Sometimes I run a Postgres query and it takes 30 seconds. Then, I immediately run the same query and it takes 2 seconds. It appears that Postgres has some sort of caching. Can I somehow see what that cache is holding? Can I force all caches to be cleared for tuning purposes?
I'm basically looking for a Postgres version of the following SQL Server command:
DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS
But I would also like to know how to see what is actually contained in that buffer.
You can see what's in the PostgreSQL buffer cache using the pg_buffercache module. I've done a presentation called "Inside the PostgreSQL Buffer Cache" that explains what you're seeing, and I show some more complicated queries to help interpret that information that go along with that.
It's also possible to look at the operating system cache too on some systems, see [pg_osmem.py] for one somewhat rough example.
There's no way to clear the caches easily. On Linux you can stop the database server and use the drop_caches facility to clear the OS cache; be sure to heed the warning there to run sync first.
I haven't seen any commands to flush the caches in PostgreSQL. What you see is likely just normal index and data caches being read from disk and held in memory. by both postgresql and the caches in the OS. To get rid of all that, the only way I know of:
What you should do is:
Shutdown the database server (pg_ctl, sudo service postgresql stop, sudo systemctl stop postgresql, etc.)
echo 3 > /proc/sys/vm/drop_caches
This will clear out the OS file/block caches - very important though I don't know how to do that on other OSs. (In case of permission denied, try sudo sh -c "echo 3 > /proc/sys/vm/drop_caches" as in that question)
Start the database server (e.g. sudo service postgresql start, sudo systemctl start postgresql)
Greg Smith's answer about drop_caches was very helpful. I did find it necessary to stop and start the postgresql service, in addition to dropping the caches. Here's a shell script that does the trick. (My environment is Ubuntu 14.04 and PostgreSQL 9.3.)
#!/usr/bin/sudo bash
service postgresql stop
sync
echo 3 > /proc/sys/vm/drop_caches
service postgresql start
I tested with a query that took 19 seconds the first time, and less than 2 seconds on subsequent attempts. After running this script, the query once again took 19 seconds.
I use this command on my linux box:
sync; /etc/init.d/postgresql-9.0 stop; echo 1 > /proc/sys/vm/drop_caches; /etc/init.d/postgresql-9.0 start
It completely gets rid of the cache.
I had this error.
psql:/cygdrive/e/test_insertion.sql:9: ERROR: type of parameter 53
(t_stat_gardien) does not match that when preparing the plan
(t_stat_avant)
I was looking for flushing the current plan and a found this:
DISCARD PLANS
I had this between my inserts and it solves my problem.
Yes, it is possible to clear both the shared buffers postgres cache AND the OS cache. Solution bellow is for Windows... others have already given the linux solution.
As many people already said, to clear the shared buffers you can just restart Postgres (no need to restart the server). But just doing this won't clear the OS cache.
To clear the OS cache used by Postgres, after stopping the service, use the excelent RamMap (https://technet.microsoft.com/en-us/sysinternals/rammap), from the excelent Sysinternals Suite.
Once you execute RamMap, just click "Empty"->"Empty Standby List" in the main menu.
Restart Postgres and you'll see now your next query will be damm slow due to no cache at all.
You can also execute the RamMap without closing Postgres, and probably will have the "no cache" results you want, since as people already said, shared buffers usually gives little impact compared to the OS cache. But for a reliable test, I would rather stop postgres as all before clearing the OS cache to make sure.
Note: AFAIK, I don't recommend clearing the other things besides "Standby list" when using RamMap, because the other data is somehow being used, and you can potentially cause problems/loose data if you do that. Remember that you are clearing memory not only used by postgres files, but any other app and OS as well.
Regards, Thiago L.
Yes, postgresql certainly has caching. The size is controlled by the setting shared_buffers. Other than that, there is as the previous answer mentions, the OS file cache which is also used.
If you want to look at what's in the cache, there is a contrib module called pg_buffercache available (in contrib/ in the source tree, in the contrib RPM, or wherever is appropriate for how you installed it). How to use it is listed in the standard PostgreSQL documentation.
There are no ways to clear out the buffer cache, other than to restart the server. You can drop the OS cache with the command mentioned in the other answer - provided your OS is Linux.
There is pg_buffercache module to look into shared_buffers cache. And at some point I needed to drop cache to make some performance tests on 'cold' cache so I wrote an pg_dropcache extension that does exactly this. Please check it out.
this is my shortcut
echo 1 > /proc/sys/vm/drop_caches; echo 2 > /proc/sys/vm/drop_caches; echo 3 > /proc/sys/vm/drop_caches; rcpostgresql stop; rcpostgresql start;
If you have a dedicated test database, you can set the parameter: shared buffers to 16. That should disable the cache for all queries.
The original heading was "See and Clear" buffers.
Postgres 13 with pg_buffercache extension provides a way to see doc page
On OSX there is a purge command for that:
sync && sudo purge
sync - force completion of pending disk writes (flush cache)
purge - force disk cache to be purged (flushed and emptied)
Credit goes to kenorb answering echo 3 > /proc/sys/vm/drop_caches on Mac OSX
Related
I have this issue that is driving me nuts. Despite all my efforts, I am not able to force my postgres server to shut down. I have followed those instructions : http://www.question-defense.com/2008/10/17/pg_ctl-server-does-not-shut-down-force-postgres-to-shutdown
but still, nothing happens and all I got in the shell is
waiting for server to shut down............................................................... failed
pg_ctl: server does not shut down
Any help much appreciated.
Update: Checking the logs, I have this recurring error :
LOG: checkpoints are occurring too frequently (25 seconds apart)
HINT: Consider increasing the configuration parameter "checkpoint_segments".
After giving it a lot of thoughts especially on the way I installed it at the first place, I realize that I set up the install so the daemon would launch postgres at the start of my machine. Thus, any manual killing would simply result in the recreation of those process by the same daemon.
To resolve this problem you need to stop the daemon from working using launchctl and remove a .plist file in your postgres directory.
Good luck if you face the same problem.
You probably run with the default setting of "checkpoint_segments = 3", that produces the warnings. Your database does many writes, right? It takes some time to write all of this to disk, and your database is quite busy rotating the logfiles, instead doing real work.
If you increase checkpint_segments, you will see performance improvements, and less I/O.
For further readings: https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
I've got a query that runs slowly in postgresql the first time it's run, subsequent runs are very fast. Restarting postgresql and apache don't reproduce the slow runtime, but restarting the entire environment/system does. Is there some other way to force whatever kind of caching is happening to expire without rebooting?
On Linux, try restarting Postgres and:
echo 3 > /proc/sys/vm/drop_caches
I'm trying to install PostgreSQL on Cygwin as a user and i'm getting stuck on the part where I have to create a database server, after i execute the pg_ctl command, it just prints "server starting" and runs in the foregorund indefinitely, here's a picture of what I'm talking about:
http://postimg.org/image/oh7ucgt9h/
Im generally a beginner to databases so any pointers would be great.
Chances you are hanging allocating shared memory. Please go with the native Windows build instead. It is far easier to manage.
However if you insist, make sure the ipc-daemon is running before you run PostgreSQL. This will probably solve your problem.
You could run:
ipc-daemon --install-as-service
net start ipc-daemon
And this should do it.
I want to make a script that will run postgres in-memory without durability.
I read this page: http://www.postgresql.org/docs/9.1/static/non-durability.html
But I didn't understand how I can set this parameters in script. Could you please, help me?
Thanks for help!
Most of those parameters, like fsync, can only be set in postgresql.conf. Changes are applied by re-starting PostgreSQL. They apply to the whole database cluster - all the databases in that PostgreSQL install. That's because the databases all share a single postmaster, write-ahead log, and set of shared system tables.
The only parameter listed there that you can set at the SQL level in a script is synchronous_commit. By setting synchronous_commit = 'off' you can say "it's OK to lose this transaction if the database crashes in the next few seconds, just make sure it still applies atomically".
I wrote more on this topic in a previous answer, Optimise PostgreSQL for fast testing.
If you want to set the other params with a script you can do so but you have to do it by opening and modifying postgresql.conf using the script, then re-starting PostgreSQL. Text-processing tools like sed make this kind of job easier.
If you're running a debian based linux distro, you can just do something like:
pg_createcluster -d /dev/shm/mypgcluster 8.4 ramcluster
to create a ram based cluster. Note that you'll have to do:
pg_drop cluster 8.4 ramcluster
and recreate it on reboot etc.
I'd like to monitor the queries getting sent to my database from an application. To that end, I've found pg_stat_activity, but more often then not, the rows which are returned read " in transaction". I'm either doing something wrong, am not fast enough to see the queries come through, am confused, or all of the above!
Can someone recommend the most idiot-proof way to monitor queries running against PostgreSQL? I'd prefer some sort of easy-to-use UI based solution (example: SQL Server's "Profiler"), but I'm not too choosy.
PgAdmin offers a pretty easy-to-use tool called server monitor
(Tools ->ServerStatus)
With PostgreSQL 8.4 or higher you can use the contrib module pg_stat_statements to gather query execution statistics of the database server.
Run the SQL script of this contrib module pg_stat_statements.sql (on ubuntu it can be found in /usr/share/postgresql/<version>/contrib) in your database and add this sample configuration to your postgresql.conf (requires re-start):
custom_variable_classes = 'pg_stat_statements'
pg_stat_statements.max = 1000
pg_stat_statements.track = top # top,all,none
pg_stat_statements.save = off
To see what queries are executed in real time you might want to just configure the server log to show all queries or queries with a minimum execution time. To do so set the logging configuration parameters log_statement and log_min_duration_statement in your postgresql.conf accordingly.
pg_activity is what we use.
https://github.com/dalibo/pg_activity
It's a great tool with a top-like interface.
You can install and run it on Ubuntu 21.10 with:
sudo apt install pg-activity
pg_activity
If you are using Docker Compose, you can add this line to your docker-compose.yaml file:
command: ["postgres", "-c", "log_statement=all"]
now you can see postgres query logs in docker-compose logs with
docker-compose logs -f
or if you want to see only postgres logs
docker-compose logs -f [postgres-service-name]
https://stackoverflow.com/a/58806511/10053470
I haven't tried it myself unfortunately, but I think that pgFouine can show you some statistics.
Although, it seems it does not show you queries in real time, but rather generates a report of queries afterwards, perhaps it still satisfies your demand?
You can take a look at
http://pgfouine.projects.postgresql.org/