Thread: pg_stat_activity doubts

On 3/22/18 8:45 AM, Debraj Manna wrote:

Hi

Can someone resolve my below doubts about pg_stat_activity :-

1. Can someone let me know how pg_stat_activity is populated? If I am executing a query like `Select * from pg_stat_activity` does it produce the details from some snapshot maintained by postgres or it checks the current connections open in postgres to provide the details?
2. Can some let me know what does the state IDLE denotes in the row of pg_stat_activity? Is it like it checks all the open connections to postgres and if the connection is not executing any query then IDLE denotes the last query executed by the connection?
3. In the output of select * from pg_stat_activity I am seeing rows with state IDLE and the query_start_date more than 2 days old? What does this signify ? Should I be worried about this?
4.  

You can use psql -E to trace the source :

template1=# \d pg_stat_activity

********* QUERY **********
SELECT c.oid,
  n.nspname,
  c.relname
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE c.relname OPERATOR(pg_catalog.~) '^(pg_stat_activity)$'
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 2, 3;
**************************

********* QUERY **********
SELECT c.relchecks, c.relkind, c.relhasindex, c.relhasrules, c.relhastriggers, c.relhasoids, '', c.reltablespace, CASE WHEN c.reloftype = 0 THEN '' ELSE c.reloftype::pg_catalog.regtype::pg_catalog.text END, c.relpersistence, c.relreplident
FROM pg_catalog.pg_class c
 LEFT JOIN pg_catalog.pg_class tc ON (c.reltoastrelid = tc.oid)
WHERE c.oid = '11283';
**************************

********* QUERY **********
SELECT a.attname,
  pg_catalog.format_type(a.atttypid, a.atttypmod),
  (SELECT substring(pg_catalog.pg_get_expr(d.adbin, d.adrelid) for 128)
   FROM pg_catalog.pg_attrdef d
   WHERE d.adrelid = a.attrelid AND d.adnum = a.attnum AND a.atthasdef),
  a.attnotnull, a.attnum,
  (SELECT c.collname FROM pg_catalog.pg_collation c, pg_catalog.pg_type t
   WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation) AS attcollation,
  NULL AS indexdef,
  NULL AS attfdwoptions
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = '11283' AND a.attnum > 0 AND NOT a.attisdropped
ORDER BY a.attnum;
**************************

           View "pg_catalog.pg_stat_activity"
      Column      |           Type           | Modifiers
------------------+--------------------------+-----------
 datid            | oid                      |
 datname          | name                     |
 pid              | integer                  |
 usesysid         | oid                      |
 usename          | name                     |
 application_name | text                     |
 client_addr      | inet                     |
 client_hostname  | text                     |
 client_port      | integer                  |
 backend_start    | timestamp with time zone |
 xact_start       | timestamp with time zone |
 query_start      | timestamp with time zone |
 state_change     | timestamp with time zone |
 waiting          | boolean                  |
 state            | text                     |
 backend_xid      | xid                      |
 backend_xmin     | xid                      |
 query            | text                     |