postgres=# \d+ pg_stat_activity
View "pg_catalog.pg_stat_activity"
Column | Type | Modifiers | Storage | Description
------------------+--------------------------+-----------+----------+-------------
datid | oid | | plain |
datname | name | | plain |
pid | integer | | plain |
usesysid | oid | | plain |
usename | name | | plain |
application_name | text | | extended |
client_addr | inet | | main |
client_hostname | text | | extended |
client_port | integer | | plain |
backend_start | timestamp with time zone | | plain |
xact_start | timestamp with time zone | | plain |
query_start | timestamp with time zone | | plain |
state_change | timestamp with time zone | | plain |
waiting | boolean | | plain |
state | text | | extended |
query | text | | extended |
View deFinition:
SELECT s.datid,d.datname,s.pid,s.usesysid,u.rolname AS usename,s.application_name,s.client_addr,s.client_hostname,s.client_port,s.backend_start,s.xact_start,s.query_start,s.state_change,s.waiting,s.state,s.query
FROM pg_database d,pg_stat_get_activity(NULL::integer) s(datid,pid,usesysid,application_name,state,query,waiting,xact_start,query_start,backend_start,state_change,client_addr,client_hostname,client_port),pg_authid u
WHERE s.datid = d.oid AND s.usesysid = u.oid;
可以看出，pg_stat_activity是一个系统视图。
1. 官方解释：
One row per server process,showing information related to the current activity of that process,such as state and current query.
每一行都表示一个系统进程，显示与当前会话的活动进程的一些信息，比如当前回话的状态和查询等。
2. 详细信息。
| 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,itindicates(表明) either that the client is connected via a Unixsocket(插座) on the server machine or that this is aninternal(内部的) process such as autovacuum. |
| client_hostname | text | Host name of the connected client,as reported by a reverse(相反) DNS lookup(查找) of client_addr. This field will only be non-null for IP connections,and only whenlog_hostname is enabled. |
| client_port | integer | TCP port number that the client is using for communication with this backend,or-1 if a Unixsocket(插座) is used |
| backend_start | timestamp(时间戳) 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(交易) 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(时间戳) 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:
|
| query | text | Text of this backend's most recent query. Ifstate isactive this field shows the currently executing query. In all other states,it shows the last query that was executed. |
3. 举例说明（讲解waiting，state，query的意义）
postgres=# select * from pg_stat_activity ; datid | datname | pid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_start | xact_start | query_start | state_change | waiting | state | query -------+----------+------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+-------------------------------+----------------------------- --+-------------------------------+---------+--------+---------------------------------- 12860 | postgres | 2534 | 10 | postgres | psql | | | -1 | 2015-03-24 08:15:42.099642+08 | 2015-03-24 09:15:41.756177+08 | 2015-03-24 09:15:41.756177+0 8 | 2015-03-24 09:15:41.756181+08 | f | active | select * from pg_stat_activity ; (1 row)当前用户waiting=f，stat=active，query=select * from pg_stat_activity.
> idle：表示当前用户空闲。
postgres=# select * from pg_stat_activity ; datid | datname | pid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_start | xact_start | query_start | state_change | waiting | state | query -------+----------+------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+-------------------------------+----------------------------- --+-------------------------------+---------+--------+---------------------------------- 12860 | postgres | 2534 | 10 | postgres | psql | | | -1 | 2015-03-24 08:15:42.099642+08 | 2015-03-24 09:19:55.516083+08 | 2015-03-24 09:19:55.516083+0 8 | 2015-03-24 09:19:55.516087+08 | f | active | select * from pg_stat_activity ; 12860 | postgres | 5084 | 10 | postgres | psql | | | -1 | 2015-03-24 09:19:39.404898+08 | | | 2015-03-24 09:19:39.407193+08 | f | idle | (2 rows)可以看到，第二个postgres 的stat=idle,表示的是空闲状态，等待命令的输入。
>idle in transaction:表示当前用户在事务中。
postgres=# begin ; BEGIN postgres=# select * from book; id | name | tag ----+------+---------- 1 | java | aa,bb,cc 2 | C++ | dd,ee (2 rows)另一个客户端启动，查询：
postgres=# select * from pg_stat_activity ; datid | datname | pid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_start | xact_start | query_start | state_change | waiting | state | query -------+----------+------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+-------------------------------+----------------------------- --+-------------------------------+---------+---------------------+---------------------------------- 12860 | postgres | 5500 | 10 | postgres | psql | | | -1 | 2015-03-24 10:22:21.344358+08 | 2015-03-24 10:23:44.615123+08 | 2015-03-24 10:23:49.750407+0 8 | 2015-03-24 10:23:49.76975+08 | f | idle in transaction | select * from book; 12860 | postgres | 5510 | 10 | postgres | psql | | | -1 | 2015-03-24 10:23:53.79722+08 | 2015-03-24 10:24:02.64689+08 | 2015-03-24 10:24:02.64689+08 | 2015-03-24 10:24:02.646894+08 | f | active | select * from pg_stat_activity ; (2 rows)可以看到，一个postgres用户的stat=idle in transcation,表示在事务中。
> idle in transaction (aborted)：表示当前用户在事务中，但是已经发生错误。
postgres=# begin ; BEGIN postgres=# select * from book; id | name | tag ----+------+---------- 1 | java | aa,ee (2 rows) postgres=# s; ERROR: Syntax error at or near "s" LINE 1: s; ^另一个客户端启动，查询：
postgres=# select * from pg_stat_activity ; datid | datname | pid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_start | xact_start | query_start | state_change | waiting | state | query -------+----------+------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+-------------------------------+----------------------------- --+-------------------------------+---------+-------------------------------+---------------------------------- 12860 | postgres | 5500 | 10 | postgres | psql | | | -1 | 2015-03-24 10:22:21.344358+08 | | 2015-03-24 10:28:55.763925+0 8 | 2015-03-24 10:28:55.764049+08 | f | idle in transaction (aborted) | s; 12860 | postgres | 5510 | 10 | postgres | psql | | | -1 | 2015-03-24 10:23:53.79722+08 | 2015-03-24 10:28:58.587982+08 | 2015-03-24 10:28:58.587982+0 8 | 2015-03-24 10:28:58.588028+08 | f | active | select * from pg_stat_activity ; (2 rows)可以看到，一个postgres用户的stat=idle in transcation（aborted）,表示在事务中发生错误了。
一个进程等待的例子：
postgres=# begin ; BEGIN postgres=# alter table book add column addr character varying; ALTER TABLE postgres=#注意还没有提交；
postgres=# select * from book;
发现查询语句等在那里，是因为在进行增加字段操作的时候，会有表锁，锁没有释放之前其他进程无法访问该表。
查看进程状态：
postgres=# select * from pg_stat_activity ; datid | datname | pid | usesysid | usename | application_name | client_addr | client_hostname | client_port | backend_start | xact_start | query_start | state_change | waiting | state | query -------+----------+------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+-------------------------------+----------------------------- --+-------------------------------+---------+---------------------+------------------------------------------------------ 12860 | postgres | 5500 | 10 | postgres | psql | | | -1 | 2015-03-24 10:22:21.344358+08 | 2015-03-24 10:55:29.238829+08 | 2015-03-24 10:55:34.670946+0 8 | 2015-03-24 10:55:34.816089+08 | f | idle in transaction | alter table book add column addr character varying; 12860 | postgres | 5510 | 10 | postgres | psql | | | -1 | 2015-03-24 10:23:53.79722+08 | 2015-03-24 10:55:46.919415+08 | 2015-03-24 10:55:46.919415+0 8 | 2015-03-24 10:55:46.919419+08 | t | active | select * from book; 12860 | postgres | 5764 | 10 | postgres | psql | | | -1 | 2015-03-24 11:02:37.09896+08 | 2015-03-24 11:02:52.233883+08 | 2015-03-24 11:02:52.233883+0 8 | 2015-03-24 11:02:52.233886+08 | f | active | select * from pg_stat_activity ; (3 rows)可以看到第二个进程的waiting=t。
