| | EnterpriseDB'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 command currently being executed by other server
processes.
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 Section 30.4 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_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 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.)
Note: Since the parameters stats_command_string,
stats_block_level, and
stats_row_level default to false,
very few statistics are collected in the default
configuration. Enabling one or more of these configuration
variables will significantly enhance the amount of useful data
produced by the statistics collector, at the expense of
additional run-time overhead.
Several predefined views are available to show the results of
statistics collection, listed in Table 37-1. 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 gsstat_stat_interval milliseconds
(500 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 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.
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 37-1. 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, 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 only available if the
parameter stats_command_string has been
turned on. Furthermore, these columns read as null unless the
user examining the view is a superuser or the same as the user
owning the process being reported on. (Note that because of the
collector's reporting delay, the current query will only be
up-to-date for long-running queries.) | | 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,
and numbers of row insertions, updates, and deletions. | | 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.
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 EnterpriseDB
handles disk I/O, data that is not in the
EnterpriseDB 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 EnterpriseDB I/O behavior are
advised to use the EnterpriseDB 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 37-2. 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-backend process access functions take a backend process ID
number, which ranges from one to the number of currently active
backend processes.
Table 37-2. Statistics Access Functions | Function | Return Type | Description |
|---|
pg_stat_get_db_numbackends(oid) | integer | Number of active backend processes for 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 fetch 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 rows read by sequential scans when argument is a table,
or number of index rows read when argument is an index
| pg_stat_get_tuples_fetched(oid) | bigint | Number of valid (unexpired) table rows 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 rows inserted into table
| pg_stat_get_tuples_updated(oid) | bigint | Number of rows updated in table
| pg_stat_get_tuples_deleted(oid) | bigint | Number of rows 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 process IDs (from 1 to the
number of active backend processes). See usage example in the text.
| pg_backend_pid() | integer | Process ID of the backend process attached to the current session
| pg_stat_get_backend_pid(integer) | integer | Process ID of the given backend process
| pg_stat_get_backend_dbid(integer) | oid | Database ID of the given backend process
| pg_stat_get_backend_userid(integer) | oid | User ID of the given backend process
| pg_stat_get_backend_activity(integer) | text | Active command of the given backend process (null if the
current user is not a superuser nor the same user as that of
the session being queried, or
stats_command_string is not on)
| pg_stat_get_backend_activity_start(integer) | timestamp with time zone | The time at which the given backend process' currently
executing query was started (null if the
current user is not a superuser nor the same user as that of
the session being queried, or
stats_command_string is not on)
| pg_stat_reset() | boolean | Reset all currently collected statistics
|
Note: pg_stat_get_db_blocks_fetched minus
pg_stat_get_db_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 process. For
example, to show the PIDs and current queries of all backend 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;
| |
|---|
