Rails PostgreSQL database performance insights. Locks, index usage, buffer cache hit ratios, vacuum stats and more.
Rails PG Extras 
Rails port of Heroku PG Extras with several additions and improvements. The goal of this project is to provide powerful insights into the PostgreSQL database for Ruby on Rails apps that are not using the Heroku PostgreSQL plugin.
Included rake tasks and Ruby methods can be used to obtain information about a Postgres instance, that may be useful when analyzing performance issues. This includes information about locks, index usage, buffer cache hit ratios and vacuum statistics. Ruby API enables developers to easily integrate the tool into e.g. automatic monitoring tasks.
You can read this blog post for detailed step by step tutorial on how to optimize PostgreSQL using PG Extras library.
Shameless plug: rails-pg-extras is just one of the tools that I use when conducting Rails performance audits. Check out my offer if you need help with optimizing your application.
Optionally you can enable a visual interface:
Alternative versions:
Installation
In your Gemfile
gem "rails-pg-extras"
calls and outliers queries require pg_stat_statements extension.
You can check if it is enabled in your database by running:
RailsPgExtras.extensions
You should see the similar line in the output:
| pg_stat_statements | 1.7 | 1.7 | track execution statistics of all SQL statements executed |
ssl_used requires sslinfo extension, and buffercache_usage/buffercache_usage queries need pg_buffercache. You can enable them all by running:
RailsPgExtras.add_extensions
By default a primary ActiveRecord database connection is used for running metadata queries, rake tasks and web UI. To connect to a different database you can specify an ENV['RAILS_PG_EXTRAS_DATABASE_URL'] value in the following format:
ENV["RAILS_PG_EXTRAS_DATABASE_URL"] = "postgresql://postgres:secret@localhost:5432/database_name"
Usage
Each command can be used as a rake task, or a directly from the Ruby code.
rake pg_extras:cache_hit
RailsPgExtras.cache_hit
+----------------+------------------------+
| Index and table hit rate |
+----------------+------------------------+
| name | ratio |
+----------------+------------------------+
| index hit rate | 0.97796610169491525424 |
| table hit rate | 0.96724294813466787989 |
+----------------+------------------------+
By default the ASCII table is displayed, to change to format you need to specify the in_format parameter ([:display_table, :hash, :array, :raw] options are available):
RailsPgExtras.cache_hit(in_format: :hash) =>
[{"name"=>"index hit rate", "ratio"=>"0.97796610169491525424"}, {"name"=>"table hit rate", "ratio"=>"0.96724294813466787989"}]
RailsPgExtras.cache_hit(in_format: :array) =>
[["index hit rate", "0.97796610169491525424"], ["table hit rate", "0.96724294813466787989"]]
RailsPgExtras.cache_hit(in_format: :raw) =>
#<PG::Result:0x00007f75777f7328 status=PGRES_TUPLES_OK ntuples=2 nfields=2 cmd_tuples=2>
Some methods accept an optional args param allowing you to customize queries:
RailsPgExtras.long_running_queries(args: { threshold: "200 milliseconds" })
By default, queries target the public schema of the database. You can specify a different schema by passing the schema argument:
RailsPgExtras.table_cache_hit(args: { schema: "my_schema" })
You can customize the default public schema by setting ENV['PG_EXTRAS_SCHEMA'] value.
Diagnose report
The simplest way to start using pg-extras is to execute a diagnose method. It runs a set of checks and prints out a report highlighting areas that may require additional investigation:
RailsPgExtras.diagnose
$ rake pg_extras:diagnose
Keep reading to learn about methods that diagnose uses under the hood.
Visual interface
You can enable UI using a Rails engine by adding the following code in config/routes.rb:
mount RailsPgExtras::Web::Engine, at: 'pg_extras'
You can enable HTTP basic auth by specifying RAILS_PG_EXTRAS_USER and RAILS_PG_EXTRAS_PASSWORD variables. Authentication is mandatory unless you specify RAILS_PG_EXTRAS_PUBLIC_DASHBOARD=true or setting RailsPgExtras.configuration.public_dashboard to true.
You can configure available web actions in config/initializers/rails_pg_extras.rb:
RailsPgExtras.configure do |config|
# Rails-pg-extras does not enable all the web actions by default. You can check all available actions via `RailsPgExtras::Web::ACTIONS`.
# For example, you may want to enable the dangerous `kill_all` action.
config.enabled_web_actions = %i[kill_all pg_stat_statements_reset add_extensions]
end
Available methods
measure_queries
This method displays query types executed when running a provided Ruby snippet, with their avg., min., max., and total duration in miliseconds. It also outputs info about the snippet execution duration and the portion spent running SQL queries (total_duration/sql_duration). It can help debug N+1 issues and review the impact of configuring eager loading:
RailsPgExtras.measure_queries { User.limit(10).map(&:team) }
{:count=>11,
:queries=>
{"SELECT \"users\".* FROM \"users\" LIMIT $1"=>
{:count=>1,
:total_duration=>1.9,
:min_duration=>1.9,
:max_duration=>1.9,
:avg_duration=>1.9},
"SELECT \"teams\".* FROM \"teams\" WHERE \"teams\".\"id\" = $1 LIMIT $2"=>
{:count=>10,
:total_duration=>0.94,
:min_duration=>0.62,
:max_duration=>1.37,
:avg_duration=>0.94}},
:total_duration=>13.35,
:sql_duration=>11.34}
RailsPgExtras.measure_queries { User.limit(10).includes(:team).map(&:team) }
{:count=>2,
:queries=>
{"SELECT \"users\".* FROM \"users\" LIMIT $1"=>
{:count=>1,
:total_duration=>3.43,
:min_duration=>3.43,
:max_duration=>3.43,
:avg_duration=>3.43},
"SELECT \"teams\".* FROM \"teams\" WHERE \"teams\".\"id\" IN ($1, $2, $3, $4, $5, $6, $7, $8)"=>
{:count=>1,
:total_duration=>2.59,
:min_duration=>2.59,
:max_duration=>2.59,
:avg_duration=>2.59}},
:total_duration=>9.75,
:sql_duration=>6.02}
Optionally, by including Marginalia gem and configuring it to display query backtraces:
config/development.rb
Marginalia::Comment.components = [:line]
you can add this info to the output:
table_info
This method displays metadata metrics for all or a selected table. You can use it to check the table’s size, its cache hit metrics, and whether it is correctly indexed. Many sequential scans or no index scans are potential indicators of misconfigured indexes. This method aggregates data provided by other methods in an easy to analyze summary format.
RailsPgExtras.table_info(args: { table_name: "users" })
| Table name | Table size | Table cache hit | Indexes cache hit | Estimated rows | Sequential scans | Indexes scans |
+------------+------------+-------------------+--------------------+----------------+------------------+---------------+
| users | 2432 kB | 0.999966685701511 | 0.9988780464661853 | 16650 | 2128 | 512496 |
index_info
This method returns summary info about database indexes. You can check index size, how often it is used and what percentage of its total size are NULL values. Like the previous method, it aggregates data from other helper methods in an easy-to-digest format.
RailsPgExtras.index_info(args: { table_name: "users" })
| Index name | Table name | Columns | Index size | Index scans | Null frac |
+-------------------------------+------------+----------------+------------+-------------+-----------+
| users_pkey | users | id | 1152 kB | 163007 | 0.00% |
| index_users_on_slack_id | users | slack_id | 1080 kB | 258870 | 0.00% |
| index_users_on_team_id | users | team_id | 816 kB | 70962 | 0.00% |
| index_users_on_uuid | users | uuid | 1032 kB | 0 | 0.00% |
| index_users_on_block_uuid | users | block_uuid | 776 kB | 19502 | 100.00% |
| index_users_on_api_auth_token | users | api_auth_token | 1744 kB | 156 | 0.00% |
cache_hit
RailsPgExtras.cache_hit
$ rake pg_extras:cache_hit
name | ratio
----------------+------------------------
index hit rate | 0.99957765013541945832
table hit rate | 1.00
(2 rows)
This command provides information on the efficiency of the buffer cache, for both index reads (index hit rate) as well as table reads (table hit rate). A low buffer cache hit ratio can be a sign that the Postgres instance is too small for the workload.
index_cache_hit
RailsPgExtras.index_cache_hit
$ rake pg_extras:index_cache_hit
| name | buffer_hits | block_reads | total_read | ratio |
+-----------------------+-------------+-------------+------------+-------------------+
| teams | 187665 | 109 | 187774 | 0.999419514948821 |
| subscriptions | 5160 | 6 | 5166 | 0.99883855981417 |
| plans | 5718 | 9 | 5727 | 0.998428496595076 |
(truncated results for brevity)
The same as cache_hit with each table’s indexes cache hit info displayed separately.
table_cache_hit
RailsPgExtras.table_cache_hit
$ rake pg_extras:table_cache_hit
| name | buffer_hits | block_reads | total_read | ratio |
+-----------------------+-------------+-------------+------------+-------------------+
| plans | 32123 | 2 | 32125 | 0.999937743190662 |
| subscriptions | 95021 | 8 | 95029 | 0.999915815172211 |
| teams | 171637 | 200 | 171837 | 0.99883610631005 |
(truncated results for brevity)
The same as cache_hit with each table’s cache hit info displayed seperately.
db_settings
RailsPgExtras.db_settings
$ rake pg_extras:db_settings
name | setting | unit |
------------------------------+---------+------+
checkpoint_completion_target | 0.7 | |
default_statistics_target | 100 | |
effective_cache_size | 1350000 | 8kB |
effective_io_concurrency | 1 | |
(truncated results for brevity)
This method displays values for selected PostgreSQL settings. You can compare them with settings recommended by PGTune and tweak values to improve performance.
ssl_used
RailsPgExtras.ssl_used
| ssl_is_used |
+---------------------------------+
| t |
Returns boolean indicating if an encrypted SSL is currently used. Connecting to the database via an unencrypted connection is a critical security risk.
index_usage
RailsPgExtras.index_usage
$ rake pg_extras:index_usage
relname | percent_of_times_index_used | rows_in_table
---------------------+-----------------------------+---------------
events | 65 | 1217347
app_infos | 74 | 314057
app_infos_user_info | 0 | 198848
user_info | 5 | 94545
delayed_jobs | 27 | 0
(5 rows)
This command provides information on the efficiency of indexes, represented as what percentage of total scans were index scans. A low percentage can indicate under indexing, or wrong data being indexed.
locks
RailsPgExtras.locks(args: { limit: 20 })
$ rake pg_extras:locks
procpid | relname | transactionid | granted | query_snippet | mode | age | application |
---------+---------+---------------+---------+-----------------------+------------------------------------------------------
31776 | | | t | <IDLE> in transaction | ExclusiveLock | 00:19:29.837898 | bin/rails
31776 | | 1294 | t | <IDLE> in transaction | RowExclusiveLock | 00:19:29.837898 | bin/rails
31912 | | | t | select * from hello; | ExclusiveLock | 00:19:17.94259 | bin/rails
3443 | | | t | +| ExclusiveLock | 00:00:00 | bin/sidekiq
| | | | select +| | |
| | | | pg_stat_activi | | |
(4 rows)
This command displays queries that have taken out an exclusive lock on a relation. Exclusive locks typically prevent other operations on that relation from taking place, and can be a cause of “hung” queries that are waiting for a lock to be granted.
all_locks
RailsPgExtras.all_locks
$ rake pg_extras:all_locks
This command displays all the current locks, regardless of their type.
outliers
RailsPgExtras.outliers(args: { limit: 20 })
$ rake pg_extras:outliers
qry | exec_time | prop_exec_time | ncalls | sync_io_time
-----------------------------------------+------------------+----------------+-------------+--------------
SELECT * FROM archivable_usage_events.. | 154:39:26.431466 | 72.2% | 34,211,877 | 00:00:00
COPY public.archivable_usage_events (.. | 50:38:33.198418 | 23.6% | 13 | 13:34:21.00108
COPY public.usage_events (id, reporte.. | 02:32:16.335233 | 1.2% | 13 | 00:34:19.784318
INSERT INTO usage_events (id, retaine.. | 01:42:59.436532 | 0.8% | 12,328,187 | 00:00:00
SELECT * FROM usage_events WHERE (alp.. | 01:18:10.754354 | 0.6% | 102,114,301 | 00:00:00
UPDATE usage_events SET reporter_id =.. | 00:52:35.683254 | 0.4% | 23,786,348 | 00:00:00
(truncated results for brevity)
This command displays statements, obtained from pg_stat_statements, ordered by the amount of time to execute in aggregate. This includes the statement itself, the total execution time for that statement, the proportion of total execution time for all statements that statement has taken up, the number of times that statement has been called, and the amount of time that statement spent on synchronous I/O (reading/writing from the file system).
Typically, an efficient query will have an appropriate ratio of calls to total execution time, with as little time spent on I/O as possible. Queries that have a high total execution time but low call count should be investigated to improve their performance. Queries that have a high proportion of execution time being spent on synchronous I/O should also be investigated.
calls
RailsPgExtras.calls(args: { limit: 10 })
$ rake pg_extras:calls
qry | exec_time | prop_exec_time | ncalls | sync_io_time
-----------------------------------------+------------------+----------------+-------------+--------------
SELECT * FROM usage_events WHERE (alp.. | 01:18:11.073333 | 0.6% | 102,120,780 | 00:00:00
BEGIN | 00:00:51.285988 | 0.0% | 47,288,662 | 00:00:00
COMMIT | 00:00:52.31724 | 0.0% | 47,288,615 | 00:00:00
SELECT * FROM archivable_usage_event.. | 154:39:26.431466 | 72.2% | 34,211,877 | 00:00:00
UPDATE usage_events SET reporter_id =.. | 00:52:35.986167 | 0.4% | 23,788,388 | 00:00:00
(truncated results for brevity)
This command is much like pg:outliers, but ordered by the number of times a statement has been called.
blocking
RailsPgExtras.blocking
$ rake pg_extras:blocking
blocked_pid | blocking_statement | blocking_duration | blocking_pid | blocked_statement | blocked_duration
-------------+--------------------------+-------------------+--------------+------------------------------------------------------------------------------------+------------------
461 | select count(*) from app | 00:00:03.838314 | 15682 | UPDATE "app" SET "updated_at" = '2013-03-04 15:07:04.746688' WHERE "id" = 12823149 | 00:00:03.821826
(1 row)
This command displays statements that are currently holding locks that other statements are waiting to be released. This can be used in conjunction with pg:locks to determine which statements need to be terminated in order to resolve lock contention.
total_index_size
RailsPgExtras.total_index_size
$ rake pg_extras:total_index_size
size
-------
28194 MB
(1 row)
This command displays the total size of all indexes on the database, in MB. It is calculated by taking the number of pages (reported in relpages) and multiplying it by the page size (8192 bytes).
index_size
RailsPgExtras.index_size
$ rake pg_extras:index_size
name | size | schema |
---------------------------------------------------------------+-------------------
idx_activity_attemptable_and_type_lesson_enrollment | 5196 MB | public |
index_enrollment_attemptables_by_attempt_and_last_in_group | 4045 MB | public |
index_attempts_on_student_id | 2611 MB | public |
enrollment_activity_attemptables_pkey | 2513 MB | custom |
index_attempts_on_student_id_final_attemptable_type | 2466 MB | custom |
attempts_pkey | 2466 MB | custom |
index_attempts_on_response_id | 2404 MB | public |
index_attempts_on_enrollment_id | 1957 MB | public |
index_enrollment_attemptables_by_enrollment_activity_id | 1789 MB | public |
enrollment_activities_pkey | 458 MB | public |
(truncated results for brevity)
This command displays the size of each each index in the database, in MB. It is calculated by taking the number of pages (reported in relpages) and multiplying it by the page size (8192 bytes).
table_size
RailsPgExtras.table_size
$ rake pg_extras:table_size
name | size | schema |
---------------------------------------------------------------+-------------------
learning_coaches | 196 MB | public |
states | 145 MB | public |
grade_levels | 111 MB | custom |
charities_customers | 73 MB | custom |
charities | 66 MB | public |
(truncated results for brevity)
This command displays the size of each table and materialized view in the database, in MB. It is calculated by using the system administration function pg_table_size(), which includes the size of the main data fork, free space map, visibility map and TOAST data.
table_indexes_size
RailsPgExtras.table_indexes_size
$ rake pg_extras:table_indexes_size
table | indexes_size
---------------------------------------------------------------+--------------
learning_coaches | 153 MB
states | 125 MB
charities_customers | 93 MB
charities | 16 MB
grade_levels | 11 MB
(truncated results for brevity)
This command displays the total size of indexes for each table and materialized view, in MB. It is calculated by using the system administration function pg_indexes_size().
total_table_size
RailsPgExtras.total_table_size
$ rake pg_extras:total_table_size
name | size
---------------------------------------------------------------+---------
learning_coaches | 349 MB
states | 270 MB
charities_customers | 166 MB
grade_levels | 122 MB
charities | 82 MB
(truncated results for brevity)
This command displays the total size of each table and materialized view in the database, in MB. It is calculated by using the system administration function pg_total_relation_size(), which includes table size, total index size and TOAST data.
unused_indexes
RailsPgExtras.unused_indexes(args: { max_scans: 50 })
$ rake pg_extras:unused_indexes
table | index | index_size | index_scans
---------------------+--------------------------------------------+------------+-------------
public.grade_levels | index_placement_attempts_on_grade_level_id | 97 MB | 0
public.observations | observations_attrs_grade_resources | 33 MB | 0
public.messages | user_resource_id_idx | 12 MB | 0
(3 rows)
This command displays indexes that have < 50 scans recorded against them, and are greater than 5 pages in size, ordered by size relative to the number of index scans. This command is generally useful for eliminating indexes that are unused, which can impact write performance, as well as read performance should they occupy space in memory.
duplicate_indexes
RailsPgExtras.duplicate_indexes
| size | idx1 | idx2 | idx3 | idx4 |
+------------+--------------+----------------+----------+-----------+
| 128 k | users_pkey | index_users_id | | |
This command displays multiple indexes that have the same set of columns, same opclass, expression and predicate - which make them equivalent. Usually it’s safe to drop one of them.
null_indexes
RailsPgExtras.null_indexes(args: { min_relation_size_mb: 10 })
$ rake pg_extras:null_indexes
oid | index | index_size | unique | indexed_column | null_frac | expected_saving
---------+--------------------+------------+--------+----------------+-----------+-----------------
183764 | users_reset_token | 1445 MB | t | reset_token | 97.00% | 1401 MB
88732 | plan_cancelled_at | 539 MB | f | cancelled_at | 8.30% | 44 MB
9827345 | users_email | 18 MB | t | email | 28.67% | 5160 kB
This command displays indexes that contain NULL values. A high ratio of NULL values means that using a partial index excluding them will be beneficial in case they are not used for searching.
seq_scans
RailsPgExtras.seq_scans
$ rake pg_extras:seq_scans
name | count
-----------------------------------+----------
learning_coaches | 44820063
states | 36794975
grade_levels | 13972293
charities_customers | 8615277
charities | 4316276
messages | 3922247
contests_customers | 2915972
classroom_goals | 2142014
(truncated results for brevity)
This command displays the number of sequential scans recorded against all tables, descending by count of sequential scans. Tables that have very high numbers of sequential scans may be under-indexed, and it may be worth investigating queries that read from these tables.
long_running_queries
RailsPgExtras.long_running_queries(args: { threshold: "200 milliseconds" })
$ rake pg_extras:long_running_queries
pid | duration | query
-------+-----------------+---------------------------------------------------------------------------------------
19578 | 02:29:11.200129 | EXPLAIN SELECT "students".* FROM "students" WHERE "students"."id" = 1450645 LIMIT 1
19465 | 02:26:05.542653 | EXPLAIN SELECT "students".* FROM "students" WHERE "students"."id" = 1889881 LIMIT 1
19632 | 02:24:46.962818 | EXPLAIN SELECT "students".* FROM "students" WHERE "students"."id" = 1581884 LIMIT 1
(truncated results for brevity)
This command displays currently running queries, that have been running for longer than 5 minutes, descending by duration. Very long running queries can be a source of multiple issues, such as preventing DDL statements completing or vacuum being unable to update relfrozenxid.
records_rank
RailsPgExtras.records_rank
$ rake pg_extras:records_rank
name | estimated_count
-----------------------------------+-----------------
tastypie_apiaccess | 568891
notifications_event | 381227
core_todo | 178614
core_comment | 123969
notifications_notification | 102101
django_session | 68078
(truncated results for brevity)
This command displays an estimated count of rows per table, descending by estimated count. The estimated count is derived from n_live_tup, which is updated by vacuum operations. Due to the way n_live_tup is populated, sparse vs. dense pages can result in estimations that are significantly out from the real count of rows.
bloat
RailsPgExtras.bloat
$ rake pg_extras:bloat
type | schemaname | object_name | bloat | waste
-------+------------+-------------------------------+-------+----------
table | public | bloated_table | 1.1 | 98 MB
table | public | other_bloated_table | 1.1 | 58 MB
index | public | bloated_table::bloated_index | 3.7 | 34 MB
table | public | clean_table | 0.2 | 3808 kB
table | public | other_clean_table | 0.3 | 1576 kB
(truncated results for brevity)
This command displays an estimation of table “bloat” – space allocated to a relation that is full of dead tuples, that has yet to be reclaimed. Tables that have a high bloat ratio, typically 10 or greater, should be investigated to see if vacuuming is aggressive enough, and can be a sign of high table churn.
vacuum_stats
RailsPgExtras.vacuum_stats
$ rake pg_extras:vacuum_stats
schema | table | last_vacuum | last_autovacuum | rowcount | dead_rowcount | autovacuum_threshold | expect_autovacuum
--------+-----------------------+-------------+------------------+----------------+----------------+----------------------+-------------------
public | log_table | | 2013-04-26 17:37 | 18,030 | 0 | 3,656 |
public | data_table | | 2013-04-26 13:09 | 79 | 28 | 66 |
public | other_table | | 2013-04-26 11:41 | 41 | 47 | 58 |
public | queue_table | | 2013-04-26 17:39 | 12 | 8,228 | 52 | yes
(truncated results for brevity)
This command displays statistics related to vacuum operations for each table, including an estimation of dead rows, last autovacuum and the current autovacuum threshold. This command can be useful when determining if current vacuum thresholds require adjustments, and to determine when the table was last vacuumed.
kill_all
RailsPgExtras.kill_all
This commands kills all the currently active connections to the database. It can be useful as a last resort when your database is stuck in a deadlock.
kill_pid
RailsPgExtras.kill_pid(args: { pid: 4657 })
This commands kills currently active database connection by its pid number. You can use connections method to find the correct pid values.
pg_stat_statements_reset
RailsPgExtras.pg_stat_statements_reset
This command discards all statistics gathered so far by pg_stat_statements.
buffercache_stats
RailsPgExtras.buffercache_stats(args: { limit: 10 })
This command shows the relations buffered in database share buffer, ordered by percentage taken. It also shows that how much of the whole relation is buffered.
buffercache_usage
RailsPgExtras.buffercache_usage(args: { limit: 20 })
This command calculates how many blocks from which table are currently cached.
extensions
RailsPgExtras.extensions
$ rake pg_extras:extensions
| pg_stat_statements | 1.7 | 1.7 | track execution statistics of all SQL statements executed
(truncated results for brevity)
This command lists all the currently installed and available PostgreSQL extensions.
connections
RailsPgExtras.connections
+----------------------------------------------------------------+
| Returns the list of all active database connections |
+------------------+--------------------------+------------------+
| username | pid | client_address | application_name |
+------------------+--------------------------+------------------+
| postgres | 15962 | 172.31.69.166/32 | sidekiq |
| postgres | 16810 | 172.31.69.166/32 | bin/rails |
+------------------+--------------------------+------------------+
This command returns the list of all active database connections.
mandelbrot
RailsPgExtras.mandelbrot
$ rake pg_extras:mandelbrot
This command outputs the Mandelbrot set, calculated through SQL.
Testing
cp docker-compose.yml.sample docker-compose.yml
docker compose up -d
rake test_all