- On 5 Apr, 2019, at 4:14 AM, Jeff Janes wrote:
> On Thu, Apr 4, 2019 at 3:42 AM Duncan Kinnear < [
> mailto:[email protected] | [email protected] ] >
> wrote:
>> the EXPLAIN (ANALYSE, TIMING TRUE) of this query gives:
>> Update on next_id (cost=0.14..8.16 rows=1 width=36) (actual time=0.057..0.057
>> rows=0 loops=1)
>> -> Index Scan using next_id_pk on next_id (cost=0.14..8.16 rows=1 width=36)
>> (actual time=0.039..0.040 rows=1 loops=1)
>> Index Cond: ((id)::text = 'Session'::text)
>> Planning Time: 0.083 ms
>> Execution Time: 0.089 ms
>> which is significantly less than 50ms.
> The EXPLAIN ANALYZE doesn't include the time needed to fsync the transaction
> logs. It measures only the update itself, not the implicit commit at the end.
> DBeaver is seeing the fsync-inclusive time. 50ms is pretty long, but some file
> systems and OSes seem to be pretty inefficient at this and take several disk
> revolutions to get the data down.
>> Now, if I point DBeaver to a VM server on the same gigabit network switch,
>> running version:
>> 9.5.3 on i386-pc-solaris2.11, compiled by cc: Sun C 5.10 SunOS_i386 Patch
>> 142363-07 2010/12/09, 64-bit
>> then the same query executes in about 2-3ms
> That machine probably has hardware to do a fast fsync, has fsync turned off,
> or
> is lying about the safety of its data.
Just a quick update. I tried performing a sequence of BEGIN; UPDATE ...;
COMMIT; and I got the following log entries:
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.639 NZST [29887]
LOG: duration: 0.025 ms parse : begin
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.639 NZST [29887]
LOG: duration: 0.014 ms bind : begin
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.639 NZST [29887]
LOG: duration: 0.003 ms execute : begin
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.639 NZST [29887]
LOG: duration: 0.045 ms parse : update NEXT_ID set
next_value=next_value+1 where id='Session'
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.640 NZST [29887]
LOG: duration: 0.055 ms bind : update NEXT_ID set
next_value=next_value+1 where id='Session'
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.640 NZST [29887]
LOG: duration: 0.059 ms execute : update NEXT_ID set
next_value=next_value+1 where id='Session'
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.640 NZST [29887]
LOG: duration: 0.004 ms parse : commit
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.640 NZST [29887]
LOG: duration: 0.003 ms bind : commit
Apr 10 09:02:40 duncanpc postgres[7656]: 2019-04-10 09:02:40.690 NZST [29887]
LOG: duration: 50.237 ms execute : commit
So this confirms that the overhead is indeed happening in the COMMIT part. But
how do I get more detailed logging to see what it is doing?
Note, in a previous reply to Jeff (which I forgot to CC to the list) I
explained that the slow machines are both using BTRFS as the filesystem, and a
bit of googling has revealed that using PostgreSQL on BTRFS filesystems is
(don't cross the streams) bad.
Jeff, I will try adding the wait event stuff to see if that it what it is doing.
Cheers, Duncan
