Hosting Operations
Read-onlyCheck PostgreSQL Lock Waits
Writes are stuck and you need to see whether sessions are waiting on locks.
Command
psql -X -c "select pid, wait_event_type, wait_event, state, left(query, 80) as query from pg_stat_activity where wait_event_type is not null order by pid;"Before you run this
System impact: Read-only. Low when scoped to the shown target.
When not to use it: Do not use this alone to decide which backend to terminate.
Expected output
Rows showing waiting PIDs, wait event type, wait event, state, and query prefix.
System impact
Read-only. Nothing changes. PostgreSQL prints sessions with wait events.
Recovery / rollback: no state is changed.
When to use it
Use when requests hang, migrations stall, or writes stop moving.
When not to use it
Do not use this alone to decide which backend to terminate.
Watch this command run
Command transcript
This sanitized transcript shows the commands and output shape without exposing host details.
$ psql -X -c "select pid, wait_event_type, wait_event, state, left(query, 80) as query from pg_stat_activity where wait_event_type is not null order by pid;"
pid | wait_event_type | wait_event | state | query
-----+-----------------+--------------+--------+----------------------------------
518 | Lock | relation | active | alter table orders add column
522 | IO | DataFileRead | active | select * from reports
(2 rows)$ psql -X -c "select wait_event_type, count(*) from pg_stat_activity where wait_event_type is not null group by wait_event_type;"
wait_event_type | count
-----------------+-------
Lock | 1
IO | 1
(2 rows)View commands shown
These are the commands shown in the sanitized transcript.
Commands shown
psql -X -c "select pid, wait_event_type, wait_event, state, left(query, 80) as query from pg_stat_activity where wait_event_type is not null order by pid;"psql -X -c "select wait_event_type, count(*) from pg_stat_activity where wait_event_type is not null group by wait_event_type;"
next steps
Related commands
Find Long-Running PostgreSQL Queries
One query can make the whole app look broken.
psql -X -c "select pid, now() - query_start as age, state, left(query, 80) as query from pg_stat_activity where query_start is not null order by age desc limit 10;"
Show Active PostgreSQL Connections
The database was not down. It was full.
psql -X -A -F '|' -c "select pid,usename,datname,state,client_addr from pg_stat_activity order by state, pid;"
Find Long-Running MySQL Queries
One old query explained the whole slowdown.
mysql -e "select id,user,host,db,command,time,state,left(info,80) as info from information_schema.processlist where command <> 'Sleep' order by time desc limit 10;"
Show PostgreSQL Database Sizes
Disk pressure starts with knowing what grew.
psql -X -c "select datname, pg_size_pretty(pg_database_size(datname)) as size from pg_database order by pg_database_size(datname) desc;"
Show MySQL Database Sizes
The storage alert needed a database name.
mysql -e "select table_schema, round(sum(data_length + index_length)/1024/1024, 1) as mb from information_schema.tables group by table_schema order by mb desc;"
Study mapping
Use this as independent command practice: read the notes, predict the output, then compare it with the example before using a real shell.
Useful for
- LPIC-1 style command-line practice
- LFCS style performance tasks
- Linux+ style troubleshooting review
Independent study support only. No affiliation, endorsement, exam dumps, or real exam questions.