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Chapter 27. Monitoring Database Activity
Tip: Solarisrequires special handling. You must use
/usr/ucb/ps
,rather than
/bin/ps
.Youalso
must usetwo
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 willbethe original
postgres
commandline.
27.2. The Statistics Collector
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 tracks the total number of rows in each table, and information
about vacuum and analyze actions for each table. It can also count calls to user-defined functions and
the total time spent in each one.
PostgreSQL also supports reporting of the exact command currently being executed by other server
processes. This facility is independent of the collector process.
27.2.1. 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 controlledby configuration parameters that are normallyset
in
postgresql.conf
.(See Chapter 18 for details about setting configuration parameters.)
The parameter track_activities enables monitoring of the current command being executed by any
server process.
The parameter track_counts controls whether statistics are collected about table and index accesses.
The parameter track_functions enables tracking of usage of user-defined functions.
The parameter track_io_timing enables monitoring of block read and write times.
Normally these parameters 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 sessions using the SET command. (To prevent
ordinaryusers from hidingtheir activity from theadministrator, onlysuperusers are allowedtochange
these parameters with
SET
.)
The statistics collector transmits the collected information to other PostgreSQL processes through
temporary files. These files are stored in the directory named by the stats_temp_directory parameter,
pg_stat_tmp
by default. For better performance,
stats_temp_directory
can be pointed at a
RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly,
apermanent copy of the statistics data is stored in the
pg_stat
subdirectory, so that statistics can
be retained across server restarts. When recovery is performed at server start (e.g. after immediate
shutdown, server crash, and point-in-time recovery), all statistics counters are reset.
27.2.2. Viewing Collected Statistics
Several predefined views, listed in Table 27-1, are availableto show the results of statistics collection.
Alternatively, one can build custom views using the underlying statistics functions, as discussed in
Section 27.2.3.
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 statistical counts to
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Chapter 27. Monitoring Database Activity
the collector just before going idle; so a query or transaction still in progress does not affect the dis-
played totals. Also, thecollector itself emits a new reportat most once per
PGSTAT_STAT_INTERVAL
milliseconds (500 ms unless altered while building the server). So the displayed information lags be-
hind actual activity. However, current-query information collected by
track_activities
is always
up-to-date.
Another important point is that when a server process is asked to display any of these statistics, it
first fetches the most recent report emitted by the collector process and then continues to use this
snapshot for allstatistical views and functions until the end of its current transaction. So the statistics
will show static information as long as you continue the current transaction. Similarly, information
about the current queries of all sessions is collected when any such information is first requested
within a transaction, and the same information will be displayed throughout the 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, besure to dothe queries outside anytransaction block. Alternatively, you can
invoke
pg_stat_clear_snapshot
(), which willdiscardthe current transaction’s statistics snapshot
(if any). The next use of statistical information will cause a new snapshot to be fetched.
A transaction can also see its own statistics (as yet untransmitted to the collector) in the views
pg_stat_xact_all_tables
,
pg_stat_xact_sys_tables
,
pg_stat_xact_user_tables
,
and
pg_stat_xact_user_functions
.These numbers do not act as stated above; instead they
update continuously throughout the transaction.
Table 27-1. Standard Statistics Views
View Name
Description
pg_stat_activity
One row per server process, showing
information related to the current activity of that
process, such as state and current query. See
pg_stat_activity for details.
pg_stat_archiver
One row only, showing statistics about the WAL
archiver process’s activity. See pg_stat_archiver
for details.
pg_stat_bgwriter
One row only, showing statistics about the
background writer process’s activity. See
pg_stat_bgwriter for details.
pg_stat_database
One row per database, showing database-wide
statistics. See pg_stat_database for details.
pg_stat_all_tables
One row for each table in the current database,
showing statistics about accesses to that specific
table. See pg_stat_all_tables for details.
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.
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Chapter 27. Monitoring Database Activity
View Name
Description
pg_stat_xact_all_tables
Similar to
pg_stat_all_tables
,but counts
actions taken so far within the current
transaction (which are not yet included in
pg_stat_all_tables
and related views). The
columns for numbers of live and dead rows and
vacuum and analyze actions are not present in
this view.
pg_stat_xact_sys_tables
Same as
pg_stat_xact_all_tables
,except
that onlysystem tables are shown.
pg_stat_xact_user_tables
Same as
pg_stat_xact_all_tables
,except
that onlyuser tables are shown.
pg_stat_all_indexes
One row for each index inthe current database,
showing statistics about accesses to that specific
index. See pg_stat_all_indexes for details.
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
One row for each table in the current database,
showing statistics about I/O on that specific
table. See pg_statio_all_tables for details.
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
One row for each index inthe current database,
showing statistics about I/O on that specific
index. See pg_statio_all_indexes for details.
pg_statio_sys_indexes
Same as
pg_statio_all_indexes
,except
that onlyindexes on system tables are shown.
pg_statio_user_indexes
Same as
pg_statio_all_indexes
,except
that onlyindexes on user tables are shown.
pg_statio_all_sequences
One row for each sequence in the current
database, showing statistics about I/O on that
specific sequence. See pg_statio_all_sequences
for details.
pg_statio_sys_sequences
Same as
pg_statio_all_sequences
,except
that onlysystem 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 onlyuser sequences are shown.
pg_stat_user_functions
One row for each tracked function, showing
statistics about executions of that function. See
pg_stat_user_functions for details.
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Chapter 27. Monitoring Database Activity
View Name
Description
pg_stat_xact_user_functions
Similar to
pg_stat_user_functions
,but
counts only calls during the current transaction
(which are not yet included in
pg_stat_user_functions
).
pg_stat_replication
One row per WAL sender process, showing
statistics about replication to that sender’s
connectedstandby server. See
pg_stat_replication for details.
pg_stat_database_conflicts
One row per database, showing database-wide
statistics about query cancels due to conflict
with recovery on standby servers. See
pg_stat_database_conflicts for details.
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. However, these statistics do not give the
entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL
buffer cache might still reside in the kernel’s I/O cache, and might therefore still be fetched without
requiring a physical read. Users interested inobtaining more detailed information onPostgreSQL I/O
behavior are advised to use the PostgreSQL statistics collector in combination with operating system
utilities that allow insight into the kernel’s handling of I/O.
Table 27-2.
pg_stat_activity
View
Column
Type
Description
datid
oid
OID of the database this
backend is connected to
datname
name
Name of the database this
backend is connected to
pid
integer
Process ID of this backend
usesysid
oid
OID of the user logged into this
backend
usename
name
Name of the user logged into
this backend
application_name
text
Name of the application that is
connected to this backend
client_addr
inet
IP address of the client
connected to this backend. If
this field is null, it indicates
either that the client is
connected via a Unix socket on
the server machine or that this
is an internal process such as
autovacuum.
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Chapter 27. Monitoring Database Activity
Column
Type
Description
client_hostname
text
Host name of the connected
client, as reported by a reverse
DNS lookup of
client_addr
.
This field will only be non-null
for IP connections, and only
when log_hostname is enabled.
client_port
integer
TCP port number that the client
is using for communication
with this backend, or
-1
if a
Unix socket is used
backend_start
timestamp with time
zone
Time when this process was
started, i.e., when the client
connected to the server
xact_start
timestamp with time
zone
Time when this process’ current
transaction was started, or null
if no transaction is active. If the
current query is the first of its
transaction, this column is equal
to the
query_start
column.
query_start
timestamp with time
zone
Time when the currently active
query was started, or if
state
is not
active
,when the last
query was started
state_change
timestamp with time
zone
Time when the
state
was last
changed
waiting
boolean
True if this backend is currently
waiting on a lock
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Chapter 27. Monitoring Database Activity
Column
Type
Description
state
text
Current overall state of this
backend. Possible values are:
active
:The backend is
executing a query.
idle
:The backendis
waiting for a new client
command.
idle in
transaction
:The
backendis in a transaction,
but is not currently
executing a query.
idle in transaction
(aborted)
:This state is
similar to
idle in
transaction
,except one
of the statements in the
transaction caused an error.
fastpath function
call
:The backend is
executing a fast-path
function.
disabled
:This state is
reported if track_activities is
disabled in this backend.
backend_xid
xid
Top-level transaction identifier
of this backend, if any.
backend_xmin
xid
The current backend’s
xmin
horizon.
query
text
Text of this backend’s most
recent query. If
state
is
active
this field shows the
currently executing query. In all
other states, it shows the last
query that was executed.
The
pg_stat_activity
view will have one row per server process, showing information related to
the current activity of that process.
Note: The
waiting
and
state
columns are independent. If a backend is in the
active
state, it
may or may not be
waiting
.If the state is
active
and
waiting
is true, it means that a query is
being executed, but is being blocked by a lock somewhere in the system.
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Chapter 27. Monitoring Database Activity
Table 27-3.
pg_stat_archiver
View
Column
Type
Description
archived_count
bigint
Number of WAL files that have
been successfully archived
last_archived_wal
text
Name of the last WAL file
successfully archived
last_archived_time
timestamp with time
zone
Time of the last successful
archive operation
failed_count
bigint
Number of failed attempts for
archiving WAL files
last_failed_wal
text
Name of the WAL file of the
last failed archival operation
last_failed_time
timestamp with time
zone
Time of the last failed archival
operation
stats_reset
timestamp with time
zone
Time at which these statistics
were last reset
The
pg_stat_archiver
view will always have a single row, containing data about the archiver
process of the cluster.
Table 27-4.
pg_stat_bgwriter
View
Column
Type
Description
checkpoints_timed
bigint
Number of scheduled
checkpoints that have been
performed
checkpoints_req
bigint
Number of requested
checkpoints that have been
performed
checkpoint_write_time
double precision
Total amount of time that has
been spent in the portion of
checkpoint processing where
files are written to disk, in
milliseconds
checkpoint_sync_time
double precision
Total amount of time that has
been spent in the portion of
checkpoint processing where
files are synchronized to disk,
in milliseconds
buffers_checkpoint
bigint
Number of buffers written
during checkpoints
buffers_clean
bigint
Number of buffers written by
the background writer
maxwritten_clean
bigint
Number of times the
background writer stopped a
cleaning scan because it had
written too many buffers
614
Chapter 27. Monitoring Database Activity
Column
Type
Description
buffers_backend
bigint
Number of buffers written
directly by a backend
buffers_backend_fsync
bigint
Number of times a backend had
to execute its own
fsync
call
(normally the background
writer handles those even when
the backend does its own write)
buffers_alloc
bigint
Number of buffers allocated
stats_reset
timestamp with time
zone
Time at which these statistics
were last reset
The
pg_stat_bgwriter
view will always have a single row, containing global data for the cluster.
Table 27-5.
pg_stat_database
View
Column
Type
Description
datid
oid
OID of a database
datname
name
Name of this database
numbackends
integer
Number of backends currently
connected to this database. This
is the only column in this view
that returns a value reflecting
current state; all other columns
return the accumulated values
since the last reset.
xact_commit
bigint
Number of transactions in this
database that have been
committed
xact_rollback
bigint
Number of transactions in this
database that have been rolled
back
blks_read
bigint
Number of disk blocks read in
this database
blks_hit
bigint
Number of times disk blocks
were foundalready in the buffer
cache, so that a read was not
necessary (this only includes
hits in the PostgreSQL buffer
cache, not the operating
system’s file system cache)
tup_returned
bigint
Number of rows returned by
queries in this database
tup_fetched
bigint
Number of rows fetched by
queries in this database
tup_inserted
bigint
Number of rows inserted by
queries in this database
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Chapter 27. Monitoring Database Activity
Column
Type
Description
tup_updated
bigint
Number of rows updated by
queries in this database
tup_deleted
bigint
Number of rows deleted by
queries in this database
conflicts
bigint
Number of queries canceled
due to conflicts with recoveryin
this database. (Conflicts occur
only on standby servers; see
pg_stat_database_conflicts for
details.)
temp_files
bigint
Number of temporary files
created by queries in this
database. All temporary files
are counted, regardless of why
the temporary file was created
(e.g., sorting or hashing), and
regardless of the log_temp_files
setting.
temp_bytes
bigint
Total amount of data written to
temporary files by queries in
this database. All temporary
files are counted, regardless of
why the temporary file was
created, and regardless of the
log_temp_files setting.
deadlocks
bigint
Number of deadlocks detected
in this database
blk_read_time
double precision
Time spent reading data file
blocks by backends in this
database, in milliseconds
blk_write_time
double precision
Time spent writing data file
blocks by backends in this
database, in milliseconds
stats_reset
timestamp with time
zone
Time at which these statistics
were last reset
The
pg_stat_database
view will contain one row for each database in the cluster, showing
database-wide statistics.
Table 27-6.
pg_stat_all_tables
View
Column
Type
Description
relid
oid
OID of a table
schemaname
name
Name of the schema that this
table is in
relname
name
Name of this table
seq_scan
bigint
Number of sequential scans
initiated on this table
616
Chapter 27. Monitoring Database Activity
Column
Type
Description
seq_tup_read
bigint
Number of live rows fetched by
sequential scans
idx_scan
bigint
Number of index scans initiated
on this table
idx_tup_fetch
bigint
Number of live rows fetched by
index scans
n_tup_ins
bigint
Number of rows inserted
n_tup_upd
bigint
Number of rows updated
n_tup_del
bigint
Number of rows deleted
n_tup_hot_upd
bigint
Number of rows HOT updated
(i.e., with no separate index
update required)
n_live_tup
bigint
Estimated number of live rows
n_dead_tup
bigint
Estimatednumber of dead rows
n_mod_since_analyze
bigint
Estimated number of rows
modified since this table was
last analyzed
last_vacuum
timestamp with time
zone
Last time at which this table
was manually vacuumed (not
counting
VACUUM FULL
)
last_autovacuum
timestamp with time
zone
Last time at which this table
was vacuumed by the
autovacuum daemon
last_analyze
timestamp with time
zone
Last time at which this table
was manually analyzed
last_autoanalyze
timestamp with time
zone
Last time at which this table
was analyzedby the
autovacuum daemon
vacuum_count
bigint
Number of times this table has
been manually vacuumed (not
counting
VACUUM FULL
)
autovacuum_count
bigint
Number of times this table has
been vacuumed by the
autovacuum daemon
analyze_count
bigint
Number of times this table has
been manually analyzed
autoanalyze_count
bigint
Number of times this table has
been analyzed by the
autovacuum daemon
The
pg_stat_all_tables
view will contain one row for each table in the current database
(including TOAST tables), showing statistics about accesses to that specific table. The
pg_stat_user_tables
and
pg_stat_sys_tables
views contain the same information, but
filtered to only show user and system tables respectively.
Table 27-7.
pg_stat_all_indexes
View
617
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