docs/content/v2024.1/launch-and-manage/monitor-and-alert/query-tuning/pg-stat-statements.md
Databases can be resource-intensive, consuming a lot of memory CPU, IO, and network resources. Optimizing your SQL can be very helpful in minimizing resource utilization. The pg_stat_statements module helps you track planning and execution statistics for all the SQL statements executed by a server. It is installed by default.
The columns of the pg_stat_statements view are described in the following table.
| Column | Type | Description |
|---|---|---|
| userid | oid | OID of user who executed the statement |
| dbid | oid | OID of database in which the statement was executed |
| queryid | bigint | Internal hash code, computed from the statement's parse tree |
| query | text | Text of a representative statement |
| calls | bigint | Number of times executed |
| total_time | double precision | Total time spent in the statement, in milliseconds |
| min_time | double precision | Minimum time spent in the statement, in milliseconds |
| max_time | double precision | Maximum time spent in the statement, in milliseconds |
| mean_time | double precision | Mean time spent in the statement, in milliseconds |
| stddev_time | double precision | Population standard deviation of time spent in the statement, in milliseconds |
| rows | bigint | Total number of rows retrieved or affected by the statement |
| shared_blks_hit | bigint | Not populated in YugabyteDB |
| shared_blks_read | bigint | Not populated in YugabyteDB |
| shared_blks_dirtied | bigint | Not populated in YugabyteDB |
| shared_blks_written | bigint | Not populated in YugabyteDB |
| local_blks_hit | bigint | Total number of local block cache hits by the statement |
| local_blks_read | bigint | Total number of local blocks read by the statement |
| local_blks_dirtied | bigint | Total number of local blocks dirtied by the statement |
| local_blks_written | bigint | Total number of local blocks written by the statement |
| temp_blks_read | bigint | Total number of temp blocks read by the statement |
| temp_blks_written | bigint | Total number of temp blocks written by the statement |
| blk_read_time | double precision | Not populated in YugabyteDB |
| blk_write_time | double precision | Not populated in YugabyteDB |
| yb_latency_histogram | jsonb | List of key value pairs where key is the latency range and value is the count of times a query was executed |
You can configure the following parameters in postgresql.conf:
| Column | Type | Default | Description |
|---|---|---|---|
pg_stat_statements.max | integer | 5000 | Maximum number of statements tracked by the module. |
pg_stat_statements.track | enum | top | Controls which statements the module tracks. Valid values are top (track statements issued directly by clients), all (track top-level and nested statements), and none (disable statement statistics collection). |
pg_stat_statements.track_utility | boolean | on | Controls whether the module tracks utility commands. |
pg_stat_statements.save | boolean | on | Specifies whether to save statement statistics across server shutdowns. |
pg_stat_statements.yb_hdr_bucket_factor | integer | 16 | Changes the number of latency range buckets. |
The module requires additional shared memory proportional to pg_stat_statements.max. Note that this memory is consumed whenever the module is loaded, even if pg_stat_statements.track is set to none.
pg_stat_statements.max = 10000
pg_stat_statements.track = all
pg_stat_statements.track_utility = off
pg_stat_statements.save = on
To track IO elapsed time, turn on the track_io_timing parameter in postgresql.conf:
track_io_timing = on
The track_activity_query_size parameter sets the number of characters to display when reporting a SQL query. Raise this value if you're not seeing longer queries in their entirety. For example:
track_activity_query_size = 2048
The extension is created by default. To add or remove it manually, use the following statements:
yugabyte=# create extension pg_stat_statements;
yugabyte=# drop extension pg_stat_statements;
Each row in the pg_stat_statements view in YugabyteDB includes the yb_latency_histogram column, of type JSONB.
yb_latency_histogram consists of a list of key value pairs where key is the latency range and value is the count of times a query was executed. After each query execution, based on the execution time of the query, the execution counter for a specific key (latency range) for that query is incremented. Each key field represents time span in milliseconds (ms). For the resolution of each latency range, refer to Latency range buckets.
The value associated with each key is the count of that query's execution latencies falling in that time span. The time spans are beginning-inclusive and end-exclusive. For example, {"[12.8, 25.6)": 4} means the query has been recorded as taking between 12.8 ms and 25.6 ms, 4 times.
Queries taking longer than the max query duration (default 1677721.5 ms or ~28 min) are recorded in the overflow bucket. The overflow bucket has the format {"[max_latency,)": n}, indicating that it covers all values beyond the scope of the histogram. Currently the max_latency tracked is 1677721.6 ms.
To view latency histograms, use the following command:
yugabyte=# select query, calls, yb_latency_histogram from pg_stat_statements;
-[ RECORD 1 ]-----------------------------------------------------------------
query | insert into table1 values($1, $2)
calls | 6
yb_latency_histogram | [{"[0.1,0.2)": 4}, {"[0.2,0.3)": 2}]
-[ RECORD 2 ]-----------------------------------------------------------------
query | select pg_sleep($1)
calls | 9
yb_latency_histogram | [{"[0.8,0.9)": 6}, {"[1677721.6,)": 3}]
In this example, Record 1 shows that the insert query took between 0.1 to 0.2 ms 4 times and 0.2 to 0.3 ms 2 times. Record 2 shows that the pg_sleep query took between 0.8 to 0.9 ms 6 times and 3 times it took more than 1677721.6 ms.
The following table shows the time resolution for each latency range. For example, for queries finishing between 25.6-51.2 the resolution of the bucket is 3.2ms. If the query finishes in 28 ms then it is recorded in the sub-bucket {25.6, 28.8).
| Time Span (ms) | Resolution (ms) |
|---|---|
| 0-1.6 | 0.1 |
| 1.6-3.2 | 0.2 |
| 3.2-6.4 | 0.4 |
| 6.4-12.8 | 0.8 |
| 12.8-25.6 | 1.6 |
| 25.6-51.2 | 3.2 |
| 51.2-102.4 | 6.4 |
| 102.4-204.8 | 12.8 |
| 204.8-409.6 | 25.6 |
| 409.6-819.2 | 51.2 |
| 819.2-1638.4 | 102.4 |
| 1638.4-3276.8 | 204.8 |
| 3276.8-6553.6 | 409.6 |
| 6553.6-13107.2 | 819.2 |
| 13107.2-26214.4 | 1638.4 |
| 26214.4-52428.8 | 3276.8 |
| 52428.8-104857.6 | 6553.6 |
| 104857.6-209715.2 (104.8-209.7 s) | 13107.2 (~13s) |
| 209715.2-419430.4 (3.49-6.99 min) | 26214.4 (~26s) |
| 419430.4-838860.8 (6.99-13.98 min) | 52428.8 (~52s) |
| 838860.8-1677721.6 (13.98-27.96 min) | 104857.6 (~104s) |
You can change the number of buckets using the yb_hdr_bucket_factor parameter in the postgresql.conf file. Valid values are 8, 16 (default), or 32. For example:
pg_stat_statements.yb_hdr_bucket_factor = 32
This changes the buckets as shown in the following table.
| Time Span (ms) | Resolution (ms) |
|---|---|
| 0-3.2 | 0.1 |
| 3.2-6.4 | 0.2 |
| 6.4-12.8 | 0.4 |
| 12.8-25.6 | 0.8 |
| 25.6-51.2 | 1.6 |
| 51.2-102.4 | 3.2 |
| 102.4-204.8 | 6.4 |
| 204.8-409.6 | 12.8 |
| 409.6-819.2 | 25.6 |
| 819.2-1638.4 | 51.2 |
| ... | |
| 209715.2-419430.4 (3.49-6.99 min) | 13107.2 |
| 419430.4-838860.8 (6.99-13.98 min) | 26214.4 |
| 838860.8-1677721.6 (13.98-27.96 min) | 52428.8 |
Use the yb_get_percentile function to show percentiles for a given histogram. This function takes a JSONB histogram (including yb_latency_histogram) and a target percentile (double precision) as input.
\df yb_get_percentile
Schema | Name | Result data type | Argument data types | Type
----------- +-------------------+------------------+-----------------------------------------+------
pg_catalog | yb_get_percentile | double precision | hist jsonb, percentile double precision | func
(1 row)
The following examples show how to display various percentile latencies for a given JSONB histogram.
SELECT yb_get_percentile('[{"[0.1,0.2)": 4}, {"[0.2,0.3)": 1}, {"[0.8,0.9)": 1}, {"[1.1,1.2)": 3}, {"[1.5,1.6)": 1}, {"[2.0,2.2)": 4}, {"[3.2,3.6)": 1}]', 99);
yb_get_percentile
-------------------
3.6
(1 row)
SELECT yb_get_percentile('[{"[0.1,0.2)": 4}, {"[0.2,0.3)": 1}, {"[0.8,0.9)": 1}, {"[1.1,1.2)": 3}, {"[1.5,1.6)": 1}, {"[2.0,2.2)": 4}, {"[3.2,3.6)": 1}]', 90);
yb_get_percentile
-------------------
2.2
(1 row)
SELECT yb_get_percentile('[{"[0.1,0.2)": 4}, {"[0.2,0.3)": 1}, {"[0.8,0.9)": 1}, {"[1.1,1.2)": 3}, {"[1.5,1.6)": 1}, {"[2.0,2.2)": 4}, {"[3.2,3.6)": 1}]', 50);
yb_get_percentile
-------------------
1.2
(1 row)
The following example displays the P99, P95, and P90 latency by augmenting the pg_stat_statements output for a specific query:
SELECT query, calls, total_time, min_time, max_time, mean_time, rows,
yb_latency_histogram,
yb_get_percentile(yb_latency_histogram, 99),
yb_get_percentile(yb_latency_histogram, 95),
yb_get_percentile(yb_latency_histogram, 90)
FROM pg_stat_statements WHERE query like '%select v from foo where customer_id%';
query | select v from foo where customer_id=$1 and day=$2::date for update
calls | 33904964
total_time | 1122539431.94119
min_time | 1.005537
max_time | 524.190576
mean_time | 33.1084094925296
rows | 33904964
yb_latency_histogram | [{"[1.0,1.1)": 7658}, {"[1.1,1.2)": 326051}, {"[1.2,1.3)": 984139}, {"[1.3,1.4)": 862572}, {"[1.4,1.5)": 604798}, {"[1.5,1.6)": 662095}, {"[1.6,1.8)": 1258824}, {"[1.8,2.0)": 951168}, {"[2.0,2.2)": 373974}, {"[2.2,2.4)": 107085}, {"[2.4,2.6)": 64638}, {"[2.6,2.8)": 73508}, {"[2.8,3.0)": 81727}, {"[3.0,3.2)": 92673}, {"[3.2,3.6)": 199462}, {"[3.6,4.0)": 216870}, {"[4.0,4.4)": 188390}, {"[4.4,4.8)": 212072}, {"[4.8,5.2)": 249788}, {"[5.2,5.6)": 274928}, {"[5.6,6.0)": 329839}, {"[6.0,6.4)": 484011}, {"[6.4,7.2)": 1198917}, {"[7.2,8.0)": 830484}, {"[8.0,8.8)": 572708}, {"[8.8,9.6)": 451159}, {"[9.6,10.4)": 395841}, {"[10.4,11.2)": 373503}, {"[11.2,12.0)": 451987}, {"[12.0,12.8)": 531331}, {"[12.8,14.4)": 770149}, {"[14.4,16.0)": 632207}, {"[16.0,17.6)": 533134}, {"[17.6,19.2)": 486974}, {"[19.2,20.8)": 426922}, {"[20.8,22.4)": 517026}, {"[22.4,24.0)": 474835}, {"[24.0,25.6)": 419358}, {"[25.6,28.8)": 893763}, {"[28.8,32.0)": 852830}, {"[32.0,35.2)": 819699}, {"[35.2,38.4)": 809411}, {"[38.4,41.6)": 744626}, {"[41.6,44.8)": 769875}, {"[44.8,48.0)": 713671}, {"[48.0,51.2)": 704817}, {"[51.2,57.6)": 1373824}, {"[57.6,64.0)": 1376515}, {"[64.0,70.4)": 1402915}, {"[70.4,76.8)": 1434799}, {"[76.8,83.2)": 1426155}, {"[83.2,89.6)": 1263072}, {"[89.6,96.0)": 919922}, {"[96.0,102.4)": 492696}, {"[102.4,115.2)": 222437}, {"[115.2,128.0)": 10392}, {"[128.0,140.8)": 451}, {"[140.8,153.6)": 146}, {"[153.6,166.4)": 78}, {"[166.4,179.2)": 22}, {"[179.2,192.0)": 1}, {"[307.2,332.8)": 3}, {"[332.8,358.4)": 6}, {"[358.4,384.0)": 3}, {"[384.0,409.6)": 7}, {"[409.6,460.8)": 6}, {"[460.8,512.0)": 15}, {"[512.0,563.2)": 2}]
p99 | 102.4
p95 | 89.6
p90 | 83.2
{{% explore-setup-single %}}
Describe the columns in the view:
yugabyte=# \d pg_stat_statements;
View "public.pg_stat_statements"
Column | Type | Collation | Nullable | Default
----------------------+------------------+-----------+----------+---------
userid | oid | | |
dbid | oid | | |
queryid | bigint | | |
query | text | | |
calls | bigint | | |
total_time | double precision | | |
min_time | double precision | | |
max_time | double precision | | |
mean_time | double precision | | |
stddev_time | double precision | | |
rows | bigint | | |
shared_blks_hit | bigint | | |
shared_blks_read | bigint | | |
shared_blks_dirtied | bigint | | |
shared_blks_written | bigint | | |
local_blks_hit | bigint | | |
local_blks_read | bigint | | |
local_blks_dirtied | bigint | | |
local_blks_written | bigint | | |
temp_blks_read | bigint | | |
temp_blks_written | bigint | | |
blk_read_time | double precision | | |
blk_write_time | double precision | | |
yb_latency_histogram | jsonb | | |
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by (blk_read_time+blk_write_time)/calls desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
(3 rows)
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by (blk_read_time+blk_write_time) desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) des
c limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
(4 rows)
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by mean_time desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) des
c limit $1
(4 rows)
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by total_time desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) des
c limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by mean_time desc limit $1
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
(5 rows)
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by stddev_time desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) des
c limit $1
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by mean_time desc limit $1
(5 rows)
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by (shared_blks_hit+shared_blks_dirtied) desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by stddev_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) des
c limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by mean_time desc limit $1
(6 rows)
yugabyte=# select userid::regrole, dbid, query
from pg_stat_statements
order by temp_blks_written desc
limit 10;
userid | dbid | query
----------+-------+--------------------------------------------------------------------------------------------------------
yugabyte | 12463 | select pg_stat_statements_reset()
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by stddev_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/cal
ls desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) des
c limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by mean_time desc limit $1
yugabyte | 12463 | select userid::regrole, dbid, query from pg_stat_statements order by (shared_blks_hit+shared_blks_dirti
ed) desc limit $1
(7 rows)
pg_stat_statements_reset discards all statistics gathered so far by pg_stat_statements. By default, this function can only be executed by superusers.
yugabyte=# select pg_stat_statements_reset();
pg_stat_statements_reset
--------------------------
(1 row)