Deep dive into PostgreSQL statistics

Deep dive into PostgreSQL statistics.PGConf US 2016, New York

Alexey Lesovskylesovsky@pgco.me

What is PostgreSQL activity statistics.

How to use statistics effectively.

How to solve problems with statistics.

http://goo.gl/uDuSvs

Agenda

Nothing about query planner statistics.

No tools overview.

Don't talk about

Statistics are useful.

Statistics are not scary.

Where to use statistics.

$ ps hf -u postgres -o cmd/usr/pgsql-9.5/bin/postgres -D /var/lib/pgsql/9.5/data \_ postgres: logger process \_ postgres: checkpointer process \_ postgres: writer process \_ postgres: wal writer process \_ postgres: autovacuum launcher process \_ postgres: stats collector process \_ postgres: postgres pgbench [local] idle in transaction \_ postgres: postgres pgbench [local] idle \_ postgres: postgres pgbench [local] UPDATE \_ postgres: postgres pgbench [local] UPDATE waiting \_ postgres: postgres pgbench [local] UPDATE

Black box

Write Ahead Log

SharedBuffers

Buffers IO Autovacuum Workers

Autovacuum Launcher

Background Workers

Indexes IO

Query Execution

Query Planning

Client Backends Postmaster

Relations IO

Logger Process Stats Collector

LogicalReplication

WAL SenderProcess

ArchiverProcess

BackgroundWriter

CheckpointerProcess

Network Storage

Recovery Process

WAL Receiver ProcessTables/Indexes Data Files

Where PostgreSQL spends its time

Too much information (more than 100 counters in 9.5).

Statistics are provided as an online counters.

No history (but reset functions are available).

No native handy stat tools in PostgreSQL.

A lot of 3rd party tools and programs.

Problems

Too much information (more than 100 counters in 9.5).

Statistics are provided as an online counters.

No history (but reset functions are available).

No native handy stat tools in PostgreSQL.

A lot of 3rd party tools and programs.

Important to use stats directly from PostgreSQL.

Basic SQL skills are required.

Problems

Counters in shared memory.

Functions.

Builtin views.

Official extensions in contribs package.

Unofficial extensions.

Statistics sources

pg_stat_all_tables

pg_current_xlog_location()

pg_replication_slots

pg_stat_replication

pg_stat_archiverpg_stat_database_conflicts

pg_buffercache

pg_stat_database

pg_stat_user_tables

pg_stat_database

pg_stat_bgwriter

pgstattuple

pg_table_size()

pg_database_size

pg_index_size()

pg_stat_activity

EXPLAIN

pg_locks

pg_stat_user_functions

pg_stat_user_indexes

pg_statio_all_indexes

pg_xlog_location_diff()

pg_stat_kcache

pg_stat_statements

pg_roles

pg_database

pg_class

pg_relation_size()

pg_total_relation_size()

current_setting()

pg_current_xlog_insert_location()

pg_last_xact_replay_timestamp()

pg_last_xlog_replay_location()pg_tablespace_size()

pg_ls_dir()

pg_statio_all_sequences

pg_statio_sys_tables

Write Ahead Log

SharedBuffers

Autovacuum Launcher

Background Workers

Indexes IO

Query Execution

Query Planning

Tables IO

LogicalReplication

WAL SenderProcess

ArchiverProcess

BackgroundWriter

CheckpointerProcess

Network Storage

Recovery Process

pg_stat_all_tables

pg_current_xlog_location

pg_stat_replication

pg_stat_archiver

pg_stat_database_conflicts

pg_buffercachepg_stat_database

pg_stat_activity

pg_stat_user_tables

pg_stat_database

pg_stat_bgwrier

pgstattuplepg_table_size

pg_database_size

pg_index_size

pg_stat_activity

EXPLAIN

pg_stat_statements

pg_locks

pg_stat_all_indexes

pg_stat_database

pg_statio_all_tablespg_xlog_location_diff

nicstat* iostat*

pg_stat_kcache

pg_stat_statements

* - system utils

pg_stat_database

$ select * from pg_stat_database;...blks_read | 7978770895blks_hit | 9683551077519...

$ select sum(blks_hit)*100/sum(blks_hit+blks_read) as hit_ratio from pg_stat_database;

More is better, and not less than 90%

Cache hit ratio

$ select datname, (xact_commit*100)/(xact_commit+xact_rollback) as c_ratio, deadlocks, conflicts, temp_file, pg_size_pretty(temp_bytes) as temp_size from pg_stat_database;

Anomalies

$ select datname, (xact_commit*100)/(xact_commit+xact_rollback) as c_ratio, deadlocks, conflicts, temp_file, pg_size_pretty(temp_bytes) as temp_size from pg_stat_database;

Anomalies

Conflicts: pg_stat_database_conflicts, server logs.

Deadlocks: server logs + log_lock_waits.

Temp files: server logs + log_temp_files, pg_stat_statements.

Anomalies

pg_stat_bgwriter

$ select * from pg_stat_bgwriter;...checkpoints_timed | 3267checkpoints_req | 6...

checkpoints_req > checkpoints_timed = bad

postgresql.conf:

● min_wal_size, max_wal_size (checkpoint_segments, >= 9.4)

● checkpoint_timeout

● checkpoint_completion_target

Checkpoints

$ select * from pg_stat_bgwriter;...buffers_clean | 99602799maxwritten_clean | 157 > 0buffers_backend_fsync | 0 > 0...

Clean dirty pages for backends.

Helps checkpointer.

postgresql.conf:● bgwriter_delay● bgwriter_lru_maxpages● bgwriter_lru_multiplier

Background Writer

pg_stat_replication

$ select * from pg_stat_replication;...sent_location | 1691/EEE65900write_location | 1691/EEE65900flush_location | 1691/EEE65900replay_location | 1691/EEE658D0...

1692/EEE65900 — location in transaction log (WAL)

All values are equal = ideal

Replication lag

Lag causes: Networking Storage CPU

How many bytes written in WAL$ select pg_xlog_location_diff(pg_current_xlog_location(),'0/00000000');

Replication lag in bytes$ select client_addr, pg_xlog_location_diff(pg_current_xlog_location(), replay_location) from pg_stat_replication;

Replication lag in seconds$ select extract(epoch from now() - pg_last_xact_replay_timestamp());

Replication lag

$ select client_addr as client, pg_size_pretty(pg_xlog_location_diff(pg_current_xlog_location(),sent_location)) as pending, pg_size_pretty(pg_xlog_location_diff(sent_location,write_location)) as write, pg_size_pretty(pg_xlog_location_diff(write_location,flush_location)) as flush, pg_size_pretty(pg_xlog_location_diff(flush_location,replay_location)) as replay, pg _size_pretty(pg_xlog_location_diff(pg_current_xlog_location(),replay_location)) as total from pg_stat_replication;

Replication lag

pg_stat_all_tables

$ select * from pg_stat_all_tables;...seq_scan | 192seq_tup_read | 364544695 > 1000 (seq_tup_avg)...

$ select relname, pg_size_pretty(pg_relation_size(relname::regclass)) as size, seq_scan, seq_tup_read, seq_scan / seq_tup_read as seq_tup_avg from pg_stat_user_tables where seq_tup_read > 0 order by 3,4 desc limit 5;

Sequential scans

$ select relname, pg_size_pretty(pg_total_relation_size(relname::regclass)) as full_size, pg_size_pretty(pg_relation_size(relname::regclass)) as table_size, pg_size_pretty(pg_total_relation_size(relname::regclass) - pg_relation_size(relname::regclass)) as index_size from pg_stat_user_tables order by pg_total_relation_size(relname::regclass) desc limit 10;

psql meta-commands: \dt+ and \di+

Tables size

$ select * from pg_stat_all_tables;... n_tup_upd | 736119030n_tup_hot_upd | 497688344...

UPDATE = DELETE + INSERT

UPDATE = TABLE UPDATE + INDEX UPDATE

Write activity

$ select s.relname, pg_size_pretty(pg_relation_size(relid)), coalesce(n_tup_ins,0) + 2 * coalesce(n_tup_upd,0) - coalesce(n_tup_hot_upd,0) + coalesce(n_tup_del,0) AS total_writes, (coalesce(n_tup_hot_upd,0)::float * 100 / (case when n_tup_upd > 0 then n_tup_upd else 1 end)::float)::numeric(10,2) AS hot_rate, (select v[1] FROM regexp_matches(reloptions::text,E'fillfactor=(\\d+)') as r(v) limit 1) AS fillfactor from pg_stat_all_tables s join pg_class c ON c.oid=relid order by total_writes desc limit 50;

What is Heap-Only Tuples?HOT does not cause index update.HOT is only for non-indexed columns.

Big n_tup_hot_upd = good.How to increase n_tup_hot_upd?

Write activity

| pg_size_pretty | total_writes | hot_rate | fillfactor -+----------------+--------------+----------+------------ | 9418 MB | 391459091 | 5.03 | | 417 MB | 285948760 | 0.00 | | 76 MB | 232031972 | 87.31 | 70 | 7123 MB | 124281107 | 99.36 | 70

Fillfactor determines page packing ratio (between 10 and 100).

Tables (and indexes) size overhead.

ALTER TABLE table_name SET (fillfactor = 70);

Write activity

$ select c.relname, current_setting('autovacuum_vacuum_threshold') as av_base_thresh, current_setting('autovacuum_vacuum_scale_factor') as av_scale_factor, (current_setting('autovacuum_vacuum_threshold')::int + (current_setting('autovacuum_vacuum_scale_factor')::float * c.reltuples)) as av_thresh, s.n_dead_tup from pg_stat_user_tables s join pg_class c ON s.relname = c.relname where s.n_dead_tup > (current_setting('autovacuum_vacuum_threshold')::int + (current_setting('autovacuum_vacuum_scale_factor')::float * c.reltuples));

| av_base_thresh | av_scale_factor | av_thresh | n_dead_tup +----------------+-----------------+-----------+------------| 50 | 0.01 | 60302 | 85445| 50 | 0.01 | 50 | 1349| 50 | 0.01 | 55 | 12479| 50 | 0.01 | 103 | 3134| 50 | 0.01 | 52716 | 173100

Autovacuum queue

$ select c.relname, current_setting('autovacuum_vacuum_threshold') as av_base_thresh, current_setting('autovacuum_vacuum_scale_factor') as av_scale_factor, (current_setting('autovacuum_vacuum_threshold')::int + (current_setting('autovacuum_vacuum_scale_factor')::float * c.reltuples)) as av_thresh, s.n_dead_tup from pg_stat_user_tables s join pg_class c ON s.relname = c.relname where s.n_dead_tup > (current_setting('autovacuum_vacuum_threshold')::int + (current_setting('autovacuum_vacuum_scale_factor')::float * c.reltuples));

| av_base_thresh | av_scale_factor | av_thresh | n_dead_tup +----------------+-----------------+-----------+------------| 50 | 0.01 | 60302 | 60545| 50 | 0.01 | 50 | 1349| 50 | 0.01 | 55 | 12479| 50 | 0.01 | 103 | 3134| 50 | 0.01 | 52716 | 173100

Avito Experiencehttps://github.com/eshkinkot/avito/blob/master/munin/vacuum_queue

Autovacuum queue

pg_stat_all_indexes

$ select * from pg_stat_all_indexes where idx_scan = 0;...indexrelname | products_special2_idxidx_scan | 0 0 = bad...

Unused indexes are bad.

Uses storage.

Slow down UPDATE, DELETE, INSERT operations.

Extra work for VACUUM.

Unused indexes

$ select * from pg_stat_all_indexes where idx_scan = 0;...indexrelname | products_special2_idxidx_scan | 0 0 = bad...

Unused indexes are bad.

Uses storage.

Slow down UPDATE, DELETE, INSERT operations.

Extra work for VACUUM.

https://goo.gl/0qXDjl

http://goo.gl/5QxTm4

Unused indexes

pg_stat_activity

$ select * from pg_stat_activity;...datname | tsusename | tsagentclient_addr | 172.17.0.37...

Used connections ratio$ select count(*)*100/(select current_setting('max_connections')::int) from pg_stat_activity;

Summary activity

$ select * from pg_stat_activity;...datname | tsusename | tsagentclient_addr | 172.17.0.37...

$ select client_addr, usename, datname, count(*) from pg_stat_activity group by 1,2,3 order by 4 desc;

client_addr | usename | datname | count -------------+-----------+---------+------- 127.0.0.1 | cron_role | ts | 508 127.0.0.1 | tsagent | ts | 60 127.0.0.1 | postgres | ts | 1 127.0.0.1 | pupkin | ts | 1

Summary activity

$ select * from pg_stat_activity;...backend_start | 2015-10-14 15:18:03.01039+00xact_start | 2015-10-14 15:21:15.336325+00query_start | 2015-10-14 15:21:30.336325+00state_change | 2015-10-14 15:21:30.33635+00...

Long queries and xacts

$ select client_addr, usename, datname, clock_timestamp() - xact_start as xact_age, clock_timestamp() - query_start as query_age, query from pg_stat_activity order by xact_start, query_start;

clock_timestamp() for calculating query or transaction age.Long queries: remember, terminate, optimize.

$ select * from pg_stat_activity where state in ('idle in transaction', 'idle in transaction (aborted)';...xact_start | 2015-10-14 15:21:21.128192+00query_start | 2015-10-14 15:21:30.336325+00state_change | 2015-10-14 15:21:30.33635+00state | idle in transaction...

Bad xacts

$ select * from pg_stat_activity where state in ('idle in transaction', 'idle in transaction (aborted)';...xact_start | 2015-10-14 15:21:21.128192+00query_start | 2015-10-14 15:21:30.336325+00state_change | 2015-10-14 15:21:30.33635+00state | idle in transaction...

idle in transaction, idle in transaction (aborted) = bad

Warning value: > 5

clock_timestamp() for calculate xact age.

Bad xacts: remember, terminate, optimize app.

Bad xacts

$ select * from pg_stat_activity where waiting;...xact_start | 2015-10-14 15:21:21.128192+00query_start | 2015-10-14 15:21:30.336325+00state_change | 2015-10-14 15:21:30.33635+00waiting | t...

Waiting clients

$ select * from pg_stat_activity where waiting;...xact_start | 2015-10-14 15:21:21.128192+00query_start | 2015-10-14 15:21:30.336325+00state_change | 2015-10-14 15:21:30.33635+00waiting | t...

waiting = true = bad.

clock_timestamp() for calculating query or xact age.

Enable log_lock_waits GUC, examine server logs.

Use pg_locks for searching blocking query or xact.

Waiting queries: remember, terminate, optimize app.

Waiting clients

https://github.com/lesovsky/uber-scripts/blob/master/postgresql/sql/c4_06_show_locked_queries.sql

http://big-elephants.com/2013-09/exploring-query-locks-in-postgres/

Blocked queries

pg_stat_statements

Statements average time in ms$ select (sum(total_time) / sum(calls))::numeric(6,3) from pg_stat_statements;

The most writing (to shared_buffers) queries$ select query, shared_blks_dirtied from pg_stat_statements where shared_blks_dirtied > 0 order by 2 desc;

pg_stat_statements

query total time: 15:43:07 (14.9%, CPU: 18.2%, IO: 9.0%)сalls: 476 (0.00%) rows: 476,000avg_time: 118881.54ms (IO: 21.2%)user: app_user db: ustatsquery: SELECT filepath, type, deviceuid FROM imvevents WHERE state = ?::eventstate AND servertime BETWEEN $1 AND $2 ORDER BY servertime DESC LIMIT $3 OFFSET $4

https://goo.gl/6025wZ

Query reports

query total time: 15:43:07 (14.9%, CPU: 18.2%, IO: 9.0%)сalls: 476 (0.00%) rows: 476,000avg_time: 118881.54ms (IO: 21.2%)user: app_user db: ustatsquery: SELECT filepath, type, deviceuid FROM imvevents WHERE state = ?::eventstate AND servertime BETWEEN $1 AND $2 ORDER BY servertime DESC LIMIT $3 OFFSET $4

Use sum() for calculating totals.Calculate queries «contribution» in totals.Resource usage (CPU, IO).

Query reports

pg_statio_all_tables, pg_statio_all_indexes.pg_stat_user_functions.Size functions - \df *size*

pgstattuple (in official contribs package)● Bloat estimation for tables and indexes.● Estimation time depends on table (or index) size.

pg_buffercache (in official contribs package)● Shared buffers inspection.● Heavy performance impact (buffers lock).

Behind this talk

pgfincore (3rd party module)● Low-level operations with tables using mincore().● OS page cache inspection.

pg_stat_kcache (3rd party module)● Using getrusage() before and after query.● CPU usage and real filesystem operations stats.● Requires pg_stat_statements and postgresql >= 9.4.● No performance impact.

Behind this talk

● The ability to use statistics is useful.

● Statistics are not difficult.

● Statistics help to answer the questions.

● Do experiments.

Resume

Author, write more!!www.blog.postgresql-consulting.com

The Statistics Collector http://www.postgresql.org/docs/current/static/monitoring-stats.html

System Administration Functions http://www.postgresql.org/docs/current/static/functions-admin.html

Contrib modules http://www.postgresql.org/docs/current/static/pgstatstatements.html http://www.postgresql.org/docs/current/static/pgstattuple.html http://www.postgresql.org/docs/current/static/pgbuffercache.html https://github.com/klando/pgfincore https://github.com/dalibo/pg_stat_kcache

SQL utils and sql code examples https://github.com/PostgreSQL-Consulting/pg-utils

Unused indexes http://goo.gl/5QxTm4 https://goo.gl/0qXDjl

Query reports https://goo.gl/6025wZ

Write Ahead Log

SharedBuffers

Autovacuum Launcher

Background Workers

Indexes IO

Query Execution

Query Planning

Tables IO

LogicalReplication

WAL SenderProcess

ArchiverProcess

BackgroundWriter

CheckpointerProcess

Network Storage

Recovery Process

pg_stat_all_tables

pg_current_xlog_location

pg_stat_replication

pg_stat_archiver

pg_stat_database_conflicts

pg_buffercachepg_stat_database

pg_stat_activity

pg_stat_user_tables

pg_stat_database

pg_stat_bgwrier

pgstattuplepg_table_size

pg_database_size

pg_index_size

pg_stat_activity

EXPLAIN

pg_stat_statementspg_locks

pg_stat_all_indexes

pg_stat_database

pg_statio_all_tablespg_xlog_location_diff

nicstat* iostat*

pg_stat_kcache

pg_stat_statements

Questions

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