AW: query performance after database rename

Hi,

Customer claims about application slowdown after such an database exchange.
I will debug the cluster to get more in-depth in-sight.
No connection pooling used.

Thanks,
Markus

Von: srinivas oguri <srinivasoguri7(at)gmail(dot)com>
Gesendet: Mittwoch, 21. November 2018 13:13
An: Zwettler Markus (OIZ) <Markus(dot)Zwettler(at)zuerich(dot)ch>
Cc: pgsql-admin(at)lists(dot)postgresql(dot)org
Betreff: Re: query performance after database rename

How you are identifying the slowdown in database system ? Are you executing some queries through psql or pgadmin and checking execution time or from application and find slowness ? Are you using connection pooling ? If so which one it is application or postgresql tools (pgpool, pgbouncer etc).

Thanks
Srinivas

On Wed, Nov 21, 2018 at 5:30 PM Zwettler Markus (OIZ) <Markus(dot)Zwettler(at)zuerich(dot)ch<mailto:Markus(dot)Zwettler(at)zuerich(dot)ch>> wrote:
Hi,

We have to exchange two databases db1 <=> db2.

alter database [db1|db2] with allow_connections=false;
select pg_terminate_backend (pg_stat_activity.pid) from pg_stat_activity where pg_stat_activity.datname in ('db1', 'db2') and pid <> pg_backend_pid();
alter database [db1|db2|temp] rename to [temp|db1|db2];
alter database [db1|db2] with allow_connections=true;

Version 9.6

Cheers, Markus

Von: srinivas oguri <srinivasoguri7(at)gmail(dot)com<mailto:srinivasoguri7(at)gmail(dot)com>>
Gesendet: Mittwoch, 21. November 2018 12:24
An: Zwettler Markus (OIZ) <Markus(dot)Zwettler(at)zuerich(dot)ch<mailto:Markus(dot)Zwettler(at)zuerich(dot)ch>>
Cc: pgsql-admin(at)lists(dot)postgresql(dot)org<mailto:pgsql-admin(at)lists(dot)postgresql(dot)org>
Betreff: Re: query performance after database rename

Hi Markus,

Can you give an example ? what is the postgres version your are using? what exactly you are doing ?

PostgreSQL do not perform any flush operation after renaming. Below is the debug log of postgresql

< 2018-11-21 11:12:19.481 UTC > DEBUG: postmaster child[14261]: starting with (
< 2018-11-21 11:12:19.481 UTC > DEBUG: postgres
< 2018-11-21 11:12:19.481 UTC > DEBUG: )
< 2018-11-21 11:12:19.481 UTC > DEBUG: InitPostgres
< 2018-11-21 11:12:19.482 UTC > DEBUG: my backend ID is 3
< 2018-11-21 11:12:19.482 UTC > DEBUG: StartTransaction
< 2018-11-21 11:12:19.482 UTC > DEBUG: name: unnamed; blockState: DEFAULT; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children:
< 2018-11-21 11:12:19.483 UTC > DEBUG: CommitTransaction
< 2018-11-21 11:12:19.483 UTC > DEBUG: name: unnamed; blockState: STARTED; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children:
< 2018-11-21 11:12:19.483 UTC > DEBUG: StartTransactionCommand
< 2018-11-21 11:12:19.483 UTC > DEBUG: StartTransaction
< 2018-11-21 11:12:19.483 UTC > DEBUG: name: unnamed; blockState: DEFAULT; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children:
< 2018-11-21 11:12:19.484 UTC > DEBUG: CommitTransactionCommand
< 2018-11-21 11:12:19.484 UTC > DEBUG: CommitTransaction
< 2018-11-21 11:12:19.484 UTC > DEBUG: name: unnamed; blockState: STARTED; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children:
< 2018-11-21 11:12:19.484 UTC > LOG: duration: 0.367 ms statement: SELECT pg_backend_pid()
< 2018-11-21 11:12:19.484 UTC > DEBUG: StartTransactionCommand
< 2018-11-21 11:12:19.484 UTC > DEBUG: StartTransaction
< 2018-11-21 11:12:19.484 UTC > DEBUG: name: unnamed; blockState: DEFAULT; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children:
< 2018-11-21 11:12:19.484 UTC > DEBUG: ProcessUtility
< 2018-11-21 11:12:19.484 UTC > DEBUG: CommitTransactionCommand
< 2018-11-21 11:12:19.484 UTC > DEBUG: CommitTransaction
< 2018-11-21 11:12:19.484 UTC > DEBUG: name: unnamed; blockState: STARTED; state: INPROGR, xid/subid/cid: 151784/1/0 (used), nestlvl: 1, children:
< 2018-11-21 11:12:19.485 UTC > LOG: duration: 1.007 ms statement: alter database test rename to test_old
< 2018-11-21 11:12:19.485 UTC > DEBUG: shmem_exit(0): 1 before_shmem_exit callbacks to make
< 2018-11-21 11:12:19.485 UTC > DEBUG: shmem_exit(0): 6 on_shmem_exit callbacks to make
< 2018-11-21 11:12:19.485 UTC > DEBUG: proc_exit(0): 3 callbacks to make
< 2018-11-21 11:12:19.485 UTC > DEBUG: exit(0)
< 2018-11-21 11:12:19.485 UTC > DEBUG: shmem_exit(-1): 0 before_shmem_exit callbacks to make
< 2018-11-21 11:12:19.485 UTC > DEBUG: shmem_exit(-1): 0 on_shmem_exit callbacks to make
< 2018-11-21 11:12:19.485 UTC > DEBUG: proc_exit(-1): 0 callbacks to make
< 2018-11-21 11:12:19.486 UTC > DEBUG: reaping dead processes
< 2018-11-21 11:12:19.486 UTC > DEBUG: server process (PID 14261) exited with exit code 0

On Wed, Nov 21, 2018 at 2:50 PM Zwettler Markus (OIZ) <Markus(dot)Zwettler(at)zuerich(dot)ch<mailto:Markus(dot)Zwettler(at)zuerich(dot)ch>> wrote:
Hi,

We have a postgresql cluster with two databases.
We rename (switch) the databases daily (alter database x rename to y;)
The query performance slows down after each renaming.
Question: Why? Does postgresql flush the db-cache?

Thanks,
Markus