pg_stat_activity View explanation in postgresSql

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

Note: The waiting and state columns are independent. If a backend is in the active state, it may or may not be waiting. If the state is active and waiting is true, it means that a query is being executed, but is being blocked by a lock somewhere in the system.

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
wait_event_type	text	The type of event for which the backend is waiting, if any; otherwise NULL. Possible values are: LWLockNamed: The backend is waiting for a specific named lightweight lock. Each such lock protects a particular data structure in shared memory.wait_event will contain the name of the lightweight lock. LWLockTranche: The backend is waiting for one of a group of related lightweight locks. All locks in the group perform a similar function; wait_eventwill identify the general purpose of locks in that group. Lock: The backend is waiting for a heavyweight lock. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. wait_event will identify the type of lock awaited. BufferPin: The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question.
wait_event	text	Wait event name if backend is currently waiting, otherwise NULL. See Table 28-4 for details.
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.
backend_xid	xid	Top-level transaction identifier of this backend, if any.
backend_xmin	xid	The current backend's xmin horizon.
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.

The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.
Note: The wait_event and state columns are independent. If a backend is in the activestate, it may or may not be waiting on some event. If the state is active and wait_eventis non-null, it means that a query is being executed, but is being blocked somewhere in the system.
current running query find out

database1=# SELECT count(*) as cnt, usename, current_query FROM pg_stat_activity GROUP BY usename,current_query ORDER BY cnt DESC;

 cnt |    usename    | current_query
-----+---------------+---------------
   7 | freddykrueger | <IDLE>
   3 | freddykrueger | SELECT name FROM users WHERE id=50;
   1 | postgres      | <IDLE>
(3 rows)

Here is an example of how wait events can be viewed

SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
 pid  | wait_event_type |  wait_event
------+-----------------+---------------
 2540 | Lock            | relation
 6644 | LWLockNamed     | ProcArrayLock
(2 rows)

Note: For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. It is quite possible that user has registered the tranche in one of the backends (by having allocation in dynamic shared memory) in which case other backends won't have that information, so we display extension for such cases.
wait_event Description:

Wait Event Type	Wait Event Name	Description
LWLockNamed	ShmemIndexLock	Waiting to find or allocate space in shared memory.
	OidGenLock	Waiting to allocate or assign an OID.
	XidGenLock	Waiting to allocate or assign a transaction id.
	ProcArrayLock	Waiting to get a snapshot or clearing a transaction id at transaction end.
	SInvalReadLock	Waiting to retrieve or remove messages from shared invalidation queue.
	SInvalWriteLock	Waiting to add a message in shared invalidation queue.
	WALBufMappingLock	Waiting to replace a page in WAL buffers.
	WALWriteLock	Waiting for WAL buffers to be written to disk.
	ControlFileLock	Waiting to read or update the control file or creation of a new WAL file.
	CheckpointLock	Waiting to perform checkpoint.
	CLogControlLock	Waiting to read or update transaction status.
	SubtransControlLock	Waiting to read or update sub-transaction information.
	MultiXactGenLock	Waiting to read or update shared multixact state.
	MultiXactOffsetControlLock	Waiting to read or update multixact offset mappings.
	MultiXactMemberControlLock	Waiting to read or update multixact member mappings.
	RelCacheInitLock	Waiting to read or write relation cache initialization file.
	CheckpointerCommLock	Waiting to manage fsync requests.
	TwoPhaseStateLock	Waiting to read or update the state of prepared transactions.
	TablespaceCreateLock	Waiting to create or drop the tablespace.
	BtreeVacuumLock	Waiting to read or update vacuum-related information for a B-tree index.
	AddinShmemInitLock	Waiting to manage space allocation in shared memory.
	AutovacuumLock	Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers.
	AutovacuumScheduleLock	Waiting to ensure that the table it has selected for a vacuum still needs vacuuming.
	SyncScanLock	Waiting to get the start location of a scan on a table for synchronized scans.
	RelationMappingLock	Waiting to update the relation map file used to store catalog to filenode mapping.
	AsyncCtlLock	Waiting to read or update shared notification state.
	AsyncQueueLock	Waiting to read or update notification messages.
	SerializableXactHashLock	Waiting to retrieve or store information about serializable transactions.
	SerializableFinishedListLock	Waiting to access the list of finished serializable transactions.
	SerializablePredicateLockListLock	Waiting to perform an operation on a list of locks held by serializable transactions.
	OldSerXidLock	Waiting to read or record conflicting serializable transactions.
	SyncRepLock	Waiting to read or update information about synchronous replicas.
	BackgroundWorkerLock	Waiting to read or update background worker state.
	DynamicSharedMemoryControlLock	Waiting to read or update dynamic shared memory state.
	AutoFileLock	Waiting to update the postgresql.auto.conf file.
	ReplicationSlotAllocationLock	Waiting to allocate or free a replication slot.
	ReplicationSlotControlLock	Waiting to read or update replication slot state.
	CommitTsControlLock	Waiting to read or update transaction commit timestamps.
	CommitTsLock	Waiting to read or update the last value set for the transaction timestamp.
	ReplicationOriginLock	Waiting to setup, drop or use replication origin.
	MultiXactTruncationLock	Waiting to read or truncate multixact information.
	OldSnapshotTimeMapLock	Waiting to read or update old snapshot control information.
LWLockTranche	clog	Waiting for I/O on a clog (transaction status) buffer.
	commit_timestamp	Waiting for I/O on commit timestamp buffer.
	subtrans	Waiting for I/O a subtransaction buffer.
	multixact_offset	Waiting for I/O on a multixact offset buffer.
	multixact_member	Waiting for I/O on a multixact_member buffer.
	async	Waiting for I/O on an async (notify) buffer.
	oldserxid	Waiting to I/O on an oldserxid buffer.
	wal_insert	Waiting to insert WAL into a memory buffer.
	buffer_content	Waiting to read or write a data page in memory.
	buffer_io	Waiting for I/O on a data page.
	replication_origin	Waiting to read or update the replication progress.
	replication_slot_io	Waiting for I/O on a replication slot.
	proc	Waiting to read or update the fast-path lock information.
	buffer_mapping	Waiting to associate a data block with a buffer in the buffer pool.
	lock_manager	Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query).
	predicate_lock_manager	Waiting to add or examine predicate lock information.
Lock	relation	Waiting to acquire a lock on a relation.
	extend	Waiting to extend a relation.
	page	Waiting to acquire a lock on page of a relation.
	tuple	Waiting to acquire a lock on a tuple.
	transactionid	Waiting for a transaction to finish.
	virtualxid	Waiting to acquire a virtual xid lock.
	speculative token	Waiting to acquire a speculative insertion lock.
	object	Waiting to acquire a lock on a non-relation database object.
	userlock	Waiting to acquire a userlock.
	advisory	Waiting to acquire an advisory user lock.
BufferPin	BufferPin	Waiting to acquire a pin on a buffer.