allows a tablespace to be examined. It returns the set of OIDs of databases that have objects stored in the tablespace. If this function returns any rows, 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 9-48 extract comments previously stored with the COMMENT command. A null value is returned if no comment could be found matching the specified parameters.

Table 9-48. Comment Information Functions

Name | Return Type | Description
--- | --- | ---
``col_description`(`table_oid`, `column_number`)` | `text` | get comment for a table column
``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)
``shobj_description`(`object_oid`, `catalog_name`)` | `text` | get comment for a shared database object

`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.

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.

`shobj_description` is used just like `obj_description` only that it is used for retrieving comments on shared objects. Some system catalogs are global to all databases within each cluster and their descriptions are stored globally as well.

The functions shown in Table 9-49 export server internal transaction information to user level. The main use of these functions is to determine which transactions were committed between two snapshots.

Table 9-49. Transaction IDs and snapshots

Name | Return Type | Description
--- | --- | ---
``txid_current`()` | `bigint` | get current transaction ID
``txid_current_snapshot`()` | `txid_snapshot` | get current snapshot
``txid_snapshot_xmin`(`txid_snapshot`)` | `bigint` | get xmin of snapshot
``txid_snapshot_xmax`(`txid_snapshot`)` | `bigint` | get xmax of snapshot
``txid_snapshot_xip`(`txid_snapshot`)` | `setof bigint` | get in-progress transaction IDs in snapshot
``txid_visible_in_snapshot`(`bigint`, `txid_snapshot`)` | `boolean` | is transaction ID visible in snapshot?

The internal transaction ID type (`xid`) is 32 bits wide and so it wraps around every 4 billion transactions. However, these functions export a 64-bit format that is extended with an "epoch" counter so that it will not wrap around for the life of an installation. The data type used by these functions, `txid_snapshot`, stores information about transaction ID visibility at a particular moment in time. Its components are described in Table 9-50.

Table 9-50. Snapshot components

Name | Description
--- | ---
`xmin` | Earliest transaction ID (txid) that is still active. All earlier transactions will either be committed and visible, or rolled back and dead.
`xmax` | First as-yet-unassigned txid. All txids later than this one are not yet started as of the time of the snapshot, and thus invisible.
`xip_list` | Active txids at the time of the snapshot. All of them are between `xmin` and `xmax`. A txid that is `xmin <= txid < xmax` and not in this list was already completed at the time of the snapshot, and thus either visible or dead according to its commit status.

`txid_snapshot`'s textual representation is ``xmin`:`xmax`:`xip_list``. For example `10:20:10,14,15` means `xmin=10, xmax=20, xip_list=10, 14, 15`.

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