PostgreSQL: Monitoring the status of your queries currently running in the database

The pg_stat_activity view has one row per server process, showing information related to the current activity of that process.

pg_stat_activity view

Column	Type	Description
datid	oid	OID of the database this backend is connected to
datname	name	Name of the database this backend is connected to
pid	integer	Process ID of this backend
usesysid	oid	OID of the user logged into this backend
usename	name	Name of the user logged into this backend
application_name	text	Name of the application that is connected to this backend
client_addr	inet	IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum.
client_hostname	text	Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.
client_port	integer	TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used
backend_start	timestamp with time zone	Time when this process was started, i.e., when the client connected to the server
xact_start	timestamp with time zone	Time when this process' current transaction was started, or null if no transaction is active. If the current query is the first of its transaction, this column is equal to the query_start column.
query_start	timestamp with time zone	Time when the currently active query was started, or if state is not active, when the last query was started
state_change	timestamp with time zone	Time when the state was last changed
waiting	boolean	True if this backend is currently waiting on a lock
state	text	Current overall state of this backend. Possible values are: active: The backend is executing a query. idle: The backend is waiting for a new client command. idle in transaction: The backend is in a transaction, but is not currently executing a query. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. fastpath function call: The backend is executing a fast-path function. disabled: This state is reported if track_activities is disabled in this backend.
query	text	Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed.

For example, you can use the following query to see the state, text, and running period of your statement.

SELECT pid, usename, query, state, age(clock_timestamp(), query_start)
FROM pg_stat_activity
WHERE usename = '<your database login>';

If you want to see if your query is holding or waiting on any locks, you can run the query below.

SELECT a.pid, a.state, L.mode, L.locktype, L.granted
FROM pg_stat_activity a
INNER JOIN pg_locks L on a.pid = L.pid
WHERE a.usename = '<your database login>'
order by L.granted desc;

References:

1. Monitoring Database Activity

2.Useful PostgreSQL Queries For Monitoring Purpose

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postgresql alter table hangs

When the SQL, "Alter table Fruit add Column Location VarChar(30))", is executed at the psql terminal, it hangs.

It turns out that there is a lock on a table. The SQL below helps find all the locks in your database.

SELECT blocked_locks.pid AS blocked_pid, blocked_activity.usename AS blocked_user, blocking_locks.pid AS blocking_pid, blocking_activity.usename AS blocking_user, blocked_activity.query AS blocked_statement, blocking_activity.query AS current_statement_in_blocking_process
FROM pg_catalog.pg_locks blocked_locks
JOIN pg_catalog.pg_stat_activity blocked_activity ON blocked_activity.pid = blocked_locks.pid
JOIN pg_catalog.pg_locks blocking_locks
ON blocking_locks.locktype = blocked_locks.locktype
AND blocking_locks.DATABASE IS NOT DISTINCT FROM blocked_locks.DATABASE AND blocking_locks.relation IS NOT DISTINCT FROM blocked_locks.relation
AND blocking_locks.page IS NOT DISTINCT FROM blocked_locks.page
AND blocking_locks.tuple IS NOT DISTINCT FROM blocked_locks.tuple
AND blocking_locks.virtualxid IS NOT DISTINCT FROM blocked_locks.virtualxid
AND blocking_locks.transactionid IS NOT DISTINCT FROM blocked_locks.transactionid
AND blocking_locks.classid IS NOT DISTINCT FROM blocked_locks.classid
AND blocking_locks.objid IS NOT DISTINCT FROM blocked_locks.objid
AND blocking_locks.objsubid IS NOT DISTINCT FROM blocked_locks.objsubid
AND blocking_locks.pid != blocked_locks.pid
JOIN pg_catalog.pg_stat_activity blocking_activity ON blocking_activity.pid = blocking_locks.pid
WHERE NOT blocked_locks.GRANTED;

Then use the SQL below to kill the blocking process.

select pg_terminate_backend(<blocking_pid>);

Reference:

Lock Monitoring

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