pg\_wait\_sampling – sampling based statistics of wait events

`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

`pg_wait_sampling` is an extension for collecting sampling statistics of wait

events.

The module must be loaded by addingpg\_wait\_sampling to

shared\_preload\_libraries in postgresql.conf, because it requires additional

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.

Whenpg\_wait\_sampling is enabled, it collects two kinds of statistics.

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.

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. 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.

intensivity of wait events among time.

pg\_wait\_sampling launches special background worker for gathering the

`pg_wait_sampling` launches special background worker for gathering the

statistics above.

Authors

Availability

pg\_wait\_sampling is realized as an extension and not available in default

`pg_wait_sampling` is realized 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

Installation

pg\_wait\_sampling is PostgreSQL extension which requires PostgreSQL 9.6 or

higher.Before build and install you should ensure following:

`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 thatpg\_config command available, or

* Your PATH variable is configured so that`pg_config` command available, or

set PG_CONFIG variable.

Typical installation procedure may look like this:

Complilation on Windows is not supported, since the extension uses symbols from PostgreSQL

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` interacts with user by set of views and functions.

pg\_wait\_sampling\_current view – information about current wait events for

`pg_wait_sampling_current` view – information about current wait events for

all processed including background workers.

| Column name| Column type| Description|

| event_type| text| Name of wait event type|

| event| text| Name of wait event|

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

`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

`pg_wait_sampling_history` view – history of wait events obtained by sampling into

in-memory ring buffer.

| Column name| Column type| Description|

| event_type| text| Name of wait event type|

| event| text| Name of wait event|

pg\_wait\_sampling\_profile view – profile of wait events obtained by sampling into

`pg_wait_sampling_profile` view – profile of wait events obtained by sampling into

in-memory hash table.

| Column name| Column type| Description|

| event| text| Name of wait event|

| count| text| Count of samples|

pg_wait_sampling_reset_profile() function resets the profile.

`pg_wait_sampling_reset_profile()` function resets the profile.

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

GUCs.

| 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|

Ifpg\_wait\_sampling.profile\_pid is set to false, sampling profile wouldn't be

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.

Contribution

Please, notice, thatpg\_wait\_sampling is still under development and while

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 inpg\_wait\_sampling and feeling power

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.