This repo contains a few tools for monitoring Postgres in production:
- pgsqlstat: report top-level postgres stats
- pgsqlslower: print details about queries taking longer than N milliseconds
- pgsqlslowest: print details about N slowest queries
- pgsqllat: print details about query latency distribution
- pgsqltxslower: print details about transactions taking longer than N milliseconds
- pglockwaits: print counts of events where Postgres blocked waiting for a lock
All of these use Postgres's built-in DTrace probes under the hood, which means:
- You don't have to reconfigure or restart Postgres to start using them.
- They instrument all Postgres processes visible on the system.
- These tools require privileges to use DTrace on postgres processes on your system. If you don't see any output (or see all zeroes) but you think your database is doing work, check whether you have the right privileges.
These tools are a work in progress. The output format for all of these tools may change.
For more information about Postgres's DTrace support, see the Dynamic Tracing chapter in the Postgres manual.
The Postgres Wiki has more information about monitoring Postgres and watching slow queries. The tools here exist because they're completely standalone and don't require reconfiguring or even restarting Postgres in order to use them.
pgsqlstat: report top-level postgres stats
pgsqlstat NSECONDS
Prints out stats every NSECONDS about all postgresql instances visible on this system. Use CTRL-C to stop.
Here's an example run on a postgres database that started idle and then had "pgbench" run against it for 10 seconds:
$ pgsqlstat 1
QS QD TxnS TxCm TxAb XLI Sort BufRd BufWd WBD
0 0 0 0 0 0 0 0 0 0
1897 1888 293 283 0 2029 0 11262 0 0
7426 7426 1061 1061 0 7893 0 34121 0 0
8058 8058 1151 1151 0 8606 0 39672 0 0
3741 3740 537 537 0 3999 0 17796 0 0
7546 7546 1078 1078 0 8064 0 36109 0 0
7601 7602 1086 1086 0 8143 0 36483 0 0
6881 6880 983 983 0 7321 0 33734 0 0
4972 4972 710 710 0 5307 0 24645 0 0
7163 7165 1024 1024 0 7636 0 33960 0 0
7562 7560 1080 1080 0 8070 0 30905 0 0
3230 3240 458 468 0 3475 0 13251 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
^C
Output columns:
QS Queries started
QD Queries completed (both successes and failures)
TxnS Transactions started
TxCm Transactions committed
TxAb Transactions aborted
XLI WAL records inserted into the transaction log
Srt Sort operations completed
BufRd Buffer read operations completed
BufWd Dirty buffers written. If this number is frequently non-zero,
that usually indicates that shared_buffers or the bgwriter
control parameters should be tuned. See the Postgres manual for
details.
WBD Dirty WAL buffers written. If this number is frequently
non-zero, that may indicate that wal_buffers needs to be tuned.
See the Postgres manual for details.
pgsqlslower: print details about slow queries
pgsqlslower NMILLISECONDS
Print details about queries taking longer than NMILLISECONDS from start to finish on all postgresql instances on this system. Note that since this tool traces query start to query done, it will never see queries taking longer than the command has been running.
This is similar to pgsqltxslower, except that it's tracing simple queries. For extended queries that are part of transactions, see pgsqltxslower.
Here's an example running this with a 40ms threshold during a "pgbench" run:
$ pgsqlslower 40
QUERY: UPDATE pgbench_tellers SET tbalance = tbalance + 1763 WHERE tid = 8;
total time: 42650 us (parse/plan/execute = 16us/62us/42519us)
txns: 0 started, 0 committed, 0 aborted
buffers: 20 read (20 hit, 0 missed), 0 flushed
QUERY: UPDATE pgbench_tellers SET tbalance = tbalance + 4281 WHERE tid = 3;
total time: 49118 us (parse/plan/execute = 18us/80us/48950us)
txns: 0 started, 0 committed, 0 aborted
buffers: 18 read (18 hit, 0 missed), 0 flushed
QUERY: END;
total time: 233183 us (parse/plan/execute = 9us/0us/5us)
txns: 0 started, 1 committed, 0 aborted
buffers: 0 read (0 hit, 0 missed), 0 flushed
This shows that there were three queries that took longer than 40ms. The tool prints out the time required to parse, plan, and execute each query; the number of transactions started, committed, or aborted; and the number of internal postgres buffers read and flushed.
pgsqlslowest: print details about N slowest queries
pgsqlslowest NSECONDS [MAXQUERIES]
Traces all queries for NSECONDS seconds on all postgresql instances on this system and prints the MAXQUERIES slowest queries. Note that because this tool traces from query start to query completion, it will never see queries that take longer than the command has been running.
For example, tracing the 15 slowest queries over five seconds while running "pgbench":
$ pgsqlslowest 5 15
queries failed 0
queries ok 35702
queries started 35711
elapsed time (us) 5006660
Slowest queries: latency (us):
30393 UPDATE pgbench_tellers SET tbalance = tbalance + -2025 WHERE tid = 8;
30774 UPDATE pgbench_tellers SET tbalance = tbalance + -3603 WHERE tid = 3;
32275 UPDATE pgbench_tellers SET tbalance = tbalance + 152 WHERE tid = 10;
33840 UPDATE pgbench_branches SET bbalance = bbalance + 1282 WHERE bid = 1;
34364 UPDATE pgbench_tellers SET tbalance = tbalance + 4838 WHERE tid = 4;
35507 UPDATE pgbench_branches SET bbalance = bbalance + 2263 WHERE bid = 1;
36241 UPDATE pgbench_tellers SET tbalance = tbalance + 3174 WHERE tid = 2;
36260 UPDATE pgbench_tellers SET tbalance = tbalance + -4164 WHERE tid = 5;
36911 UPDATE pgbench_tellers SET tbalance = tbalance + 4609 WHERE tid = 7;
37583 UPDATE pgbench_branches SET bbalance = bbalance + -4310 WHERE bid = 1;
42188 UPDATE pgbench_tellers SET tbalance = tbalance + -922 WHERE tid = 5;
45902 UPDATE pgbench_tellers SET tbalance = tbalance + 2607 WHERE tid = 6;
46313 UPDATE pgbench_tellers SET tbalance = tbalance + -2672 WHERE tid = 5;
118198 BEGIN;
2083346 END;
pgsqllat: print details about query latency
pgsqllat NSECONDS
Traces all queries for NSECONDS seconds on all postgresql instances on this system and prints distributions of query latency over time and overall query latency. Note that because this tool traces from query start to query completion, it will never see queries that take longer than the command has been running.
For example, tracing queries over ten seconds while running "pgbench":
$ pgsqllat 10
elapsed time (us) 10001952
queries failed 0
queries ok 73301
queries started 73311
All queries: latency (in microseconds), per second:
key min .------------------. max | count
1413409840 4 : ▁▃▁▅█▃▁▂▂▂▂▁▁ : 524288 | 2459
1413409841 4 : ▁▃▁▆█▃▁▂▂▂▁▁▁ : 524288 | 8597
1413409842 4 : ▂▃▁▅█▂▁▂▂▂▁▁ ▁ : 524288 | 7420
1413409843 4 : ▁▃▁▄█▂▁▂▂▂▁▁▁ : 524288 | 8141
1413409844 4 : ▂▃▁▅█▂▁▂▂▂▁▁ : 524288 | 8714
1413409845 4 : ▁▃▁▅█▂▁▂▂▂▁▁ : 524288 | 8730
1413409846 4 : ▁▃▁▃█▃▁▂▂▂▂▁▁ : 524288 | 7474
1413409847 4 : ▃▁▂█▄▁▂▂▂▂▁▁ : 524288 | 6744
1413409848 4 : ▁▃▁▄█▃▁▂▂▂▁▁▁ : 524288 | 8063
1413409849 4 : ▃▂▂█▆▁▂▂▂▂▁▁ ▁▁ : 524288 | 4695
1413409850 4 : ▃▁▃█▄▁▂▂▂▁▁▁▁▁ : 524288 | 2264
All queries: latency (in microseconds), over all 10 seconds:
value ------------- Distribution ------------- count
9 | 0
10 |@@@@ 2669
20 |@@@@@@@@@ 6102
30 |@@ 1431
40 | 75
50 | 57
60 | 36
70 |@ 810
80 |@@ 1335
90 |@@@@ 2764
100 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 25431
200 |@@@@@@@@@@@@@@@@@@@ 12950
300 |@@@@ 2380
400 |@ 391
500 | 179
600 | 147
700 | 160
800 | 206
900 | 326
1000 |@@@@@@ 4349
2000 |@@@@ 3009
3000 |@@@ 2013
4000 |@@ 1509
5000 |@@ 1121
6000 |@ 841
7000 |@ 651
8000 |@ 522
9000 |@ 390
10000 |@@ 1243
20000 | 147
30000 | 21
40000 | 3
50000 | 2
60000 | 1
70000 | 0
80000 | 0
90000 | 0
100000 | 20
200000 | 10
300000 | 0
pgsqltxslower: print details about slow transactions
pgsqltxslower NMILLISECONDS
Print details about transactions taking longer than NMILLISECONDS from start to finish on all postgresql instances on this system. Note that since this tool traces transaction start to commit/abort, it will never see transactions taking longer than the command has been running.
This is similar to pgsqlslower, except that it's tracing transactions, which may be made up of multiple queries.
Here's example output from tracing "pgbench" again:
TIME PID GAP(ms) ELAP(ms) EVENT
6920.155 78403 0.003 0.000 transaction-start
6920.178 78403 0.021 0.005 query-parse BEGIN;
6920.193 78403 0.003 0.002 query-rewrite
6920.212 78403 0.005 0.003 query-execute 0 buffer reads, 0 internal sorts, 0 external sorts
6920.275 78403 0.042 0.011 query-parse UPDATE pgbench_accounts SET abalance = abalance + -161 WHERE aid = 27491;
6920.310 78403 0.004 0.021 query-rewrite
6920.376 78403 0.003 0.052 query-plan
6920.495 78403 0.006 0.100 query-execute 5 buffer reads, 0 internal sorts, 0 external sorts
6920.558 78403 0.043 0.011 query-parse SELECT abalance FROM pgbench_accounts WHERE aid = 27491;
6920.589 78403 0.004 0.015 query-rewrite
6920.638 78403 0.003 0.035 query-plan
6920.686 78403 0.016 0.020 query-execute 4 buffer reads, 0 internal sorts, 0 external sorts
6920.755 78403 0.048 0.011 query-parse UPDATE pgbench_tellers SET tbalance = tbalance + -161 WHERE tid = 7;
6920.789 78403 0.004 0.018 query-rewrite
6920.846 78403 0.003 0.043 query-plan
6920.914 78403 0.005 0.050 query-execute 3 buffer reads, 0 internal sorts, 0 external sorts
6920.978 78403 0.042 0.011 query-parse UPDATE pgbench_branches SET bbalance = bbalance + -161 WHERE bid = 1;
6921.011 78403 0.004 0.017 query-rewrite
6921.064 78403 0.003 0.039 query-plan
6926.959 78403 0.005 5.877 query-execute 14 buffer reads, 0 internal sorts, 0 external sorts
6927.066 78403 0.072 0.022 query-parse INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (7, 1, 27491, -161, CURRENT_TIMESTAMP);
6927.113 78403 0.006 0.028 query-rewrite
6927.145 78403 0.004 0.015 query-plan
6927.201 78403 0.006 0.036 query-execute 1 buffer reads, 0 internal sorts, 0 external sorts
6927.257 78403 0.042 0.005 query-parse END;
6927.271 78403 0.003 0.001 query-rewrite
6927.288 78403 0.004 0.002 query-execute 0 buffer reads, 0 internal sorts, 0 external sorts
6929.587 78403 2.292 9.435 transaction-commit
This transaction took 9.4ms, of which 5.8ms was spent executing the longest query.
All timestamps in this output are in milliseconds. Columns include:
- TIME: milliseconds since the script started tracing
- PID: postgres worker process id
- GAP: time between this event and the previous one (see below)
- ELAP: elapsed time for this event. For transaction-commit, this is the time for the whole transaction.
- EVENT: describes what happened
This tool breaks transactions into a few discrete operations: query parsing, rewriting, planning, and execution. There may be multiple queries processed in a transaction, and each query may skip one or more of these phases. Moreover, there are gaps between these operations. Most notable are gaps between query-execute and query-parse, which is presumably time that postgres spent waiting for the client to send the next query for parsing. So if you take these two lines:
6926.959 78403 0.005 5.877 query-execute 14 buffer reads, 0 internal sorts, 0 external sorts
6927.066 78403 0.072 0.022 query-parse INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (7, 1, 27491, -161, CURRENT_TIMESTAMP);
The first line denotes execution of the previous query. Execution took 5.8ms and included 14 buffer reads and no sorts. Then it was 72 microseconds before postgres started parsing the next command. Parsing the command took 22 microseconds.
Caveat: Even when using the "temporal" DTrace option, it's possible for events to be printed out of order. This is annoying, but one way to work around it is to:
-
Save the output to a file.
-
Find the transaction of interest (usually based on the latency for the transaction-commit event). Find the PID.
-
Select only the lines of the file for that PID and sort them by timestamp:
awk '{ $2 == YOURPID }' < YOURFILE | sort -n -k1,1
Then the events for all transactions handled by that process will be in order and you can find all the events for the transaction you're interested in.
pglockwaits: print counts of lock wait events
pglockwaits NSECONDS
Prints out stats every NSECONDS about all cases where a postgresql backend blocked waiting for a lock. The results are broken out by lock type. Use CTRL-C to stop.
Here's an example run on a postgres database that started idle and then had load applied for a few seconds:
$ pglockwaits 1
AS RS RX SUX S SRX X AX
2015 Aug 13 21:19:27 0 0 0 0 0 0 0 0
2015 Aug 13 21:19:28 0 0 0 0 0 0 0 0
2015 Aug 13 21:19:29 0 0 0 0 0 0 0 0
2015 Aug 13 21:19:30 0 0 0 0 591 0 22 0
2015 Aug 13 21:19:31 0 0 0 0 16966 0 165 0
2015 Aug 13 21:19:32 0 0 0 0 13762 0 82 0
2015 Aug 13 21:19:33 0 0 0 0 19739 0 95 0
Output columns correspond to lock types:
AS ACCESS SHARE
RS ROW SHARE
RX ROW EXCLUSIVE
SUX SHARE UPDATE EXCLUSIVE
S SHARE
SRX SHARE ROW EXCLUSIVE
X EXCLUSIVE
AX ACCESS EXCLUSIVE
See http://www.postgresql.org/docs/current/static/explicit-locking.html for more information about these.
The relationship between queries and transactions is not as simple as it seems. If you imagine a simple psql(1) command-line session: unless the user types the "BEGIN" command, each query maps directly to one transaction, and we'd expect to see these probes each time you hit "enter" with a valid, complete SQL line:
query-start
transaction-start
transaction-commit
query-done
This makes sense. If the query fails (e.g., because of bad syntax), we might instead see:
query-start
transaction-start
transaction-abort
Note the conspicuous absence of a "query-done" probe. Trickier, but manageable.
Now suppose you run this three-command sequence:
BEGIN;
select NOW();
COMMIT;
We see these probes:
query-start querystring = "BEGIN;"
transaction-start
query-done
query-start querystring = "select NOW();"
query-done
query-start querystring = "COMMIT;"
transaction-commit
query-done
And if instead you run this three-command sequence:
BEGIN;
select NOW();
ABORT;
We see these probes:
query-start querystring = "BEGIN;"
transaction-start
query-done
query-start querystring = "select NOW();"
query-done
query-start querystring = "ABORT;"
transaction-abort
query-done
Unlike the syntax error case above, we get a query-done for the "ABORT" command.
Finally, all of the above assumes the simple query protocol. Clients can also use extended queries, in which case we may not see query-start for several of the intermediate queries. We will typically see query-parse and query-execute, though.
To summarize: what's subtle about this is that:
- query strings are only associated with queries, not transactions, and so are available only at query-start
- query-start probes may happen inside of a transaction or they may end up being part of a subsequently-created transaction
As a result, the total transaction time is measured from transaction-start to transaction-{commit or abort}, but if we want to keep track of the queries run during a transaction, we have to keep track of query-start firings from before the transaction started.
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