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.
PostgreSQL also supports determining the exact command currently being executed by other server processes. This is an independent facility that can be enabled or disabled whether or not block-level and row-level statistics are being collected.
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 parameters that are normally set in
postgresql.conf
. (See Chapter 17, Server Configuration for
details about setting configuration parameters.)
The parameter 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 parameters 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 tracks disk-block-level access statistics and row-level access statistics and sends these to the collector. Additionally, per-database transaction commit and abort statistics are collected if either of these parameters are set.
The parameter stats_command_string enables monitoring of the current command being executed by any server process. The statistics collector subprocess need not be running to enable this feature.
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
ordinary users from hiding their activity from the administrator,
only superusers are allowed to change these parameters with
SET
.)
Since the parameters stats_block_level
, and
stats_row_level
default to false
,
very few statistics are collected in the default
configuration. Enabling either of these configuration
variables will significantly increase the amount of useful data
produced by the statistics facilities, at the expense of
additional run-time overhead.
Several predefined views, listed in Table 25.1, “Standard Statistics 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 block and row access counts to
the collector just before going idle; so a query or transaction still in
progress does not affect the displayed totals. Also, the collector itself
emits a new report at most once per PGSTAT_STAT_INTERVAL
milliseconds (500 unless altered while building the server). So the
displayed information lags behind actual activity. However, current-query
information collected by stats_command_string
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 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. Similarly, information about the current queries of all processes 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, be sure to do the queries outside any transaction block.
Table 25.1. Standard Statistics Views
View Name | Description |
---|---|
pg_stat_activity |
One row per server process, showing database OID, database name,
process ID, user OID, user name, current query, query's
waiting status, time at
which the current query began execution, time at which the process
was started, and client's address and port number. The columns
that report data on the current query are available unless the
parameter stats_command_string has been
turned off. Furthermore, these columns are only visible if the
user examining the view is a superuser or the same as the user
owning the process being reported on.
|
pg_stat_database |
One row per database, showing database OID, database name, number of active server processes connected to that database, number of transactions committed and rolled back in that database, total disk blocks read, and total buffer hits (i.e., block read requests avoided by finding the block already in buffer cache). |
pg_stat_all_tables |
For each table in the current database (including TOAST tables), the table OID, schema and table name, number of sequential scans initiated, number of live rows fetched by sequential scans, number of index scans initiated (over all indexes belonging to the table), number of live rows fetched by index scans, numbers of row insertions, updates, and deletions, the last time the table was vacuumed manually, the last time it was vacuumed by the autovacuum daemon, the last time it was analyzed manually, and the last time it was analyzed by the autovacuum daemon. |
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 table and index OID, schema, table and index name, number of index scans initiated on that index, number of index entries returned by index scans, and number of live table rows fetched by simple index scans using that index. |
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 (including TOAST tables), the table OID, schema and table name, number of disk blocks read from that table, number of buffer hits, numbers of disk blocks read and buffer hits in all indexes of that table, numbers of disk blocks read and buffer hits from that table's auxiliary TOAST table (if any), and 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 table and index OID, schema, table and index name, 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 sequence OID, schema and sequence name, 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.
Beginning in PostgreSQL 8.1, indexes can be
used either directly or via “bitmap scans”. In a bitmap scan
the output of several indexes can be combined via AND or OR rules;
so it is difficult to associate individual heap row fetches
with specific indexes when a bitmap scan is used. Therefore, a bitmap
scan increments the
pg_stat_all_indexes
.idx_tup_read
count(s) for the index(es) it uses, and it increments the
pg_stat_all_tables
.idx_tup_fetch
count for the table, but it does not affect
pg_stat_all_indexes
.idx_tup_fetch
.
Before PostgreSQL 8.1, the
idx_tup_read
and idx_tup_fetch
counts
were essentially always equal. Now they can be different even without
considering bitmap scans, because idx_tup_read
counts
index entries retrieved from the index while idx_tup_fetch
counts live rows fetched from the table; the latter will be less if any
dead or not-yet-committed rows are fetched using the index.
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 may still reside in the
kernel's I/O cache, and may therefore still be fetched without
requiring a physical read. Users interested in obtaining more
detailed information on PostgreSQL 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.
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. These functions are listed in Table 25.2, “Statistics Access Functions”. The per-database access functions take a database OID as argument to identify which database to report on. The per-table and per-index functions take a table or index OID. (Note that only tables and indexes in the current database can be seen with these functions.) The per-server-process access functions take a server process number, which ranges from one to the number of currently active server processes.
Table 25.2. Statistics Access Functions
Function | Return Type | Description |
---|---|---|
|
integer |
Number of active server processes for database |
|
bigint |
Transactions committed in database |
|
bigint |
Transactions rolled back in database |
|
bigint |
Number of disk block fetch requests for database |
|
bigint |
Number of disk block fetch requests found in cache for database |
|
bigint |
Number of sequential scans done when argument is a table, or number of index scans done when argument is an index |
|
bigint |
Number of rows read by sequential scans when argument is a table, or number of index entries returned when argument is an index |
|
bigint |
Number of table rows fetched by bitmap scans when argument is a table, or table rows fetched by simple index scans using the index when argument is an index |
|
bigint |
Number of rows inserted into table |
|
bigint |
Number of rows updated in table |
|
bigint |
Number of rows deleted from table |
|
bigint |
Number of disk block fetch requests for table or index |
|
bigint |
Number of disk block requests found in cache for table or index |
|
timestamptz |
Time of the last vacuum initiated by the user on this table |
|
timestamptz |
Time of the last vacuum initiated by the autovacuum daemon on this table |
|
timestamptz |
Time of the last analyze initiated by the user on this table |
|
timestamptz |
Time of the last analyze initiated by the autovacuum daemon on this table |
|
setof integer |
Set of currently active server process numbers (from 1 to the number of active server processes). See usage example in the text |
|
integer |
Process ID of the server process attached to the current session |
|
integer |
Process ID of the given server process |
|
oid |
Database ID of the given server process |
|
oid |
User ID of the given server process |
|
text |
Active command of the given server process, but only if the
current user is a superuser or the same user as that of
the session being queried (and
stats_command_string is on)
|
|
boolean |
True if the given server process is waiting for a lock,
but only if the current user is a superuser or the same user as that of
the session being queried (and
stats_command_string is on)
|
|
timestamp with time zone |
The time at which the given server process' currently
executing query was started, but only if the
current user is a superuser or the same user as that of
the session being queried (and
stats_command_string is on)
|
|
timestamp with time zone |
The time at which the given server process was started, or null if the current user is not a superuser nor the same user as that of the session being queried |
|
inet |
The IP address of the client connected to the given server process. Null if the connection is over a Unix domain socket. Also null if the current user is not a superuser nor the same user as that of the session being queried |
|
integer |
The IP port number of the client connected to the given server process. -1 if the connection is over a Unix domain socket. Null if the current user is not a superuser nor the same user as that of the session being queried |
|
boolean |
Reset all block-level and row-level statistics to zero |
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 server process. For
example, to show the PIDs and current queries of all server processes:
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;