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9.25. System Information Functions

Table 9-59 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 28.2.2 for more information.

Table 9-59. Session Information Functions

Name Return Type Description
current_catalog name name of current database (called "catalog" in the SQL standard)
current_database() name name of current database
current_query() text text of the currently executing query, as submitted by the client (might contain more than one statement)
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_backend_pid() int Process ID of the server process attached to the current session
pg_blocking_pids(int) int[] Process ID(s) that are blocking specified server process ID
pg_conf_load_time() timestamp with time zone configuration load time
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_listening_channels() setof text channel names that the session is currently listening on
pg_notification_queue_usage() double fraction of the asynchronous notification queue currently occupied (0-1)
pg_postmaster_start_time() timestamp with time zone server start time
pg_trigger_depth() int current nesting level of PostgreSQL triggers (0 if not called, directly or indirectly, from inside a trigger)
session_user name session user name
user name equivalent to current_user
version() text PostgreSQL version information. See also server_version_num for a machine-readable version.

Note: current_catalog, current_schema, current_user, session_user, and user have special syntactic status in SQL: they must be called without trailing parentheses. (In PostgreSQL, parentheses can optionally be used with current_schema, but not with the others.)

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".

current_schema returns the name of the schema that is first in 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 returned search path.

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_blocking_pids returns an array of the process IDs of the sessions that are blocking the server process with the specified process ID, or an empty array if there is no such server process or it is not blocked. One server process blocks another if it either holds a lock that conflicts with the blocked process's lock request (hard block), or is waiting for a lock that would conflict with the blocked process's lock request and is ahead of it in the wait queue (soft block). When using parallel queries the result always lists client-visible process IDs (that is, pg_backend_pid results) even if the actual lock is held or awaited by a child worker process. As a result of that, there may be duplicated PIDs in the result. Also note that when a prepared transaction holds a conflicting lock, it will be represented by a zero process ID in the result of this function. Frequent calls to this function could have some impact on database performance, because it needs exclusive access to the lock manager's shared state for a short time.

pg_conf_load_time returns the timestamp with time zone when the server configuration files were last loaded. (If the current session was alive at the time, this will be the time when the session itself re-read the configuration files, so the reading will vary a little in different sessions. Otherwise it is the time when the postmaster process re-read the configuration files.)

pg_my_temp_schema returns the OID of the current session's temporary schema, or zero 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 another session's temporary schema. (This can be useful, for example, to exclude other sessions' temporary tables from a catalog display.)

pg_listening_channels returns a set of names of asynchronous notification channels that the current session is listening to. pg_notification_queue_usage returns the fraction of the total available space for notifications currently occupied by notifications that are waiting to be processed, as a double in the range 0-1. See LISTEN and NOTIFY for more information.

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

version returns a string describing the PostgreSQL server's version. You can also get this information from server_version or for a machine-readable version, server_version_num. Software developers should use server_version_num (available since 8.2) or PQserverVersion instead of parsing the text version.

Table 9-60 lists functions that allow the user to query object access privileges programmatically. See Section 5.6 for more information about privileges.

Table 9-60. Access Privilege Inquiry Functions

Name Return Type Description
has_any_column_privilege(user, table, privilege) boolean does user have privilege for any column of table
has_any_column_privilege(table, privilege) boolean does current user have privilege for any column of table
has_column_privilege(user, table, column, privilege) boolean does user have privilege for column
has_column_privilege(table, column, privilege) boolean does current user have privilege for column
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_foreign_data_wrapper_privilege(user, fdw, privilege) boolean does user have privilege for foreign-data wrapper
has_foreign_data_wrapper_privilege(fdw, privilege) boolean does current user have privilege for foreign-data wrapper
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_sequence_privilege(user, sequence, privilege) boolean does user have privilege for sequence
has_sequence_privilege(sequence, privilege) boolean does current user have privilege for sequence
has_server_privilege(user, server, privilege) boolean does user have privilege for foreign server
has_server_privilege(server, privilege) boolean does current user have privilege for foreign server
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
has_type_privilege(user, type, privilege) boolean does user have privilege for type
has_type_privilege(type, privilege) boolean does current user have privilege for type
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
row_security_active(table) boolean does current user have row level security active for table

has_table_privilege checks whether a user can access a table in a particular way. The user can be specified by name, by OID (pg_authid.oid), public to indicate the PUBLIC pseudo-role, 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, TRUNCATE, REFERENCES, or TRIGGER. Optionally, WITH GRANT OPTION can be added to a privilege type to test whether the privilege is held with grant option. Also, multiple privilege types can be listed separated by commas, in which case the result will be true if any of the listed privileges is held. (Case of the privilege string is not significant, and extra whitespace is allowed between but not within privilege names.) Some examples:

SELECT has_table_privilege('myschema.mytable', 'select');
SELECT has_table_privilege('joe', 'mytable', 'INSERT, SELECT WITH GRANT OPTION');

has_sequence_privilege checks whether a user can access a sequence in a particular way. The possibilities for its arguments are analogous to has_table_privilege. The desired access privilege type must evaluate to one of USAGE, SELECT, or UPDATE.

has_any_column_privilege checks whether a user can access any column of a table in a particular way. Its argument possibilities are analogous to has_table_privilege, except that the desired access privilege type must evaluate to some combination of SELECT, INSERT, UPDATE, or REFERENCES. Note that having any of these privileges at the table level implicitly grants it for each column of the table, so has_any_column_privilege will always return true if has_table_privilege does for the same arguments. But has_any_column_privilege also succeeds if there is a column-level grant of the privilege for at least one column.

has_column_privilege checks whether a user can access a column in a particular way. Its argument possibilities are analogous to has_table_privilege, with the addition that the column can be specified either by name or attribute number. The desired access privilege type must evaluate to some combination of SELECT, INSERT, UPDATE, or REFERENCES. Note that having any of these privileges at the table level implicitly grants it for each column of the table.

has_database_privilege checks whether a user can access a database in a particular way. Its argument possibilities are analogous to has_table_privilege. The desired access privilege type must evaluate to some combination of 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. Its argument possibilities 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 8.18). The desired access privilege type must evaluate to EXECUTE. An example is:

SELECT has_function_privilege('joeuser', 'myfunc(int, text)', 'execute');

has_foreign_data_wrapper_privilege checks whether a user can access a foreign-data wrapper in a particular way. Its argument possibilities are analogous to has_table_privilege. The desired access privilege type must evaluate to USAGE.

has_language_privilege checks whether a user can access a procedural language in a particular way. Its argument possibilities 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. Its argument possibilities are analogous to has_table_privilege. The desired access privilege type must evaluate to some combination of CREATE or USAGE.

has_server_privilege checks whether a user can access a foreign server in a particular way. Its argument possibilities are analogous to has_table_privilege. The desired access privilege type must evaluate to USAGE.

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

has_type_privilege checks whether a user can access a type in a particular way. Its argument possibilities are analogous to has_table_privilege. When specifying a type by a text string rather than by OID, the allowed input is the same as for the regtype data type (see Section 8.18). The desired access privilege type must evaluate to USAGE.

pg_has_role checks whether a user can access a role in a particular way. Its argument possibilities are analogous to has_table_privilege, except that public is not allowed as a user name. The desired access privilege type must evaluate to some combination of 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.

row_security_active checks whether row level security is active for the specified table in the context of the current_user and environment. The table can be specified by name or by OID.

Table 9-61 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 9-61. Schema Visibility Inquiry Functions

Name Return Type Description
pg_collation_is_visible(collation_oid) boolean is collation visible in search path
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_opclass_is_visible(opclass_oid) boolean is operator class visible in search path
pg_operator_is_visible(operator_oid) boolean is operator visible in search path
pg_opfamily_is_visible(opclass_oid) boolean is operator family 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, materialized views, indexes, sequences and foreign tables; 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 a non-schema-qualified type name in this way — if the name can be recognized at all, it must be visible.

Table 9-62 lists functions that extract information from the system catalogs.

Table 9-62. 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(pg_node_tree, 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(pg_node_tree, 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_functiondef(func_oid) text get definition of a function
pg_get_function_arguments(func_oid) text get argument list of function's definition (with default values)
pg_get_function_identity_arguments(func_oid) text get argument list to identify a function (without default values)
pg_get_function_result(func_oid) text get RETURNS clause for function
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_keywords() setof record get list of SQL keywords and their categories
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, smallserial or bigserial column uses
pg_get_triggerdef(trigger_oid) text get CREATE [ CONSTRAINT ] TRIGGER command for trigger
pg_get_triggerdef(trigger_oid, pretty_bool) text get CREATE [ CONSTRAINT ] TRIGGER command for trigger
pg_get_userbyid(role_oid) name get role name with given OID
pg_get_viewdef(view_name) text get underlying SELECT command for view or materialized view (deprecated)
pg_get_viewdef(view_name, pretty_bool) text get underlying SELECT command for view or materialized view (deprecated)
pg_get_viewdef(view_oid) text get underlying SELECT command for view or materialized view
pg_get_viewdef(view_oid, pretty_bool) text get underlying SELECT command for view or materialized view
pg_get_viewdef(view_oid, wrap_column_int) text get underlying SELECT command for view or materialized view; lines with fields are wrapped to specified number of columns, pretty-printing is implied
pg_index_column_has_property(index_oid, column_no, prop_name) boolean test whether an index column has a specified property
pg_index_has_property(index_oid, prop_name) boolean test whether an index has a specified property
pg_indexam_has_property(am_oid, prop_name) boolean test whether an index access method has a specified property
pg_options_to_table(reloptions) setof record get the set of storage option name/value pairs
pg_tablespace_databases(tablespace_oid) setof oid get the set of database OIDs that have objects in the tablespace
pg_tablespace_location(tablespace_oid) text get the path in the file system that this tablespace is located in
pg_typeof(any) regtype get the data type of any value
collation for (any) text get the collation of the argument
to_regclass(rel_name) regclass get the OID of the named relation
to_regproc(func_name) regproc get the OID of the named function
to_regprocedure(func_name) regprocedure get the OID of the named function
to_regoper(operator_name) regoper get the OID of the named operator
to_regoperator(operator_name) regoperator get the OID of the named operator
to_regtype(type_name) regtype get the OID of the named type
to_regnamespace(schema_name) regnamespace get the OID of the named schema
to_regrole(role_name) regrole get the OID of the named role

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_keywords returns a set of records describing the SQL keywords recognized by the server. The word column contains the keyword. The catcode column contains a category code: U for unreserved, C for column name, T for type or function name, or R for reserved. The catdesc column contains a possibly-localized string describing the category.

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. If the expression might contain Vars, specify the OID of the relation they refer to as the second parameter; if no Vars are expected, zero is sufficient. 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_functiondef returns a complete CREATE OR REPLACE FUNCTION statement for a function. pg_get_function_arguments returns the argument list of a function, in the form it would need to appear in within CREATE FUNCTION. pg_get_function_result similarly returns the appropriate RETURNS clause for the function. pg_get_function_identity_arguments returns the argument list necessary to identify a function, in the form it would need to appear in within ALTER FUNCTION, for instance. This form omits default values.

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 lower cased 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 sequence functions (see Section 9.16). This association can be modified or removed with ALTER SEQUENCE OWNED BY. (The function probably should have been called pg_get_owned_sequence; its current 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_index_column_has_property, pg_index_has_property, and pg_indexam_has_property return whether the specified index column, index, or index access method possesses the named property. NULL is returned if the property name is not known or does not apply to the particular object, or if the OID or column number does not identify a valid object. Refer to Table 9-63 for column properties, Table 9-64 for index properties, and Table 9-65 for access method properties. (Note that extension access methods can define additional property names for their indexes.)

Table 9-63. Index Column Properties

Name Description
asc Does the column sort in ascending order on a forward scan?
desc Does the column sort in descending order on a forward scan?
nulls_first Does the column sort with nulls first on a forward scan?
nulls_last Does the column sort with nulls last on a forward scan?
orderable Does the column possess any defined sort ordering?
distance_orderable Can the column be scanned in order by a "distance" operator, for example ORDER BY col <-> constant ?
returnable Can the column value be returned by an index-only scan?
search_array Does the column natively support col = ANY(array) searches?
search_nulls Does the column support IS NULL and IS NOT NULL searches?

Table 9-64. Index Properties

Name Description
clusterable Can the index be used in a CLUSTER command?
index_scan Does the index support plain (non-bitmap) scans?
bitmap_scan Does the index support bitmap scans?
backward_scan Can the index be scanned backwards?

Table 9-65. Index Access Method Properties

Name Description
can_order Does the access method support ASC, DESC and related keywords in CREATE INDEX?
can_unique Does the access method support unique indexes?
can_multi_col Does the access method support indexes with multiple columns?
can_exclude Does the access method support exclusion constraints?

pg_options_to_table returns the set of storage option name/value pairs (option_name/option_value) when passed pg_class.reloptions or pg_attribute.attoptions.

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.

pg_typeof returns the OID of the data type of the value that is passed to it. This can be helpful for troubleshooting or dynamically constructing SQL queries. The function is declared as returning regtype, which is an OID alias type (see Section 8.18); this means that it is the same as an OID for comparison purposes but displays as a type name. For example:

SELECT pg_typeof(33);

 pg_typeof 
-----------
 integer
(1 row)

SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
 typlen 
--------
      4
(1 row)

The expression collation for returns the collation of the value that is passed to it. Example:

SELECT collation for (description) FROM pg_description LIMIT 1;
 pg_collation_for 
------------------
 "default"
(1 row)

SELECT collation for ('foo' COLLATE "de_DE");
 pg_collation_for 
------------------
 "de_DE"
(1 row)

The value might be quoted and schema-qualified. If no collation is derived for the argument expression, then a null value is returned. If the argument is not of a collatable data type, then an error is raised.

The to_regclass, to_regproc, to_regprocedure, to_regoper, to_regoperator, to_regtype, to_regnamespace, and to_regrole functions translate relation, function, operator, type, schema, and role names (given as text) to objects of type regclass, regproc, regprocedure, regoper, regoperator, regtype, regnamespace, and regrole respectively. These functions differ from a cast from text in that they don't accept a numeric OID, and that they return null rather than throwing an error if the name is not found (or, for to_regproc and to_regoper, if the given name matches multiple objects).

Table 9-66 lists functions related to database object identification and addressing.

Table 9-66. Object Information and Addressing Functions

Name Return Type Description
pg_describe_object(catalog_id, object_id, object_sub_id) text get description of a database object
pg_identify_object(catalog_id oid, object_id oid, object_sub_id integer) type text, schema text, name text, identity text get identity of a database object
pg_identify_object_as_address(catalog_id oid, object_id oid, object_sub_id integer) type text, name text[], args text[] get external representation of a database object's address
pg_get_object_address(type text, name text[], args text[]) catalog_id oid, object_id oid, object_sub_id int32 get address of a database object, from its external representation

pg_describe_object returns a textual description of a database object specified by catalog OID, object OID and a (possibly zero) sub-object ID. This description is intended to be human-readable, and might be translated, depending on server configuration. This is useful to determine the identity of an object as stored in the pg_depend catalog.

pg_identify_object returns a row containing enough information to uniquely identify the database object specified by catalog OID, object OID and a (possibly zero) sub-object ID. This information is intended to be machine-readable, and is never translated. type identifies the type of database object; schema is the schema name that the object belongs in, or NULL for object types that do not belong to schemas; name is the name of the object, quoted if necessary, only present if it can be used (alongside schema name, if pertinent) as a unique identifier of the object, otherwise NULL; identity is the complete object identity, with the precise format depending on object type, and each part within the format being schema-qualified and quoted as necessary.

pg_identify_object_as_address returns a row containing enough information to uniquely identify the database object specified by catalog OID, object OID and a (possibly zero) sub-object ID. The returned information is independent of the current server, that is, it could be used to identify an identically named object in another server. type identifies the type of database object; name and args are text arrays that together form a reference to the object. These three columns can be passed to pg_get_object_address to obtain the internal address of the object. This function is the inverse of pg_get_object_address.

pg_get_object_address returns a row containing enough information to uniquely identify the database object specified by its type and object name and argument arrays. The returned values are the ones that would be used in system catalogs such as pg_depend and can be passed to other system functions such as pg_identify_object or pg_describe_object. catalog_id is the OID of the system catalog containing the object; object_id is the OID of the object itself, and object_sub_id is the object sub-ID, or zero if none. This function is the inverse of pg_identify_object_as_address.

The functions shown in Table 9-67 extract comments previously stored with the COMMENT command. A null value is returned if no comment could be found for the specified parameters.

Table 9-67. 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 the table with OID 123456. The one-parameter form of obj_description requires only the object OID. It is deprecated since there is no guarantee that OIDs are unique across different system catalogs; therefore, the wrong comment might be returned.

shobj_description is used just like obj_description except it is used for retrieving comments on shared objects. Some system catalogs are global to all databases within each cluster, and the descriptions for objects in them are stored globally as well.

The functions shown in Table 9-68 provide server transaction information in an exportable form. The main use of these functions is to determine which transactions were committed between two snapshots.

Table 9-68. Transaction IDs and Snapshots

Name Return Type Description
txid_current() bigint get current transaction ID, assigning a new one if the current transaction does not have one
txid_current_snapshot() txid_snapshot get current snapshot
txid_snapshot_xip(txid_snapshot) setof bigint get in-progress transaction IDs in snapshot
txid_snapshot_xmax(txid_snapshot) bigint get xmax of snapshot
txid_snapshot_xmin(txid_snapshot) bigint get xmin of snapshot
txid_visible_in_snapshot(bigint, txid_snapshot) boolean is transaction ID visible in snapshot? (do not use with subtransaction ids)

The internal transaction ID type (xid) is 32 bits wide and wraps around every 4 billion transactions. However, these functions export a 64-bit format that is extended with an "epoch" counter so it will not wrap around during 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 9-69.

Table 9-69. 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 greater than or equal to this are not yet started as of the time of the snapshot, and thus invisible.
xip_list Active txids at the time of the snapshot. The list includes only those active txids between xmin and xmax; there might be active txids higher than 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. The list does not include txids of subtransactions.

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.

The functions shown in Table 9-70 provide information about transactions that have been already committed. These functions mainly provide information about when the transactions were committed. They only provide useful data when track_commit_timestamp configuration option is enabled and only for transactions that were committed after it was enabled.

Table 9-70. Committed transaction information

Name Return Type Description
pg_xact_commit_timestamp(xid) timestamp with time zone get commit timestamp of a transaction
pg_last_committed_xact() xid xid, timestamp timestamp with time zone get transaction ID and commit timestamp of latest committed transaction

The functions shown in Table 9-71 print information initialized during initdb, such as the catalog version. They also show information about write-ahead logging and checkpoint processing. This information is cluster-wide, and not specific to any one database. They provide most of the same information, from the same source, as pg_controldata, although in a form better suited to SQL functions.

Table 9-71. Control Data Functions

Name Return Type Description
pg_control_checkpoint() record Returns information about current checkpoint state.
pg_control_system() record Returns information about current control file state.
pg_control_init() record Returns information about cluster initialization state.
pg_control_recovery() record Returns information about recovery state.

pg_control_checkpoint returns a record, shown in Table 9-72

Table 9-72. pg_control_checkpoint Columns

Column Name Data Type
checkpoint_location pg_lsn
prior_location pg_lsn
redo_location pg_lsn
redo_wal_file text
timeline_id integer
prev_timeline_id integer
full_page_writes boolean
next_xid text
next_oid oid
next_multixact_id xid
next_multi_offset xid
oldest_xid xid
oldest_xid_dbid oid
oldest_active_xid xid
oldest_multi_xid xid
oldest_multi_dbid oid
oldest_commit_ts_xid xid
newest_commit_ts_xid xid
checkpoint_time timestamp with time zone

pg_control_system returns a record, shown in Table 9-73

Table 9-73. pg_control_system Columns

Column Name Data Type
pg_control_version integer
catalog_version_no integer
system_identifier bigint
pg_control_last_modified timestamp with time zone

pg_control_init returns a record, shown in Table 9-74

Table 9-74. pg_control_init Columns

Column Name Data Type
max_data_alignment integer
database_block_size integer
blocks_per_segment integer
wal_block_size integer
bytes_per_wal_segment integer
max_identifier_length integer
max_index_columns integer
max_toast_chunk_size integer
large_object_chunk_size integer
bigint_timestamps boolean
float4_pass_by_value boolean
float8_pass_by_value boolean
data_page_checksum_version integer

pg_control_recovery returns a record, shown in Table 9-75

Table 9-75. pg_control_recovery Columns

Column Name Data Type
min_recovery_end_location pg_lsn
min_recovery_end_timeline integer
backup_start_location pg_lsn
backup_end_location pg_lsn
end_of_backup_record_required boolean

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