Querying a System Catalog

In addition to the created tables, a database contains many system catalogs. These system catalogs contain MogDB installation information and information about various queries and processes in MogDB. You can collect information about the database by querying system catalogs.

In System Catalogs and System Views, the description about each table indicates whether the table is visible to all users or only the initial user. To query tables that are visible only to the initial user, log in as the user.

MogDB provides the following types of system catalogs and views:

System catalogs and views inherited from PG

These system catalogs and views have the prefix PG.
New system catalogs and views of MogDB

These system catalogs and views have the prefix GS.

Querying Database Tables

For example, you can run the following command to query the PG_TABLES system catalog for all tables in the public schema:

SELECT distinct(tablename) FROM pg_tables WHERE SCHEMANAME = 'public';

Information similar to the following is displayed:

     tablename
-------------------
 err_hr_staffs
 test
 err_hr_staffs_ft3
 web_returns_p1
 mig_seq_table
 films4
(6 rows)

Viewing Database Users

You can run the PG_USER command to view the list of all users in the database, and view the user ID (USESYSID) and permissions.

SELECT * FROM pg_user; 
 usename | usesysid | usecreatedb | usesuper | usecatupd | userepl |  passwd  | valbegin |
 valuntil |   respool    | parent | spacelimit | useconfig | nodegroup | tempspacelimit |
spillspacelimit
---------+----------+-------------+----------+-----------+---------+----------+----------+
----------+--------------+--------+------------+-----------+-----------+----------------+-
----------------
 omm     |       10 | t           | t        | t         | t       | ******** |          |
          | default_pool |      0 |            |           |           |                |
 joe     |    16806 | f           | f        | f         | f       | ******** |          |
          | default_pool |      0 |            |           |           |                |
(2 rows)

Viewing and Stopping the Running Query Statements

You can view the running query statements in the PG_STAT_ACTIVITY view. You can use the following methods:

Set the parameter track_activities to on.
```
SET track_activities = on;
```
The database collects the running information about active queries only if the parameter is set to on.

View the running query statements. Run the following command to view the database names, users performing queries, query status, and the corresponding PIDs which are connected to the running query statements:

SELECT datname, usename, state,pid FROM pg_stat_activity;

 datname  | usename | state  |       pid
----------+---------+--------+-----------------
 postgres | Ruby    | active | 140298793514752
 postgres | Ruby    | active | 140298718004992
 postgres | Ruby    | idle   | 140298650908416
 postgres | Ruby    | idle   | 140298625742592
 postgres | omm | active | 140298575406848
(5 rows)

If the state column is idle, the connection is idle and requires a user to enter a command.

To identify only active query statements, run the following command:

SELECT datname, usename, state FROM pg_stat_activity WHERE state != 'idle';

To cancel queries that have been running for a long time, use the PG_TERMINATE_BACKEND function to end sessions based on the thread ID.
```
SELECT PG_TERMINATE_BACKEND(139834759993104);
```
If information similar to the following is displayed, the session is successfully terminated:
```
PG_TERMINATE_BACKEND
----------------------
 t
(1 row)
```
If information similar to the following is displayed, a user has terminated the current session:
```
FATAL:  terminating connection due to administrator command
FATAL:  terminating connection due to administrator command
```
Note:

If the PG_TERMINATE_BACKEND function is used to terminate the backend threads of the current session, the gsql client will be reconnected automatically rather than be logged out. The message “The connection to the server was lost. Attempting reset: Succeeded.” is returned.
```
FATAL:  terminating connection due to administrator command
FATAL:  terminating connection due to administrator command
The connection to the server was lost. Attempting reset: Succeeded.
```