​
current_setting()

Returns the current value of a specified runtime parameter. This function corresponds to the SQL command SHOW. 
SELECT current_setting ('server_version');
---------RESULT
 current_setting
-----------------
 8.3.0
(1 row)
You can use the SHOW ALL command to get the complete list of runtime parameters and corresponding descriptions.

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has_table_privilege()

Checks if a user has access to a table in a specific way. You can identify the user by their name, their OID (pg_authid.oid), or by using public to refer to the PUBLIC pseudo-role. If no argument is provided, it assumes the current user. To specify the table, you can use its name or OID. If needed, you can schema-qualify the table name. The desired access privilege type is specified as a text string. It must be one of the following values: SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, or TRIGGER. You can also add WITH GRANT OPTION to test if the privilege is held with the grant option. If you want to check for multiple privilege types, you can list them separated by commas. The result will be true if any of the listed privileges is held. The case of the privilege string is not important, and you can have extra whitespace between privilege names, but not within them.
Syntax
has_table_privilege([user,] table, privilege) -> boolean
Example
SELECT has_table_privilege('test_user', 'foo', 'SELECT');
----RESULT
t

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has_schema_privilege()

Checks if a user has access to a schema in a specific way. It has similar argument possibilities as the has_table_privilege function. The desired access privilege type should be a combination of CREATE and/or USAGE, such as “CREATE, USAGE”, or just a single privilege type like “CREATE” or “USAGE”.
Syntax
has_schema_privilege([user,] schema, privilege) -> boolean
Example
SELECT has_schema_privilege('test_user', 'test_schema', 'CREATE');
----RESULT
t

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has_any_column_privilege()

Checks if a user has access to any column of a table in a specific way. Currently, this function is identical to the has_table_privilege function.
Syntax
has_any_column_privilege([user,] table, privilege) -> boolean
Example
SELECT has_any_column_privilege('test_user', 'foo_view'::regclass, 'INSERT');
----RESULT
f

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has_function_privilege()

Checks if a user has access to a function. If no user is provided, it assumes the current user.
Syntax
has_function_privilege([user], function, privilege); -> boolean
Example
SELECT has_function_privilege('test_user', 'foo_func(int)', 'EXECUTE');
----RESULT
t

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rw_recovery_status()

Retrieves the current recovery status of the cluster. The return values can be one of STARTING, RECOVERING, or RUNNING.
Syntax
rw_recovery_status() -> varchar
Example
SELECT rw_recovery_status();
-- RESULT
'RUNNING'

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pg_is_in_recovery()

Checks if the PostgreSQL instance is currently in recovery mode.
Syntax
pg_is_in_recovery() -> boolean
Example
SELECT pg_is_in_recovery();
----RESULT
t

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set_config()

Syntax
set_config ( setting_name text, new_value text, is_local boolean ) → text
Sets the parameter setting_name to new_value, and returns that value. If is_local is true, the new value will only apply during the current transaction. If you want the new value to apply for the rest of the current session, use false instead. This function corresponds to the SQL command SET.
Example
SELECT set_config('rw_implicit_flush', 'true', false);
-------
true

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pg_indexes_size('table_name')

Returns the total size of all indexes associated with a particular table in bytes. 
SELECT pg_indexes_size('t1');
---------RESULT
 pg_indexes_size
-----------------
               0
(1 row)

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pg_table_size('table_name')

Returns the size of a table in bytes. 
SELECT pg_table_size('t1');
---------RESULT
 pg_table_size
---------------
           240
(1 row)

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pg_stat_get_numscans('table_name')

Returns the total number of scans (sequential and index scans) performed on a specified table since the server was started. 
SELECT pg_stat_get_numscans('my_table');
 pg_stat_get_numscans
----------------------
                    0
(1 row)
This is a dummy function intended for compatibility with third-party tools. We keep it here only for reference and it will be eventually removed. Please do not use it in production environments or any important tasks.