Deep dive into PostgreSQL statistics.

1. Deep dive into PostgreSQL Statistics. Alexey Lesovsky

2. Agenda PostgreSQL activity statistics. How to use statistics effectively. Solve problems with statistics.

3. Don't talk about Nothing about query planner statistics. Tools and programs overview.

4. Goals Statistics is useful. Using statistics are not scary. Which statistics can be used in particular situation.

5. 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

6. Where time is wasted Write Ahead Log Shared Buffers Buffers IO Autovacuum Workers Autovacuum Launcher Background Workers Indexes IO Query Execution Query Planning Client Backends Postmaster Relations IO Logger Process Stats Collector Logical Replication WAL Sender Process Archiver Process Background Writer Checkpointer Process Network Storage Recovery Process WAL Receiver Process Tables/Indexes Data Files

7. 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.

8. 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.

9. Statistics have Events happens in postgres cluster. Objects properties (databases, tables, indexes). Time spent on events.

10. Statistics sources Counters in shared memory. Functions. Builtin views. Official contribs package. Unofficial contribs.

11. Statistics sources pg_stat_all_tables pg_current_xlog_location() pg_replication_slots pg_stat_replication pg_stat_archiver pg_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_tables 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()

12. Write Ahead Log Shared Buffers Buffers IO Autovacuum Workers Autovacuum Launcher Background Workers Indexes IO Query Execution Query Planning Client Backends Postmaster Tables IO Logger Process Stats Collector Logical Replication WAL Sender Process Archiver Process Background Writer Checkpointer Process Network Storage Recovery Process WAL Receiver Process Tables/Indexes Data Files pg_stat_all_tables pg_current_xlog_location pg_replication_slots pg_stat_replication pg_stat_archiver pg_stat_database_conflicts pg_buffercache pg_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_user_functions pg_stat_all_indexes pg_stat_database pg_statio_all_indexes pg_statio_all_tables pg_xlog_location_diff nicstat* iostat* pg_stat_kcache pg_stat_statements

14. Cache hit ratio $ select * from pg_stat_database; ... blks_read | 7978770895 blks_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%

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

16. Anomalies $ select * from pg_stat_database; ... xact_commit | 51167155051 < 95 % xact_rollback | 44781878826 > 5 % conflicts | 0 > 10 temp_files | 377675 > 100 temp_bytes | 4783712399875 > 10 GB deadlocks | 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;

18. Checkpoint's $ select * from pg_stat_bgwriter; ... checkpoints_timed | 3267 checkpoints_req | 6 ... checkpoints_req > checkpoints_timed = bad postgresql.conf ● checkpoint_segments ● checkpoint_timeout ● checkpoint_completion_target

19. Background Writer $ select * from pg_stat_bgwriter; ... buffers_clean | 99602799 maxwritten_clean | 157 > 0 buffers_backend_fsync | 0 > 0 ... Clean dirty pages for backends. Helps checkpointer. postgresql.conf ● bgwriter_delay ● bgwriter_lru_maxpages ● bgwriter_lru_multiplier

21. Replication lag $ select * from pg_stat_replication; ... sent_location | 1691/EEE65900 write_location | 1691/EEE65900 flush_location | 1691/EEE65900 replay_location | 1691/EEE658D0 ... 1692/EEE65900 — location in transaction log (WAL) All values are equals = ideal

22. Replication lag What 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());

24. Sequential scans $ select * from pg_stat_all_tables; ... seq_scan | 192 seq_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;

25. 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+

26. Write activity $ select * from pg_stat_all_tables; ... n_tup_upd | 736119030 n_tup_hot_upd | 497688344 ... UPDATE = DELETE + INSERT UPDATE = TABLE UPDATE + INDEX UPDATE

27. 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?

28. 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);

29. 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

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

32. Unused indexes $ select * from pg_stat_all_indexes where idx_scan = 0; ... indexrelname | products_special2_idx idx_scan | 0 0 = bad ... https://github.com/PostgreSQL-Consulting/pg-utils/blob/master/sql/lo w_used_indexes.sql http://www.databasesoup.com/2014/05/new-finding-unused-indexes-q uery.html

33. Unused indexes $ select * from pg_stat_all_indexes where idx_scan = 0; ... indexrelname | products_special2_idx idx_scan | 0 0 = bad ... https://github.com/PostgreSQL-Consulting/pg-utils/blob/master/sql/lo w_used_indexes.sql http://www.databasesoup.com/2014/05/new-finding-unused-indexes-q uery.html Unused indexes are bad. Use storage. Slow down UPDATE, DELETE, INSERT operations. Extra work for VACUUM.

35. Summary activity $ select * from pg_stat_activity; ... datname | ts usename | tsagent client_addr | 172.17.0.37 ... Used connections ratio $ select count(*)*100/(select current_setting('max_connections')::int) from pg_stat_activity;

36. Summary activity $ select * from pg_stat_activity; ... datname | ts usename | tsagent client_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

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

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

39. Long queries $ select * from pg_stat_activity; ... backend_start | 2015-10-14 15:18:03.01039+00 xact_start | 2015-10-14 15:21:15.336325+00 query_start | 2015-10-14 15:21:30.336325+00 state_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; clock_timestamp() for calculating query or transaction age. Long queries: remember, terminate, optimize.

40. 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+00 query_start | 2015-10-14 15:21:30.336325+00 state_change | 2015-10-14 15:21:30.33635+00 state | idle in transaction ...

41. 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+00 query_start | 2015-10-14 15:21:30.336325+00 state_change | 2015-10-14 15:21:30.33635+00 state | 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.

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

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

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

47. Query reports query total time: 15:43:07 (14.9%, CPU: 18.2%, IO: 9.0%) сalls: 476 (0.00%) rows: 476,000 avg_time: 118881.54ms (IO: 21.2%) user: app_user db: ustats query: select filepath, type, deviceuid from imv5event where state = ?::eventstate and servertime between $1 and $2 order by servertime desc LIMIT $3 OFFSET $4

48. Query reports query total time: 15:43:07 (14.9%, CPU: 18.2%, IO: 9.0%) сalls: 476 (0.00%) rows: 476,000 avg_time: 118881.54ms (IO: 21.2%) user: app_user db: ustats query: 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).

49. 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).

50. 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.

51. Resume ● The ability to use statistics is useful. ● Statistics in Postgres is not difficult. ● Statistics will help answer the questions. ● Do experiment.

52. Links Author, 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

53. Write Ahead Log Shared Buffers Buffers IO Autovacuum Workers Autovacuum Launcher Background Workers Indexes IO Query Execution Query Planning Client Backends Postmaster Tables IO Logger Process Stats Collector Logical Replication WAL Sender Process Archiver Process Background Writer Checkpointer Process Network Storage Recovery Process WAL Receiver Process Tables/Indexes Data Files pg_stat_all_tables pg_current_xlog_location pg_replication_slots pg_stat_replication pg_stat_archiver pg_stat_database_conflicts pg_buffercache pg_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_user_functions pg_stat_all_indexes pg_stat_database pg_statio_all_indexes pg_statio_all_tables pg_xlog_location_diff nicstat* iostat* pg_stat_kcache pg_stat_statements Thanks. Questions?

Deep dive into PostgreSQL statistics.

Алексей Лесовский

Deep dive into PostgreSQL statistics.

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