deep dive into postgresql statistics

Deep dive intoPostgreSQLStatistics.

Alexey Lesovsky

Agenda

PostgreSQL activity statistics.

How to use statistics effectively.

Solve problems with statistics.

Don't talk about

Nothing about query planner statistics.

Tools and programs overview.

Goals

Statistics is useful.

Using statistics are not scary.

Which statistics can be used in particular situation.

Black box

$ ps hf -u postgres -o cmd

/usr/pgsql-9.4/bin/postgres -D /var/lib/pgsql/9.4/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

Where time is wasted

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

Problems

Information too much (more than 100 counters in 9.4).

Statistics provided as an online counters.

No history (but reset functions are available).

No native handy stat tools in PostgreSQL.

Too many 3rd party tools and programs.

Problems

Information too much (more than 100 counters in 9.4).

Statistics provided as an online counters.

No history (but reset functions are available).

No native handy stat tools in PostgreSQL.

Too many 3rd party tools and programs.

Important to use stats directly from Postgres.

Basic SQL skills are required.

Statistics have

Events happens in postgres cluster.

Objects properties (databases, tables, indexes).

Time spent on events.

Statistics sources

Counters in shared memory.

Functions.

Builtin views.

Official contribs package.

Unofficial contribs.

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_bgwrier

pgstattuple

pg_table_size()

pg_database_size

pg_index_size()

pg_stat_activity

EXPLAIN

pg_locks

pg_stat_user_functions

pg_stat_all_indexes

pg_statio_all_indexes

pg_statio_all_tablespg_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()

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

Cache hit ratio$ 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 = better, and not less than 90%

Anomalies$ select * from pg_stat_database;...xact_commit | 51167155051xact_rollback | 44781878826conflicts | 0temp_files | 377675temp_bytes | 4783712399875deadlocks | 151...

$ 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 * from pg_stat_database;...xact_commit | 51167155051 < 95 %xact_rollback | 44781878826 > 5 %conflicts | 0 > 10temp_files | 377675 > 100temp_bytes | 4783712399875 > 10 GBdeadlocks | 151 > 10...

$ 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;

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

checkpoints_req > checkpoints_timed = bad

postgresql.conf● checkpoint_segments

● checkpoint_timeout

● checkpoint_completion_target

Background Writer$ 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

Replication lag$ 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 equals = ideal

Replication lagWhat can cause lag: Networking Storage CPU

How much 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());

Sequential scans$ 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;

Tables size$ 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+

Write activity$ 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 do not cause index update.HOT only for non-indexed columns.

Big n_tup_hot_upd = good.How 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 determine page packing ratio (between 10 and 100).

Tables (and indexes) size overhead.

ALTER TABLE table_name SET (fillfactor = 70);

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 | 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

https://github.com/eshkinkot/avito/blob/master/munin/vacuum_queue

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

https://github.com/PostgreSQL-Consulting/pg-utils/blob/master/sql/low_used_indexes.sql

http://www.databasesoup.com/2014/05/new-finding-unused-indexes-query.html





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



Unused indexes are bad.

Use storage.

Slow down UPDATE, DELETE, INSERT operations.

Extra work for VACUUM.





Summary 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

Long queries$ 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...


$ 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;


$ 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.

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...

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.

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 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.

Use pg_locks for searching blocking query or xact.

Waiting queries: remember, terminate, optimize app.

Блокировкиhttps://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/

locked_item | productswaiting | twaiting_query | update products set avail = false where id = 154;waiting_mode | RowExclusiveLockwaiting_xact_age | 00:00:07.103685waiting_query_age | 00:00:01.813508waiting_pid | 5895waiting_user | app_rolewaiting_state | activewaiting | flocking_query | alter table products drop column avail;locking_mode | AccessExclusiveLocklocking_xact_age | 00:00:16.416441locking_query_age | 00:00:14.416441locking_pid | 5844locking_user | postgreslocking_state | idle in transaction

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

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/

pg_stat_statements$ select * from pg_stat_statements;...query | SELECT "id" FROM run_plan_xact(?)calls | 11165832total_time | 11743325.6880088rows | 11165832blk_read_time | 495425.535999976blk_write_time | 0

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

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

Query reportsquery 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 imv5event where state = ?::eventstate and servertime between $1 and $2 order by servertime desc LIMIT $3 OFFSET $4

Query reportsquery 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 imv5event where state = ?::eventstate and servertime between $1 and $2 order by servertime desc LIMIT $3 OFFSET $4

Use sum() for calculating totals.

Calculate our query «contribution» in totals.

Resource usage (CPU, IO).

Behind the scene

pg_statio_all_tables, pg_statio_all_indexes.

pg_stat_user_functions.

Size functions - \df *size*

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

pg_buffercache (official contrib)● Shared buffers inspection.● Heavy performance impact (buffers lock).

Behind the scene

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

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

Resume

● The ability to use statistics is useful.

● Statistics in Postgres is not difficult.

● Statistics will help answer the questions.

● Do experiment.

LinksAuthor, write more!!www.blog.postgresql-consulting.com (english)www.thislinux.org (russian only)

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

http://www.blog.postgresql-consulting.com/

http://www.thislinux.org/

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

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

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

https://github.com/PostgreSQL-Consulting/pg-utils

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

Thanks. Questions?

deep dive into postgresql statistics

Education