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Development Versions: devel
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9.26. System Information Functions and Operators

Table 9.63 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 27.2.2 for more information.

Table 9.63. Session Information Functions

Function

Description

current_catalogname

current_database () → name

Returns the name of the current database. (Databases are called catalogs in the SQL standard, so current_catalog is the standard's spelling.)

current_query () → text

Returns the text of the currently executing query, as submitted by the client (which might contain more than one statement).

current_rolename

This is equivalent to current_user.

current_schemaname

current_schema () → name

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 ( include_implicit boolean ) → name[]

Returns an array of the names of all schemas presently in the effective search path, in their priority order. (Items in the current search_path setting that do not correspond to existing, searchable schemas are omitted.) If the Boolean argument is true, then implicitly-searched system schemas such as pg_catalog are included in the result.

current_username

Returns the user name of the current execution context.

inet_client_addr () → inet

Returns the IP address of the current client, or NULL if the current connection is via a Unix-domain socket.

inet_client_port () → integer

Returns the IP port number of the current client, or NULL if the current connection is via a Unix-domain socket.

inet_server_addr () → inet

Returns the IP address on which the server accepted the current connection, or NULL if the current connection is via a Unix-domain socket.

inet_server_port () → integer

Returns the IP port number on which the server accepted the current connection, or NULL if the current connection is via a Unix-domain socket.

pg_backend_pid () → integer

Returns the process ID of the server process attached to the current session.

pg_blocking_pids ( integer ) → integer[]

Returns an array of the process ID(s) of the sessions that are blocking the server process with the specified process ID from acquiring a lock, 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.

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 () → timestamp with time zone

Returns the time 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_current_logfile ( [ text ] ) → text

Returns the path name of the log file currently in use by the logging collector. The path includes the log_directory directory and the individual log file name. The result is NULL if the logging collector is disabled. When multiple log files exist, each in a different format, pg_current_logfile without an argument returns the path of the file having the first format found in the ordered list: stderr, csvlog. NULL is returned if no log file has any of these formats. To request information about a specific log file format, supply either csvlog or stderr as the value of the optional parameter. The result is NULL if the log format requested is not configured in log_destination. The result reflects the contents of the current_logfiles file.

pg_my_temp_schema () → oid

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 ( oid ) → boolean

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_jit_available () → boolean

Returns true if a JIT compiler extension is available (see Chapter 31) and the jit configuration parameter is set to on.

pg_listening_channels () → setof text

Returns the set of names of asynchronous notification channels that the current session is listening to.

pg_notification_queue_usage () → double precision

Returns the fraction (0–1) of the asynchronous notification queue's maximum size that is currently occupied by notifications that are waiting to be processed. See LISTEN and NOTIFY for more information.

pg_postmaster_start_time () → timestamp with time zone

Returns the time when the server started.

pg_safe_snapshot_blocking_pids ( integer ) → integer[]

Returns an array of the process ID(s) of the sessions that are blocking the server process with the specified process ID from acquiring a safe snapshot, or an empty array if there is no such server process or it is not blocked.

A session running a SERIALIZABLE transaction blocks a SERIALIZABLE READ ONLY DEFERRABLE transaction from acquiring a snapshot until the latter determines that it is safe to avoid taking any predicate locks. See Section 13.2.3 for more information about serializable and deferrable transactions.

Frequent calls to this function could have some impact on database performance, because it needs access to the predicate lock manager's shared state for a short time.

pg_trigger_depth () → integer

Returns the current nesting level of PostgreSQL triggers (0 if not called, directly or indirectly, from inside a trigger).

session_username

Returns the session user's name.

username

This is equivalent to current_user.

version () → text

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


Note

current_catalog, current_role, 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_role and user are synonyms for current_user. (The SQL standard draws a distinction between current_role and current_user, but PostgreSQL does not, since it unifies users and roles into a single kind of entity.)

Table 9.64 lists functions that allow querying object access privileges programmatically. (See Section 5.7 for more information about privileges.) In these functions, the user whose privileges are being inquired about can be specified by name or by OID (pg_authid.oid), or if the name is given as public then the privileges of the PUBLIC pseudo-role are checked. Also, the user argument can be omitted entirely, in which case the current_user is assumed. The object that is being inquired about can be specified either by name or by OID, too. When specifying by name, a schema name can be included if relevant. The access privilege of interest is specified by a text string, which must evaluate to one of the appropriate privilege keywords for the object's type (e.g., SELECT). 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');

Table 9.64. Access Privilege Inquiry Functions

Function

Description

has_any_column_privilege ( [ user name or oid, ] table text or oid, privilege text ) → boolean

Does user have privilege for any column of table? This succeeds either if the privilege is held for the whole table, or if there is a column-level grant of the privilege for at least one column. Allowable privilege types are SELECT, INSERT, UPDATE, and REFERENCES.

has_column_privilege ( [ user name or oid, ] table text or oid, column text or smallint, privilege text ) → boolean

Does user have privilege for the specified table column? This succeeds either if the privilege is held for the whole table, or if there is a column-level grant of the privilege for the column. The column can be specified by name or by attribute number (pg_attribute.attnum). Allowable privilege types are SELECT, INSERT, UPDATE, and REFERENCES.

has_database_privilege ( [ user name or oid, ] database text or oid, privilege text ) → boolean

Does user have privilege for database? Allowable privilege types are CREATE, CONNECT, TEMPORARY, and TEMP (which is equivalent to TEMPORARY).

has_foreign_data_wrapper_privilege ( [ user name or oid, ] fdw text or oid, privilege text ) → boolean

Does user have privilege for foreign-data wrapper? The only allowable privilege type is USAGE.

has_function_privilege ( [ user name or oid, ] function text or oid, privilege text ) → boolean

Does user have privilege for function? The only allowable privilege type is EXECUTE.

When specifying a function by name rather than by OID, the allowed input is the same as for the regprocedure data type (see Section 8.19). An example is:

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

has_language_privilege ( [ user name or oid, ] language text or oid, privilege text ) → boolean

Does user have privilege for language? The only allowable privilege type is USAGE.

has_schema_privilege ( [ user name or oid, ] schema text or oid, privilege text ) → boolean

Does user have privilege for schema? Allowable privilege types are CREATE and USAGE.

has_sequence_privilege ( [ user name or oid, ] sequence text or oid, privilege text ) → boolean

Does user have privilege for sequence? Allowable privilege types are USAGE, SELECT, and UPDATE.

has_server_privilege ( [ user name or oid, ] server text or oid, privilege text ) → boolean

Does user have privilege for foreign server? The only allowable privilege type is USAGE.

has_table_privilege ( [ user name or oid, ] table text or oid, privilege text ) → boolean

Does user have privilege for table? Allowable privilege types are SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, and TRIGGER.

has_tablespace_privilege ( [ user name or oid, ] tablespace text or oid, privilege text ) → boolean

Does user have privilege for tablespace? The only allowable privilege type is CREATE.

has_type_privilege ( [ user name or oid, ] type text or oid, privilege text ) → boolean

Does user have privilege for data type? The only allowable privilege type is USAGE. When specifying a type by name rather than by OID, the allowed input is the same as for the regtype data type (see Section 8.19).

pg_has_role ( [ user name or oid, ] role text or oid, privilege text ) → boolean

Does user have privilege for role? Allowable privilege types are MEMBER and 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. This function does not allow the special case of setting user to public, because the PUBLIC pseudo-role can never be a member of real roles.

row_security_active ( table text or oid ) → boolean

Is row-level security active for the specified table in the context of the current user and current environment?


Table 9.65 shows the operators available for the aclitem type, which is the catalog representation of access privileges. See Section 5.7 for information about how to read access privilege values.

Table 9.65. aclitem Operators

Operator

Description

Example(s)

aclitem = aclitemboolean

Are aclitems equal? (Notice that type aclitem lacks the usual set of comparison operators; it has only equality. In turn, aclitem arrays can only be compared for equality.)

'calvin=r*w/hobbes'::aclitem = 'calvin=r*w*/hobbes'::aclitemf

aclitem[] @> aclitemboolean

Does array contain the specified privileges? (This is true if there is an array entry that matches the aclitem's grantee and grantor, and has at least the specified set of privileges.)

'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] @> 'calvin=r*/hobbes'::aclitemt

aclitem[] ~ aclitemboolean

This is a deprecated alias for @>.

'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] ~ 'calvin=r*/hobbes'::aclitemt


Table 9.66 shows some additional functions to manage the aclitem type.

Table 9.66. aclitem Functions

Function

Description

acldefault ( type "char", ownerId oid ) → aclitem[]

Constructs an aclitem array holding the default access privileges for an object of type type belonging to the role with OID ownerId. This represents the access privileges that will be assumed when an object's ACL entry is null. (The default access privileges are described in Section 5.7.) The type parameter must be one of 'c' for COLUMN, 'r' for TABLE and table-like objects, 's' for SEQUENCE, 'd' for DATABASE, 'f' for FUNCTION or PROCEDURE, 'l' for LANGUAGE, 'L' for LARGE OBJECT, 'n' for SCHEMA, 't' for TABLESPACE, 'F' for FOREIGN DATA WRAPPER, 'S' for FOREIGN SERVER, or 'T' for TYPE or DOMAIN.

aclexplode ( aclitem[] ) → setof record ( grantor oid, grantee oid, privilege_type text, is_grantable boolean )

Returns the aclitem array as a set of rows. If the grantee is the pseudo-role PUBLIC, it is represented by zero in the grantee column. Each granted privilege is represented as SELECT, INSERT, etc. Note that each privilege is broken out as a separate row, so only one keyword appears in the privilege_type column.

makeaclitem ( grantee oid, grantor oid, privileges text, is_grantable boolean ) → aclitem

Constructs an aclitem with the given properties.


Table 9.67 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. Thus, to list the names of all visible tables:

SELECT relname FROM pg_class WHERE pg_table_is_visible(oid);

For functions and operators, an object in the search path is said to be visible if there is no object of the same name and argument data type(s) earlier in the path. For operator classes and families, both the name and the associated index access method are considered.

Table 9.67. Schema Visibility Inquiry Functions

Function

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? (This also works for procedures and aggregates.)

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_statistics_obj_is_visible ( stat oid ) → boolean

Is statistics object visible in search path?

pg_table_is_visible ( table oid ) → boolean

Is table visible in search path? (This works for all types of relations, including views, materialized views, indexes, sequences and foreign tables.)

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?


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 9.68 lists functions that extract information from the system catalogs.

Table 9.68. System Catalog Information Functions

Function

Description

format_type ( type oid, typemod integer ) → text

Returns the SQL name for 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 ( constraint oid [, pretty boolean ] ) → text

Reconstructs the creating command for a constraint. (This is a decompiled reconstruction, not the original text of the command.)

pg_get_expr ( expr pg_node_tree, relation oid [, pretty boolean ] ) → text

Decompiles the internal form of an expression stored in the system catalogs, such as the default value for a column. If the expression might contain Vars, specify the OID of the relation they refer to as the second parameter; if no Vars are expected, passing zero is sufficient.

pg_get_functiondef ( func oid ) → text

Reconstructs the creating command for a function or procedure. (This is a decompiled reconstruction, not the original text of the command.) The result is a complete CREATE OR REPLACE FUNCTION or CREATE OR REPLACE PROCEDURE statement.

pg_get_function_arguments ( func oid ) → text

Reconstructs the argument list of a function or procedure, in the form it would need to appear in within CREATE FUNCTION (including default values).

pg_get_function_identity_arguments ( func oid ) → text

Reconstructs the argument list necessary to identify a function or procedure, in the form it would need to appear in within commands such as ALTER FUNCTION. This form omits default values.

pg_get_function_result ( func oid ) → text

Reconstructs the RETURNS clause of a function, in the form it would need to appear in within CREATE FUNCTION. Returns NULL for a procedure.

pg_get_indexdef ( index oid [, column integer, pretty boolean ] ) → text

Reconstructs the creating command for an index. (This is a decompiled reconstruction, not the original text of the command.) If column is supplied and is not zero, only the definition of that column is reconstructed.

pg_get_keywords () → setof record ( word text, catcode "char", catdesc text )

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 an unreserved keyword, C for a keyword that can be a column name, T for a keyword that can be a type or function name, or R for a fully reserved keyword. The catdesc column contains a possibly-localized string describing the category.

pg_get_ruledef ( rule oid [, pretty boolean ] ) → text

Reconstructs the creating command for a rule. (This is a decompiled reconstruction, not the original text of the command.)

pg_get_serial_sequence ( table text, column text ) → text

Returns the name of the sequence associated with a column, or NULL if no sequence is associated with the column. If the column is an identity column, the associated sequence is the sequence internally created for that column. For columns created using one of the serial types (serial, smallserial, bigserial), it is the sequence created for that serial column definition. In the latter case, the association can be modified or removed with ALTER SEQUENCE OWNED BY. (This function probably should have been called pg_get_owned_sequence; its current name reflects the fact that it has historically been used with serial-type columns.) The first parameter is a table name with optional schema, and the second parameter is a column name. Because the first parameter potentially contains both schema and table names, it is parsed per usual SQL rules, meaning it is lower-cased by default. The second parameter, being just a column name, is treated literally and so has its case preserved. The result is suitably formatted for passing to the sequence functions (see Section 9.17).

A typical use is in reading the current value of the sequence for an identity or serial column, for example:

SELECT currval(pg_get_serial_sequence('sometable', 'id'));

pg_get_statisticsobjdef ( statobj oid ) → text

Reconstructs the creating command for an extended statistics object. (This is a decompiled reconstruction, not the original text of the command.)

pg_get_triggerdef ( trigger oid [, pretty boolean ] ) → text

Reconstructs the creating command for a trigger. (This is a decompiled reconstruction, not the original text of the command.)

pg_get_userbyid ( role oid ) → name

Returns a role's name given its OID.

pg_get_viewdef ( view oid [, pretty boolean ] ) → text

Reconstructs the underlying SELECT command for a view or materialized view. (This is a decompiled reconstruction, not the original text of the command.)

pg_get_viewdef ( view oid, wrap_column integer ) → text

Reconstructs the underlying SELECT command for a view or materialized view. (This is a decompiled reconstruction, not the original text of the command.) In this form of the function, pretty-printing is always enabled, and long lines are wrapped to try to keep them shorter than the specified number of columns.

pg_get_viewdef ( view text [, pretty boolean ] ) → text

Reconstructs the underlying SELECT command for a view or materialized view, working from a textual name for the view rather than its OID. (This is deprecated; use the OID variant instead.)

pg_index_column_has_property ( index regclass, column integer, property text ) → boolean

Tests whether an index column has the named property. Common index column properties are listed in Table 9.69. (Note that extension access methods can define additional property names for their indexes.) 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.

pg_index_has_property ( index regclass, property text ) → boolean

Tests whether an index has the named property. Common index properties are listed in Table 9.70. (Note that extension access methods can define additional property names for their indexes.) NULL is returned if the property name is not known or does not apply to the particular object, or if the OID does not identify a valid object.

pg_indexam_has_property ( am oid, property text ) → boolean

Tests whether an index access method has the named property. Access method properties are listed in Table 9.71. NULL is returned if the property name is not known or does not apply to the particular object, or if the OID does not identify a valid object.

pg_options_to_table ( options_array text[] ) → setof record ( option_name text, option_value text )

Returns the set of storage options represented by a value from pg_class.reloptions or pg_attribute.attoptions.

pg_tablespace_databases ( tablespace oid ) → setof oid

Returns the set of OIDs of databases that have objects stored in the specified tablespace. If this function returns any rows, the tablespace is not empty and cannot be dropped. To identify the specific objects populating the tablespace, you will need to connect to the database(s) identified by pg_tablespace_databases and query their pg_class catalogs.

pg_tablespace_location ( tablespace oid ) → text

Returns the file system path that this tablespace is located in.

pg_typeof ( "any" ) → regtype

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 8.19); 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

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

COLLATION FOR ( "any" ) → text

Returns the name of the collation of the value that is passed to it. The value is quoted and schema-qualified if necessary. If no collation was derived for the argument expression, then NULL is returned. If the argument is not of a collatable data type, then an error is raised.

For example:

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

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

to_regclass ( text ) → regclass

Translates a textual relation name to its OID. A similar result is obtained by casting the string to type regclass (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.

to_regcollation ( text ) → regcollation

Translates a textual collation name to its OID. A similar result is obtained by casting the string to type regcollation (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.

to_regnamespace ( text ) → regnamespace

Translates a textual schema name to its OID. A similar result is obtained by casting the string to type regnamespace (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.

to_regoper ( text ) → regoper

Translates a textual operator name to its OID. A similar result is obtained by casting the string to type regoper (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found or is ambiguous. Also unlike the cast, this does not accept a numeric OID as input.

to_regoperator ( text ) → regoperator

Translates a textual operator name (with parameter types) to its OID. A similar result is obtained by casting the string to type regoperator (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.

to_regproc ( text ) → regproc

Translates a textual function or procedure name to its OID. A similar result is obtained by casting the string to type regproc (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found or is ambiguous. Also unlike the cast, this does not accept a numeric OID as input.

to_regprocedure ( text ) → regprocedure

Translates a textual function or procedure name (with argument types) to its OID. A similar result is obtained by casting the string to type regprocedure (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.

to_regrole ( text ) → regrole

Translates a textual role name to its OID. A similar result is obtained by casting the string to type regrole (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.

to_regtype ( text ) → regtype

Translates a textual type name to its OID. A similar result is obtained by casting the string to type regtype (see Section 8.19); however, this function will return NULL rather than throwing an error if the name is not found. Also unlike the cast, this does not accept a numeric OID as input.


Most of the functions that reconstruct (decompile) database objects have an optional pretty flag, which if true causes the result to be pretty-printed. Pretty-printing suppresses unnecessary parentheses and adds whitespace for legibility. 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; so avoid using pretty-printed output for dump purposes. Passing false for the pretty parameter yields the same result as omitting the parameter.

Table 9.69. 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 9.70. 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 scan direction be changed in mid-scan (to support FETCH BACKWARD on a cursor without needing materialization)?

Table 9.71. 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?
can_include Does the access method support the INCLUDE clause of CREATE INDEX?

Table 9.72 lists functions related to database object identification and addressing.

Table 9.72. Object Information and Addressing Functions

Function

Description

pg_describe_object ( classid oid, objid oid, objsubid integer ) → text

Returns a textual description of a database object identified by catalog OID, object OID, and sub-object ID (such as a column number within a table; the sub-object ID is zero when referring to a whole object). This description is intended to be human-readable, and might be translated, depending on server configuration. This is especially useful to determine the identity of an object referenced in the pg_depend catalog.

pg_identify_object ( classid oid, objid oid, objsubid integer ) → record ( type text, schema text, name text, identity text )

Returns a row containing enough information to uniquely identify the database object specified by catalog OID, object OID and 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, if the name (along with schema name, if pertinent) is sufficient to uniquely identify the object, otherwise NULL; identity is the complete object identity, with the precise format depending on object type, and each name within the format being schema-qualified and quoted as necessary.

pg_identify_object_as_address ( classid oid, objid oid, objsubid integer ) → record ( type text, object_names text[], object_args text[] )

Returns a row containing enough information to uniquely identify the database object specified by catalog OID, object OID and 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; object_names and object_args are text arrays that together form a reference to the object. These three values can be passed to pg_get_object_address to obtain the internal address of the object.

pg_get_object_address ( type text, object_names text[], object_args text[] ) → record ( classid oid, objid oid, objsubid integer )

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


The functions shown in Table 9.73 extract comments previously stored with the COMMENT command. A null value is returned if no comment could be found for the specified parameters.

Table 9.73. Comment Information Functions

Function

Description

col_description ( table oid, column integer ) → text

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.)

obj_description ( object oid, catalog name ) → text

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.

obj_description ( object oid ) → text

Returns the comment for a database object specified by its OID alone. This is deprecated since there is no guarantee that OIDs are unique across different system catalogs; therefore, the wrong comment might be returned.

shobj_description ( object oid, catalog name ) → text

Returns the comment for a shared database object specified by its OID and the name of the containing system catalog. This is just like obj_description except that it is used for retrieving comments on shared objects (that is, databases, roles, and tablespaces). 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 9.74 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 9.74. Transaction ID and Snapshot Information Functions

Function

Description

pg_current_xact_id () → xid8

Returns the current transaction's ID. It will assign a new one if the current transaction does not have one already (because it has not performed any database updates).

pg_current_xact_id_if_assigned () → xid8

Returns the current transaction's ID, or NULL if no ID is assigned yet. (It's best to use this variant if the transaction might otherwise be read-only, to avoid unnecessary consumption of an XID.)

pg_xact_status ( xid8 ) → text

Reports the commit status of a recent transaction. The result is one of in progress, committed, or aborted, provided that the transaction is recent enough that the system retains the commit status of that transaction. If it is old enough that no references to the transaction survive in the system and the commit status information has been discarded, the result is NULL. Applications might use this function, for example, to determine whether their transaction committed or aborted after the application and database server become disconnected while a COMMIT is in progress. Note that prepared transactions are reported as in progress; applications must check pg_prepared_xacts if they need to determine whether a transaction ID belongs to a prepared transaction.

pg_current_snapshot () → pg_snapshot

Returns a current snapshot, a data structure showing which transaction IDs are now in-progress.

pg_snapshot_xip ( pg_snapshot ) → setof xid8

Returns the set of in-progress transaction IDs contained in a snapshot.

pg_snapshot_xmax ( pg_snapshot ) → xid8

Returns the xmax of a snapshot.

pg_snapshot_xmin ( pg_snapshot ) → xid8

Returns the xmin of a snapshot.

pg_visible_in_snapshot ( xid8, pg_snapshot ) → boolean

Is the given transaction ID visible according to this snapshot (that is, was it completed before the snapshot was taken)? Note that this function will not give the correct answer for a subtransaction ID.


The internal transaction ID type xid is 32 bits wide and wraps around every 4 billion transactions. However, the functions shown in Table 9.74 use a 64-bit type xid8 that does not wrap around during the life of an installation, and can be converted to xid by casting if required. The data type pg_snapshot stores information about transaction ID visibility at a particular moment in time. Its components are described in Table 9.75. pg_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.

Table 9.75. Snapshot Components

Name Description
xmin Lowest transaction ID that was still active. All transaction IDs less than xmin are either committed and visible, or rolled back and dead.
xmax One past the highest completed transaction ID. All transaction IDs greater than or equal to xmax had not yet completed as of the time of the snapshot, and thus are invisible.
xip_list Transactions in progress at the time of the snapshot. A transaction ID that is xmin <= X < xmax and not in this list was already completed at the time of the snapshot, and thus is either visible or dead according to its commit status. This list does not include the transaction IDs of subtransactions.

In releases of PostgreSQL before 13 there was no xid8 type, so variants of these functions were provided that used bigint to represent a 64-bit XID, with a correspondingly distinct snapshot data type txid_snapshot. These older functions have txid in their names. They are still supported for backward compatibility, but may be removed from a future release. See Table 9.76.

Table 9.76. Deprecated Transaction ID and Snapshot Information Functions

Function

Description

txid_current () → bigint

See pg_current_xact_id().

txid_current_if_assigned () → bigint

See pg_current_xact_id_if_assigned().

txid_current_snapshot () → txid_snapshot

See pg_current_snapshot().

txid_snapshot_xip ( txid_snapshot ) → setof bigint

See pg_snapshot_xip().

txid_snapshot_xmax ( txid_snapshot ) → bigint

See pg_snapshot_xmax().

txid_snapshot_xmin ( txid_snapshot ) → bigint

See pg_snapshot_xmin().

txid_visible_in_snapshot ( bigint, txid_snapshot ) → boolean

See pg_visible_in_snapshot().

txid_status ( bigint ) → text

See pg_xact_status().


The functions shown in Table 9.77 provide information about when past transactions were committed. They only provide useful data when the track_commit_timestamp configuration option is enabled, and only for transactions that were committed after it was enabled.

Table 9.77. Committed Transaction Information Functions

Function

Description

pg_xact_commit_timestamp ( xid ) → timestamp with time zone

Returns the commit timestamp of a transaction.

pg_last_committed_xact () → record ( xid xid, timestamp timestamp with time zone )

Returns the transaction ID and commit timestamp of the latest committed transaction.


The functions shown in Table 9.78 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, not specific to any one database. These functions provide most of the same information, from the same source, as the pg_controldata application.

Table 9.78. Control Data Functions

Function

Description

pg_control_checkpoint () → record

Returns information about current checkpoint state, as shown in Table 9.79.

pg_control_system () → record

Returns information about current control file state, as shown in Table 9.80.

pg_control_init () → record

Returns information about cluster initialization state, as shown in Table 9.81.

pg_control_recovery () → record

Returns information about recovery state, as shown in Table 9.82.


Table 9.79. pg_control_checkpoint Output Columns

Column Name Data Type
checkpoint_lsn pg_lsn
redo_lsn 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

Table 9.80. pg_control_system Output Columns

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

Table 9.81. pg_control_init Output 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
float8_pass_by_value boolean
data_page_checksum_version integer

Table 9.82. pg_control_recovery Output Columns

Column Name Data Type
min_recovery_end_lsn pg_lsn
min_recovery_end_timeline integer
backup_start_lsn pg_lsn
backup_end_lsn pg_lsn
end_of_backup_record_required boolean

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