27. Monitoring Database Activity
A database administrator frequently wonders, «What is the system doing right now?» This chapter discusses how to find that out.
Several tools are available for monitoring database activity and analyzing performance. Most of this chapter is devoted to describing PostgreSQL™'s statistics collector, but one should not neglect regular Unix monitoring programs such as ps, top, iostat, and vmstat. Also, once one has identified a poorly-performing query, further investigation might be needed using PostgreSQL™'s EXPLAIN(7) command. Sezione 14.1, «Usare EXPLAIN» discusses EXPLAIN and other methods for understanding the behavior of an individual query.
27.1. Standard Unix Tools
On most Unix platforms, PostgreSQL™ modifies its command title as reported by ps, so that individual server processes can readily be identified. A sample display is
$ ps auxww | grep ^postgres postgres 960 0.0 1.1 6104 1480 pts/1 SN 13:17 0:00 postgres -i postgres 963 0.0 1.1 7084 1472 pts/1 SN 13:17 0:00 postgres: writer process postgres 965 0.0 1.1 6152 1512 pts/1 SN 13:17 0:00 postgres: stats collector process postgres 998 0.0 2.3 6532 2992 pts/1 SN 13:18 0:00 postgres: tgl runbug 127.0.0.1 idle postgres 1003 0.0 2.4 6532 3128 pts/1 SN 13:19 0:00 postgres: tgl regression [local] SELECT waiting postgres 1016 0.1 2.4 6532 3080 pts/1 SN 13:19 0:00 postgres: tgl regression [local] idle in transaction
(The appropriate invocation of ps varies across different platforms, as do the details of what is shown. This example is from a recent Linux system.) The first process listed here is the master server process. The command arguments shown for it are the same ones used when it was launched. The next two processes are background worker processes automatically launched by the master process. (The «stats collector» process will not be present if you have set the system not to start the statistics collector.) Each of the remaining processes is a server process handling one client connection. Each such process sets its command line display in the form
postgres: user database host activity
The user, database, and (client) host items remain the same for the life of the client connection, but the activity indicator changes. The activity can be idle (i.e., waiting for a client command), idle in transaction (waiting for client inside a BEgin block), or a command type name such as SELECT. Also, waiting is appended if the server process is presently waiting on a lock held by another session. In the above example we can infer that process 1003 is waiting for process 1016 to complete its transaction and thereby release some lock.
If you have turned off update_process_title then the activity indicator is not updated; the process title is set only once when a new process is launched. On some platforms this saves a measurable amount of per-command overhead; on others it's insignificant.
Solaris™ requires special handling. You must use /usr/ucb/ps, rather than /bin/ps. You also must use two w flags, not just one. In addition, your original invocation of the postgres command must have a shorter ps status display than that provided by each server process. If you fail to do all three things, the ps output for each server process will be the original postgres command line.