pgsentinel – sampling active session history

Introduction

PostgreSQL provides session activity. However, in order to gather activity behavior user have to sample the pg_stat_activity view multiple times. pgsentinel is an extension to record active session history and also link the activity with query statistics (pg_stat_statements).

The module must be loaded by adding pgsentinel to shared_preload_libraries in postgresql.conf, means that a server restart is needed to add or remove the module.

When pgsentinel is enabled, it collects the history of session activity:

• It's implemented as in-memory ring buffer where samples are written with given (configurable) period. Therefore, user can see some number of recent samples depending on history size (configurable).

In combination with pg_stat_statements this extension can also link the session activity with query statistics.

To get more granular queries statistic pgsentinel extension samples the pg_stat_statements view:

• at the same time it is sampling the active sessions
• only for the queryid that were associated to an active session (if any) during the sampling

pgsentinel launches special background worker for gathering the sessions activity.

Availability

pgsentinel is implemented as an extension and not available in default PostgreSQL installation. It is available from github under the same license as GNU AFFERO GENERAL PUBLIC LICENSE and supports PostgreSQL 9.6+.

Installation

pgsentinel is a PostgreSQL extension which requires PostgreSQL 9.6 or higher. Before build and install you should ensure the following:

• PostgreSQL version is 9.6 or higher.
• You have development package of PostgreSQL installed or you built PostgreSQL from source.
• Your PATH variable is configured so that pg_config command available, or set PG_CONFIG variable.

Typical installation procedure may look like this:

As pgsentinel uses the pg_stat_statements extension (officially bundled with PostgreSQL) for tracking which queries get executed in your database, add the following entries to your postgres.conf:

$ shared_preload_libraries = 'pg_stat_statements,pgsentinel'
$ # Icncrease the max size of the query strings Postgres records
$ track_activity_query_size = 2048
$ # Track statements generated by stored procedures as well
$ pg_stat_statements.track = all

restart the postgresql daemon and create the extension:

$ git clone https://github.com/pgsentinel/pgsentinel.git
$ cd pgsentinel/src
$ make
$ sudo make install
$ psql DB -c "CREATE EXTENSION pgsentinel;"

Usage

pgsentinel reports the active session history activity through the pg_active_session_history view:

Column	Type	Collation	Nullable	Default
ash_time	timestamp with time zone
datid	oid
datname	text
pid	integer
leader_pid	integer
usesysid	oid
usename	text
application_name	text
client_addr	text
client_hostname	text
client_port	integer
backend_start	timestamp with time zone
xact_start	timestamp with time zone
query_start	timestamp with time zone
state_change	timestamp with time zone
wait_event_type	text
wait_event	text
state	text
backend_xid	xid
backend_xmin	xid
top_level_query	text
query	text
cmdtype	text
queryid	bigint
backend_type	text
blockers	integer
blockerpid	integer
blocker_state	text

You could see it as samplings of pg_stat_activity providing more information:

• ash_time: the sampling time
• top_level_query: the top level statement (in case PL/pgSQL is used)
• query: the statement being executed (not normalised, as it is in pg_stat_statements, means you see the values)
• cmdtype: the statement type (SELECT,UPDATE,INSERT,DELETE,UTILITY,UNKNOWN,NOTHING)
• queryid: the queryid of the statement which links to pg_stat_statements
• blockers: the number of blockers
• blockerpid: the pid of the blocker (if blockers = 1), the pid of one blocker (if blockers > 1)
• blocker_state: state of the blocker (state of the blockerpid)

pgsentinel also reports the queries statistics history through the pg_stat_statements_history view:

Column	Type	Collation	Nullable	Default
ash_time	timestamp with time zone
userid	oid
dbid	oid
queryid	bigint
calls	bigint
total_exec_time	double precision
rows	bigint
shared_blks_hit	bigint
shared_blks_read	bigint
shared_blks_dirtied	bigint
shared_blks_written	bigint
local_blks_hit	bigint
local_blks_read	bigint
local_blks_dirtied	bigint
local_blks_written	bigint
temp_blks_read	bigint
temp_blks_written	bigint
blk_read_time	double precision
blk_write_time	double precision
plans	bigint
total_plan_time	double precision
wal_records	bigint
wal_fpi	bigint
wal_bytes	numeric

The fields description are the same as for pg_stat_statements (except for the ash_time one, which is the time of the active session history sampling).

The worker is controlled by the following GUCs:

Parameter name	Data type	Description	Default value	Min value
pgsentinel_ash.sampling_period	int4	Period for history sampling in seconds	1	1
pgsentinel_ash.max_entries	int4	Size of pg_active_session_history in-memory ring buffer	1000	1000
pgsentinel.db_name	char	database the worker should connect to	postgres
pgsentinel_ash.track_idle_trans	boolean	track session in idle in transaction state	false
pgsentinel_pgssh.max_entries	int4	Size of pg_stat_statements_history in-memory ring buffer	1000	1000
pgsentinel_pgssh.enable	boolean	enable pg_stat_statements_history	false

Remark

• Some fields may be NULL depending on the version (for example, leader_pid is NULL for version <= 13.0...)

See how to query the view in this short video

The videos are available on youtube

Contribution

If you're lacking of some functionality in pgsentinel
then you're welcome to make pull requests.

Author

• Bertrand Drouvot bdrouvot@gmail.com, France, Twitter