Table of Contents
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 may be needed using
PostgreSQL's EXPLAIN command.
Section 13.1, “Using EXPLAIN
” discusses EXPLAIN
and other methods for understanding the behavior of an individual
query.
On most 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 given 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 connection source host items remain the same for
the life of the client connection, but the activity indicator changes.
The activity may 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 attached if the server process is presently waiting
on a lock held by another server process. In the above example we can infer
that process 1003 is waiting for process 1016 to complete its transaction and
thereby release some lock or other.
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 useful 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.