9.26. System Information Functions and Operators
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## 9.26. System Information Functions and Operators

Table 9.66 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 28.2.24 for more information.

Table 9.66. Session Information Functions

Function Description
`current_catalog` → `name` `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_role` → `name` This is equivalent to `current_user`.
`current_schema` → `name` `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_user` → `name` 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`, `jsonlog`. `NULL` is returned if no log file has any of these formats. To request information about a specific log file format, supply either `csvlog`, `jsonlog` 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 32) 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_user` → `name` Returns the session user's name.
`user` → `name` 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 9.67 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 9.67. 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_parameter_privilege` ( [ `user` `name` or `oid`, ] `parameter` `text`, `privilege` `text` ) → `boolean` Does user have privilege for configuration parameter? The parameter name is case-insensitive. Allowable privilege types are `SET` and `ALTER SYSTEM`.
`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`. `WITH ADMIN OPTION` or `WITH GRANT OPTION` can be added to either of these privilege types to test whether the `ADMIN` privilege is held (all four spellings test the same thing). 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 9.68 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 9.68. `aclitem` Operators

Operator Description Example(s)
`aclitem` `=` `aclitem` → `boolean` Are `aclitem`s 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'::aclitem` → `f`
`aclitem[]` `@>` `aclitem` → `boolean` 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'::aclitem` → `t`
`aclitem[]` `~` `aclitem` → `boolean` This is a deprecated alias for `@>`. `'{calvin=r*w/hobbes,hobbes=r*w*/postgres}'::aclitem[] ~ 'calvin=r*/hobbes'::aclitem` → `t`

Table 9.69 shows some additional functions to manage the `aclitem` type.

Table 9.69. `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`, 'p' for `PARAMETER`, '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 9.70 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 9.70. 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 9.71 lists functions that extract information from the system catalogs.

Table 9.71. 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_char_to_encoding` ( `encoding` `name` ) → `integer` Converts the supplied encoding name into an integer representing the internal identifier used in some system catalog tables. Returns `-1` if an unknown encoding name is provided.
`pg_encoding_to_char` ( `encoding` `integer` ) → `name` Converts the integer used as the internal identifier of an encoding in some system catalog tables into a human-readable string. Returns an empty string if an invalid encoding number is provided.

`pg_get_catalog_foreign_keys` () → `setof record` ( `fktable` `regclass`, `fkcols` `text[]`, `pktable` `regclass`, `pkcols` `text[]`, `is_array` `boolean`, `is_opt` `boolean` )

Returns a set of records describing the foreign key relationships that exist within the PostgreSQL system catalogs. The `fktable` column contains the name of the referencing catalog, and the `fkcols` column contains the name(s) of the referencing column(s). Similarly, the `pktable` column contains the name of the referenced catalog, and the `pkcols` column contains the name(s) of the referenced column(s). If `is_array` is true, the last referencing column is an array, each of whose elements should match some entry in the referenced catalog. If `is_opt` is true, the referencing column(s) are allowed to contain zeroes instead of a valid reference.

`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"`, `barelabel` `boolean`, `catdesc` `text`, `baredesc` `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 `barelabel` column contains `true` if the keyword can be used as a “bare” column label in `SELECT` lists, or `false` if it can only be used after `AS`. The `catdesc` column contains a possibly-localized string describing the keyword's category. The `baredesc` column contains a possibly-localized string describing the keyword's column label status.

`pg_get_partition_constraintdef` ( `table` `oid` ) → `text` Reconstructs the definition of a partition constraint. (This is a decompiled reconstruction, not the original text of the command.)
`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 9.72. (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 9.73. (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 9.74. `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_settings_get_flags` ( `guc` `text` ) → `text[]` Returns an array of the flags associated with the given GUC, or `NULL` if it does not exist. The result is an empty array if the GUC exists but there are no flags to show. Only the most useful flags listed in Table 9.75 are exposed.

`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 9.72. 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 9.73. 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 9.74. 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 9.75. GUC Flags

Flag | Description
--- | ---
`EXPLAIN` | Parameters with this flag are included in `EXPLAIN (SETTINGS)` commands.
`NO_SHOW_ALL` | Parameters with this flag are excluded from `SHOW ALL` commands.
`NO_RESET_ALL` | Parameters with this flag are excluded from `RESET ALL` commands.
`NOT_IN_SAMPLE` | Parameters with this flag are not included in `postgresql.conf` by default.
`RUNTIME_COMPUTED` | Parameters with this flag are runtime-computed ones.

Table 9.76 lists functions related to database object identification and addressing.

Table 9.76. 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. This function returns `NULL` values for undefined objects.

`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. Undefined objects are identified with `NULL` values.

`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`. Undefined objects are identified with `NULL` values.

The functions shown in Table 9.77 extract comments previously stored with the COMMENT command. A null value is returned if no comment could be found for the specified parameters.

Table 9.77. 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 9.78 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 9.78. Transaction ID and Snapshot Information Functions

Function Description
`age` ( `xid` ) → `integer` Returns the number of transactions between the supplied transaction id and the current transaction counter.
`mxid_age` ( `xid` ) → `integer` Returns the number of multixacts IDs between the supplied multixact ID and the current multixacts counter.
`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.

`pg_get_multixact_members` ( `multixid` `xid` ) → `setof record` ( `xid` `xid`, `mode` `text` )

Returns the transaction ID and lock mode for each member of the specified multixact ID. The lock modes `forupd`, `fornokeyupd`, `sh`, and `keysh` correspond to the row-level locks `FOR UPDATE`, `FOR NO KEY UPDATE`, `FOR SHARE`, and `FOR KEY SHARE`, respectively, as described in Section 13.3.2. Two additional modes are specific to multixacts: `nokeyupd`, used by updates that do not modify key columns, and `upd`, used by updates or deletes that modify key columns.

The internal transaction ID type `xid` is 32 bits wide and wraps around every 4 billion transactions. However, the functions shown in Table 9.78, except `age`, `mxid_age`, and `pg_get_multixact_members`, 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 9.79. `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 9.79. 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 9.80.

Table 9.80. 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 9.81 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 9.81. Committed Transaction Information Functions

Function Description
`pg_xact_commit_timestamp` ( `xid` ) → `timestamp with time zone` Returns the commit timestamp of a transaction.
`pg_xact_commit_timestamp_origin` ( `xid` ) → `record` ( `timestamp` `timestamp with time zone`, `roident` `oid`) Returns the commit timestamp and replication origin of a transaction.
`pg_last_committed_xact` () → `record` ( `xid` `xid`, `timestamp` `timestamp with time zone`, `roident` `oid` ) Returns the transaction ID, commit timestamp and replication origin of the latest committed transaction.

The functions shown in Table 9.82 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 9.82. Control Data Functions

Function Description
`pg_control_checkpoint` () → `record` Returns information about current checkpoint state, as shown in Table 9.83.
`pg_control_system` () → `record` Returns information about current control file state, as shown in Table 9.84.
`pg_control_init` () → `record` Returns information about cluster initialization state, as shown in Table 9.85.
`pg_control_recovery` () → `record` Returns information about recovery state, as shown in Table 9.86.

Table 9.83. `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 9.84. `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 9.85. `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 9.86. `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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