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> and top>. Also, once one has identified a
poorly-performing query, further investigation may be needed using
PostgreSQL's EXPLAIN> command.
The User's Guide discusses EXPLAIN>
and other methods for understanding the behavior of an individual
query.
Standard Unix Tools
ps
to monitor activity
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 postmaster -i
postgres 963 0.0 1.1 7084 1472 pts/1 SN 13:17 0:00 postgres: stats buffer 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
postmaster>, the master server process. The command arguments
shown for it are the same ones given when it was launched. The next two
processes implement the statistics collector, which will be described in
detail in the next section. (These 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> (ie, 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 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.
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
postmaster must have a shorter
ps status display than that provided by each
backend. If you fail to do all three things, the ps>
output for each backend will be the original postmaster>
command line.
Statistics Collector
statistics
PostgreSQL's statistics collector>
is a subsystem that supports collection and reporting of information about
server activity. Presently, the collector can count accesses to tables
and indexes in both disk-block and individual-row terms. It also supports
determining the exact query currently being executed by other server
processes.
Statistics Collection Configuration
Since collection of statistics adds some overhead to query execution,
the system can be configured to collect or not collect information.
This is controlled by configuration variables that are normally set in
postgresql.conf> (see for
details about setting configuration variables).
The variable STATS_START_COLLECTOR must be set to
true> for the statistics collector to
be launched at all. This is the default and recommended setting,
but it may be turned off if you have no interest in statistics and
want to squeeze out every last drop of overhead. (The savings is
likely to be small, however.) Note that this option
cannot be changed while the server is running.
The variables STATS_COMMAND_STRING,
STATS_BLOCK_LEVEL,
and STATS_ROW_LEVEL control how much information is
actually sent to the collector, and thus determine how much run-time
overhead occurs. These respectively determine whether a server process
sends its current command string, disk-block-level access statistics, and
row-level access statistics to the collector. Normally these variables are
set in postgresql.conf> so that they apply to all server
processes, but it is possible to turn them on or off in individual server
processes using the SET> command. (To prevent ordinary users
from hiding their activity from the administrator, only superusers are
allowed to change these variables with SET>.)
Since the variables STATS_COMMAND_STRING,
STATS_BLOCK_LEVEL,
and STATS_ROW_LEVEL
default to false>, no statistics are actually collected
in the default configuration! You must turn one or more of them on
before you will get useful results from the statistical display
functions.
Viewing Collected Statistics
Several predefined views are available to show the results of
statistics collection. Alternatively, one can build custom views
using the underlying statistics functions.
When using the statistics to monitor current activity, it is important
to realize that the information does not update instantaneously.
Each individual server process transmits new access counts to the collector
just before waiting for another client command; so a query still in
progress does not affect the displayed totals. Also, the collector itself
emits new totals at most once per PGSTAT_STAT_INTERVAL (500 milliseconds
by default). So the displayed totals lag behind actual activity.
Another important point is that when a server process is asked to display
any of these statistics, it first fetches the most recent totals emitted by
the collector process. It then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will appear not to change as long as you continue the
current transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block.
Standard Statistics Views
View Name
Description
pg_stat_activity>
One row per server process, showing process PID, database,
user, and current query. The current query column is only available
to superusers; for others it reads as NULL. (Note that because of
the collector's reporting delay, current query will only be up-to-date
for long-running queries.)
pg_stat_database>
One row per database, showing number of active backends,
total transactions committed and total rolled back in that database,
total disk blocks read, and total number of buffer hits (ie, block
read requests avoided by finding the block already in buffer cache).
pg_stat_all_tables>
For each table in the current database, total numbers of
sequential and index scans, total numbers of tuples returned by
each type of scan, and totals of tuple insertions, updates,
and deletes.
pg_stat_sys_tables>
Same as pg_stat_all_tables, except that only system tables
are shown.
pg_stat_user_tables>
Same as pg_stat_all_tables, except that only user tables
are shown.
pg_stat_all_indexes>
For each index in the current database, the total number
of index scans that have used that index, the number of index tuples
read, and the number of successfully fetched heap tuples (this may
be less when there are index entries pointing to expired heap tuples).
pg_stat_sys_indexes>
Same as pg_stat_all_indexes, except that only indexes on
system tables are shown.
pg_stat_user_indexes>
Same as pg_stat_all_indexes, except that only indexes on
user tables are shown.
pg_statio_all_tables>
For each table in the current database, the total number of disk
blocks read from that table, the number of buffer hits, the numbers of
disk blocks read and buffer hits in all the indexes of that table,
the numbers of disk blocks read and buffer hits from the table's
auxiliary TOAST table (if any), and the numbers of disk blocks read
and buffer hits for the TOAST table's index.
pg_statio_sys_tables>
Same as pg_statio_all_tables, except that only system tables
are shown.
pg_statio_user_tables>
Same as pg_statio_all_tables, except that only user tables
are shown.
pg_statio_all_indexes>
For each index in the current database, the numbers of
disk blocks read and buffer hits in that index.
pg_statio_sys_indexes>
Same as pg_statio_all_indexes, except that only indexes on
system tables are shown.
pg_statio_user_indexes>
Same as pg_statio_all_indexes, except that only indexes on
user tables are shown.
pg_statio_all_sequences>
For each sequence object in the current database, the numbers
of disk blocks read and buffer hits in that sequence.
pg_statio_sys_sequences>
Same as pg_statio_all_sequences, except that only system
sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.)
pg_statio_user_sequences>
Same as pg_statio_all_sequences, except that only user
sequences are shown.
The per-index statistics are particularly useful to determine which
indexes are being used and how effective they are.
The pg_statio_> views are primarily useful to determine
the effectiveness of the buffer cache. When the number of actual disk
reads is much smaller than the number of buffer hits, then the cache
is satisfying most read requests without invoking a kernel call.
Other ways of looking at the statistics can be set up by writing queries
that use the same underlying statistics access functions as these standard
views do. The per-database access functions accept a database OID to
identify which database to report on. The per-table and per-index
functions accept a table or index OID (note that only tables and indexes
in the current
database can be seen with these functions). The per-backend access
functions accept a backend ID number, which ranges from one to the number
of currently active backends.
Statistics Access Functions
Function
Return Type
Description
pg_stat_get_db_numbackends(oid)
integer
Number of active backends in database
pg_stat_get_db_xact_commit(oid)
bigint
Transactions committed in database
pg_stat_get_db_xact_rollback(oid)
bigint
Transactions rolled back in database
pg_stat_get_db_blocks_fetched(oid)
bigint
Number of disk block fetch requests for database
pg_stat_get_db_blocks_hit(oid)
bigint
Number of disk block requests found in cache for database
pg_stat_get_numscans(oid)
bigint
Number of sequential scans done when argument is a table,
or number of index scans done when argument is an index
pg_stat_get_tuples_returned(oid)
bigint
Number of tuples read by sequential scans when argument is a table,
or number of index tuples read when argument is an index
pg_stat_get_tuples_fetched(oid)
bigint
Number of valid (unexpired) table tuples fetched by sequential scans
when argument is a table, or fetched by index scans using this index
when argument is an index
pg_stat_get_tuples_inserted(oid)
bigint
Number of tuples inserted into table
pg_stat_get_tuples_updated(oid)
bigint
Number of tuples updated in table
pg_stat_get_tuples_deleted(oid)
bigint
Number of tuples deleted from table
pg_stat_get_blocks_fetched(oid)
bigint
Number of disk block fetch requests for table or index
pg_stat_get_blocks_hit(oid)
bigint
Number of disk block requests found in cache for table or index
pg_stat_get_backend_idset()
set of integer
Set of currently active backend IDs (from 1 to N where N is the
number of active backends). See usage example below.
pg_backend_pid()
integer
Process ID of the attached backend
pg_stat_get_backend_pid(integer)
integer
Process ID of all backend processes
pg_stat_get_backend_dbid(integer)
oid
Database ID of backend process
pg_stat_get_backend_userid(integer)
oid
User ID of backend process
pg_stat_get_backend_activity(integer)
text
Current query of backend process (NULL if caller is not superuser)
Note: blocks_fetched minus blocks_hit gives the number of kernel read()
calls issued for the table, index, or database; but the actual number of
physical reads is usually lower due to kernel-level buffering.
The function pg_stat_get_backend_idset provides
a convenient way to generate one row for each active backend. For
example, to show the PIDs and current queries of all backends:
SELECT pg_stat_get_backend_pid(S.backendid) AS procpid,
pg_stat_get_backend_activity(S.backendid) AS current_query
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS S;
Viewing Locks
Another useful tool for monitoring database activity is the
pg_locks system catalog. This allows the
database administrator to view information about the outstanding
locks in the lock manager. For example, this capability can be
used to:
View all the locks currently outstanding, all the locks on
relations in a particular database, all the locks on a
particular relation, or all the locks held by a particular
PostgreSQL backend.
View the relation in the current database with the most
un-granted locks (which might be a source of contention among
database clients).
Determine the effect of lock contention on overall database
performance, as well as the extent to which contention varies
with overall database traffic.
For more information on locking and managing concurrency with
PostgreSQL, refer to the
Administrator's Guide.
When the pg_locks view is accessed, an
exclusive lock on an internal lock manager data structure must be
acquired to ensure that the data produced by the view is
consistent. The lock held on this structure conflicts with normal
database operations, and can therefore have an effect on overall
database performance. Nevertheless, the performance impact of
accessing this view should be minimal in most situations.
The pg_locks view contains one row per
lock. This means that if there are multiple locks on a single
relation (which may or may not conflict with one another), a
single relation may show up many times. Furthermore, only
table-level locks are displayed (not row-level ones).
Lock Status System View
Column Name
Type
Description
relation
oid
The OID of the locked relation. When querying
pg_locks, this column can be joined with the
pg_class system catalog to get more
information on the locked relation.
database
oid
The OID of the database in which the locked relation
exists. If the lock is on a globally-shared object, this value
will be 0. When querying pg_locks, this
column can be joined with the pg_database
system catalog to get more information on the locked object's
database.
backendpid
int4
The process ID of the
PostgreSQL backend that has
acquired or is attempting to acquire the lock. If you have
enabled the statistics collector, this column can be joined
with the pg_stat_activity view to access
more information on the backend holding or waiting to hold the
lock.
mode
text
The mode of the lock. For more information on the
different lock modes available in
PostgreSQL, refer to the
User's Guide.
isgranted
text
A boolean column indicating whether or not this
particular lock has been granted. If the lock has not been
granted, the backend atempting to acquire it will sleep until
the lock is released (or a deadlock situation is detected). A
single backend can be waiting to acquire at most one lock at
any given time.