pg_stat_activity

When using the statistics to monitor current activity, it is important to realize that the information does not update instantaneously. Each individual server process transmits new block and row access counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by stats_command_string is always up-to-date.

Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will appear not to change as long as you continue the current transaction. Similarly, information about the current queries of all processes is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. But if you want to see new results with each query, be sure to do the queries outside any transaction block.

pg_stat_activity can provide information about the status of query that run on the main node.If you want to see the global status of query, seepg_stat_activity_global

Table 1. pg_catalog.pg_stat_activity

column	type	references	description
datid	oid		database oid
datname	name		database name
procpid	integer		process ID
sess_id	integer		session ID
usesysid	oid		user OID
usename	name		user name
current_query	text		current query
waiting	boolean		query’s waiting status
query_start	timestamp with time zone		time at which the current query began execution
backend_start	timestamp with time zone		time at which he process was started
client_addr	inet		client’s address number
client_port	integer		client’s port number
application_name	text		the name of backend process connect to server
xact_start	timestamp with time zone		time at which the current transaction started. Null when the process is without transaction.
waiting_resource	boolean		status of waiting for resource manager.