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 the last vacuum and analyze times 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 is an facility independent of the collector process.
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 18 for details about setting configuration parameters.)
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_activities enables monitoring of the current command being executed by any server process.
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.)
The statistics collector communicates with the backends needing information (including autovacuum) through temporary files. These files are stored in the pg_stat_tmp subdirectory. When the postmaster shuts down, a permanent copy of the statistics data is stored in the global subdirectory. For increased performance, the parameter stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements.
Several predefined views, listed in Table 27-1, 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 statistical 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 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 all
statistical 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
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. Alternatively, you can invoke
pg_stat_clear_snapshot(), which will discard the
current transaction's statistics snapshot (if any). The next use of
statistical information will cause a new snapshot to be fetched.
Table 27-1. Standard Statistics Views
|pg_stat_activity||One row per server process, showing database OID, database name, process ID, user OID, user name, application name, client's address and port number, times at which the server process, current transaction, and current query began execution, process's waiting status, and text of the current query. The columns that report data on the current query are available unless the parameter track_activities 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_bgwriter||One row only, showing cluster-wide statistics from the background writer: number of scheduled checkpoints, requested checkpoints, buffers written by checkpoints and cleaning scans, and the number of times the background writer stopped a cleaning scan because it had written too many buffers. Also includes statistics about the shared buffer pool, including buffers written by backends (that is, not by the background writer) and total buffers allocated.|
|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, total buffer hits (i.e., block read requests avoided by finding the block already in buffer cache), number of rows returned, fetched, inserted, updated and deleted.|
|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, number of row updates that were HOT (i.e., no separate index update), numbers of live and dead rows, 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.|
|pg_stat_user_functions||For all tracked functions, function OID, schema, name, number of calls, total time, and self time. Self time is the amount of time spent in the function itself, total time includes the time spent in functions it called. Time values are in milliseconds.|
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.
Note: 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 might still reside in the kernel's I/O cache, and might 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 27-2. 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. The functions for function-call statistics take a function OID. (Note that only tables, indexes, and functions 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 27-2. Statistics Access Functions
|integer||Number of active server processes for database|
|bigint||Number of transactions committed in database|
|bigint||Number of 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 tuples returned for database|
|bigint||Number of tuples fetched for database|
|bigint||Number of tuples inserted in database|
|bigint||Number of tuples updated in database|
|bigint||Number of tuples deleted in 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 (includes HOT updates)|
|bigint||Number of rows deleted from table|
|bigint||Number of rows HOT-updated in table|
|bigint||Number of live rows in table|
|bigint||Number of dead rows in 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|
|integer||Process ID of the server process attached to the current session|
|setof record||Returns a record of information about the backend with the specified PID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view.|
|bigint||Number of times the function has been called|
|bigint||Total wall clock time spent in the function, in microseconds. Includes the time spent in functions called by this one.|
|bigint||Time spent in only this function. Time spent in called functions is excluded.|
|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 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 track_activities 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 track_activities 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 track_activities is on)|
|timestamp with time zone||The time at which the given server process' currently executing transaction was started, but only if the current user is a superuser or the same user as that of the session being queried (and track_activities 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 TCP 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|
|bigint||Number of times the background writer has started timed checkpoints (because the checkpoint_timeout time has expired)|
|bigint||Number of times the background writer has started checkpoints based on requests from backends because the checkpoint_segments has been exceeded or because the CHECKPOINT command has been issued|
|bigint||Number of buffers written by the background writer during checkpoints|
|bigint||Number of buffers written by the background writer for routine cleaning of dirty pages|
|bigint||Number of times the background writer has stopped its cleaning scan because it has written more buffers than specified in the bgwriter_lru_maxpages parameter|
|bigint||Number of buffers written by backends because they needed to allocate a new buffer|
|bigint||Total number of buffer allocations|
|void||Discard the current statistics snapshot|
|void||Reset all statistics counters for the current database to zero (requires superuser privileges)|
|void||Reset some of the shared statistics counters for the database cluster to zero (requires superuser privileges). Calling pg_stat_reset_shared('bgwriter') will zero all the values shown by pg_stat_bgwriter.|
|void||Reset statistics for a single table or index in the current database to zero (requires superuser privileges)|
|void||Reset statistics for a single function in the current database to zero (requires superuser privileges)|
pg_stat_get_blocks_hitgives the number of kernel
read()calls issued for the table, index, or database; the number of actual physical reads is usually lower due to kernel-level buffering. The *_blks_read statistics columns use this subtraction, i.e., fetched minus hit.
All functions to access information about backends are indexed by backend id
pg_stat_get_activity which is indexed by PID.
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;