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`pg_wait_sampling` – sampling based statistics of wait events
=============================================================

Introduction
------------

PostgreSQL 9.6+ provides an information about current wait event of particular
process.  However, in order to gather descriptive statistics of server
behavior user have to sample current wait event multiple times.
`pg_wait_sampling` is an extension for collecting sampling statistics of wait
events.

The module must be loaded by adding `pg_wait_sampling` to
`shared_preload_libraries` in postgresql.conf, because it requires additional
shared memory and launches background worker.  This means that a server restart
is needed to add or remove the module.

When `pg_wait_sampling` is enabled, it collects two kinds of statistics.

 * History of waits events.  It's implemented as in-memory ring buffer where
   samples of each process wait events are written with given (configurable)
   period.  Therefore, for each running process user can see some number of
   recent samples depending on history size (configurable).  Assuming there is
   a client who periodically read this history and dump it somewhere, user
   can have continuous history.
 * Waits profile.  It's implemented as in-memory hash table where count
   of samples are accumulated per each process and each wait event
   (and each query with `pg_stat_statements`).  This hash
   table can be reset by user request.  Assuming there is a client who
   periodically dumps profile and resets it, user can have statistics of
   intensivity of wait events among time.

In combination with `pg_stat_statements` this extension can also provide
per query statistics.

`pg_wait_sampling` launches special background worker for gathering the
statistics above.

Availability
------------

`pg_wait_sampling` is implemented as an extension and not available in default
PostgreSQL installation. It is available from
[github](https://github.com/postgrespro/pg_wait_sampling)
under the same license as
[PostgreSQL](http://www.postgresql.org/about/licence/)
and supports PostgreSQL 9.6+.

Installation
------------

Pre-built `pg_wait_sampling` packages are provided in official PostgreSQL
repository: https://download.postgresql.org/pub/repos/

Manual build
------------

`pg_wait_sampling` is PostgreSQL extension which requires PostgreSQL 9.6 or
higher. Before build and install you should ensure 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:

    $ git clone https://github.com/postgrespro/pg_wait_sampling.git
    $ cd pg_wait_sampling
    $ make USE_PGXS=1
    $ sudo make USE_PGXS=1 install
    $ make USE_PGXS=1 installcheck
    $ psql DB -c "CREATE EXTENSION pg_wait_sampling;"

Compilation on Windows is not supported, since the extension uses symbols from PostgreSQL
that are not exported.

Usage
-----

`pg_wait_sampling` interacts with user by set of views and functions.

`pg_wait_sampling_current` view – information about current wait events for
all processed including background workers.

| Column name | Column type |      Description        |
| ----------- | ----------- | ----------------------- |
| pid         | int4        | Id of process           |
| event_type  | text        | Name of wait event type |
| event       | text        | Name of wait event      |
| queryid     | int8        | Id of query             |

`pg_wait_sampling_get_current(pid int4)` returns the same table for single given
process.

`pg_wait_sampling_history` view – history of wait events obtained by sampling into
in-memory ring buffer.

| Column name | Column type |      Description        |
| ----------- | ----------- | ----------------------- |
| pid         | int4        | Id of process           |
| ts          | timestamptz | Sample timestamp        |
| event_type  | text        | Name of wait event type |
| event       | text        | Name of wait event      |
| queryid     | int8        | Id of query             |

`pg_wait_sampling_profile` view – profile of wait events obtained by sampling into
in-memory hash table.

| Column name | Column type |      Description        |
| ----------- | ----------- | ----------------------- |
| pid         | int4        | Id of process           |
| event_type  | text        | Name of wait event type |
| event       | text        | Name of wait event      |
| queryid     | int8        | Id of query             |
| count       | text        | Count of samples        |

`pg_wait_sampling_reset_profile()` function resets the profile.

The work of wait event statistics collector worker is controlled by following
GUCs.

|         Parameter name              | Data type |                  Description                | Default value |
| ----------------------------------- | --------- | ------------------------------------------- | ------------: |
| pg_wait_sampling.history_size       | int4      | Size of history in-memory ring buffer       |          5000 |
| pg_wait_sampling.history_period     | int4      | Period for history sampling in milliseconds |            10 |
| pg_wait_sampling.profile_period     | int4      | Period for profile sampling in milliseconds |            10 |
| pg_wait_sampling.profile_pid        | bool      | Whether profile should be per pid           |          true |
| pg_wait_sampling.profile_queries    | bool      | Whether profile should be per query			|          true |

If `pg_wait_sampling.profile_pid` is set to false, sampling profile wouldn't be
collected in per-process manner.  In this case the value of pid could would
be always zero and corresponding row contain samples among all the processes.

While `pg_wait_sampling.profile_queries` is set to false `queryid` field in
views will be zero.

These GUCs are allowed to be changed by superuser.  Also, they are placed into
shared memory.  Thus, they could be changed from any backend and affects worker
runtime.

See
[PostgreSQL documentation](http://www.postgresql.org/docs/devel/static/monitoring-stats.html#WAIT-EVENT-TABLE)
for list of possible wait events.

Contribution
------------

Please, notice, that `pg_wait_sampling` is still under development and while
it's stable and tested, it may contains some bugs. Don't hesitate to raise
[issues at github](https://github.com/postgrespro/pg_wait_sampling/issues) with
your bug reports.

If you're lacking of some functionality in `pg_wait_sampling` and feeling power
to implement it then you're welcome to make pull requests.

Authors
-------

 * Alexander Korotkov <a.korotkov@postgrespro.ru>, Postgres Professional,
   Moscow, Russia
 * Ildus Kurbangaliev <i.kurbangaliev@gmail.com>, Postgres Professional,
   Moscow, Russia

