Thread: Running lots of inserts from selects on 9.4.5

On Tue, Feb 9, 2016 at 4:09 PM, Dan Langille <dan@langille.org> wrote:
> I have a wee database server which regularly tries to insert 1.5 million or
> even 15 million new rows into a 400 million row table.  Sometimes these
> inserts take hours.
>
> The actual query to produces the join is fast.  It's the insert which is
> slow.
>
> INSERT INTO File (FileIndex, JobId, PathId, FilenameId, LStat, MD5,
> DeltaSeq)
>   SELECT batch_testing.FileIndex, batch_testing.JobId, Path.PathId,
> Filename.FilenameId, batch_testing.LStat, batch_testing.MD5,
> batch_testing.DeltaSeq
>     FROM batch_testing JOIN Path     ON (batch_testing.Path = Path.Path)
>                        JOIN Filename ON (batch_testing.Name =
> Filename.Name);
>
> This is part of the plan: http://img.ly/images/9374145/full  created via
> http://tatiyants.com/pev/#/plans
>
> This gist contains postgresql.conf, zfs settings, slog, disk partitions.
>
>    https://gist.github.com/dlangille/33331a8c8cc62fa13b9f

The table you are inserting into has 7 indexes, all of which have to
be maintained.  The index on the sequence column should be efficient
to maintain.  But for the rest, if the inserted rows are not naturally
ordered by any of the indexed columns then it would end up reading 6
random scattered leaf pages in order to insert row pointers.  If none
those pages are in memory, that is going to be slow to read off from
hdd in single-file.  Also, you are going dirty all of those scattered
pages, and they will be slow to write back to hdd because there
probably won't be much opportunity for write-combining.

Do you really need all of those indexes?

Won't the index on (jobid, pathid, filenameid) service any query that
(jobid) does, so you can get rid of the latter?

And unless you have range queries on fileindex, like "where jobid = 12
and fileindex between 4 and 24" then you should be able to replace
(jobid, fileindex) with (fileindex,jobid) and then get rid of the
stand-alone index on (fileindex).

If you add an "order by" to the select statement which order by the
fields of one of the remaining indexes, than you could make the
maintenance of that index become much cheaper.

Could you move the indexes for this table to SSD?

SSD is probably wasted on your WAL.  If your main concern is bulk
insertions, then WAL is going to written sequentially with few fsyncs.
That is ideal for HDD.  Even if you also have smaller transactions,
WAL is still sequentially written as long as you have a non-volatile
cache on your RAID controller which can absorb fsyncs efficiently.

Cheers,

Jeff

> On Feb 10, 2016, at 2:47 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
>
> On Tue, Feb 9, 2016 at 4:09 PM, Dan Langille <dan@langille.org> wrote:
>> I have a wee database server which regularly tries to insert 1.5 million or
>> even 15 million new rows into a 400 million row table.  Sometimes these
>> inserts take hours.
>>
>> The actual query to produces the join is fast.  It's the insert which is
>> slow.
>>
>> INSERT INTO File (FileIndex, JobId, PathId, FilenameId, LStat, MD5,
>> DeltaSeq)
>>  SELECT batch_testing.FileIndex, batch_testing.JobId, Path.PathId,
>> Filename.FilenameId, batch_testing.LStat, batch_testing.MD5,
>> batch_testing.DeltaSeq
>>    FROM batch_testing JOIN Path     ON (batch_testing.Path = Path.Path)
>>                       JOIN Filename ON (batch_testing.Name =
>> Filename.Name);
>>
>> This is part of the plan: http://img.ly/images/9374145/full  created via
>> http://tatiyants.com/pev/#/plans
>>
>> This gist contains postgresql.conf, zfs settings, slog, disk partitions.
>>
>>   https://gist.github.com/dlangille/33331a8c8cc62fa13b9f
>
> The table you are inserting into has 7 indexes, all of which have to
> be maintained.  The index on the sequence column should be efficient
> to maintain.  But for the rest, if the inserted rows are not naturally
> ordered by any of the indexed columns then it would end up reading 6
> random scattered leaf pages in order to insert row pointers.  If none
> those pages are in memory, that is going to be slow to read off from
> hdd in single-file.  Also, you are going dirty all of those scattered
> pages, and they will be slow to write back to hdd because there
> probably won't be much opportunity for write-combining.
>
> Do you really need all of those indexes?
>
> Won't the index on (jobid, pathid, filenameid) service any query that
> (jobid) does, so you can get rid of the latter?
>
> And unless you have range queries on fileindex, like "where jobid = 12
> and fileindex between 4 and 24" then you should be able to replace
> (jobid, fileindex) with (fileindex,jobid) and then get rid of the
> stand-alone index on (fileindex).
>
> If you add an "order by" to the select statement which order by the
> fields of one of the remaining indexes, than you could make the
> maintenance of that index become much cheaper.

I will make these changes one-by-one and test each.  This will be interesting.

> Could you move the indexes for this table to SSD?

Now that's a clever idea.

bacula=# select pg_size_pretty(pg_indexes_size('file'));
 pg_size_pretty
----------------
 100 GB
(1 row)

bacula=# select pg_size_pretty(pg_table_size('file'));
 pg_size_pretty
----------------
 63 GB
(1 row)

bacula=#

No suprising that the indexes are larger than the data.

The SSD is 30GB.  I don't have enough space.  Buying 2x500GB SSDs
would allow me to put all the data onto SSD.  I'm using about 306G for the
databases now.

> SSD is probably wasted on your WAL.  If your main concern is bulk
> insertions, then WAL is going to written sequentially with few fsyncs.
> That is ideal for HDD.  Even if you also have smaller transactions,

OK.

> WAL is still sequentially written as long as you have a non-volatile
> cache on your RAID controller which can absorb fsyncs efficiently.

Of note, no RAID controller or non-volatile cache here. I'm running ZFS with plain HBA controllers.

Thank you.  I have some interesting changes to test.

--
Dan Langille - BSDCan / PGCon
dan@langille.org

> On Feb 10, 2016, at 5:13 AM, Dan Langille <dan@langille.org> wrote:
>
>> On Feb 10, 2016, at 2:47 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
>>
>> On Tue, Feb 9, 2016 at 4:09 PM, Dan Langille <dan@langille.org> wrote:
>>> I have a wee database server which regularly tries to insert 1.5 million or
>>> even 15 million new rows into a 400 million row table.  Sometimes these
>>> inserts take hours.
>>>
>>> The actual query to produces the join is fast.  It's the insert which is
>>> slow.
>>>
>>> INSERT INTO File (FileIndex, JobId, PathId, FilenameId, LStat, MD5,
>>> DeltaSeq)
>>> SELECT batch_testing.FileIndex, batch_testing.JobId, Path.PathId,
>>> Filename.FilenameId, batch_testing.LStat, batch_testing.MD5,
>>> batch_testing.DeltaSeq
>>>   FROM batch_testing JOIN Path     ON (batch_testing.Path = Path.Path)
>>>                      JOIN Filename ON (batch_testing.Name =
>>> Filename.Name);
>>>
>>> This is part of the plan: http://img.ly/images/9374145/full  created via
>>> http://tatiyants.com/pev/#/plans
>>>
>>> This gist contains postgresql.conf, zfs settings, slog, disk partitions.
>>>
>>>  https://gist.github.com/dlangille/33331a8c8cc62fa13b9f
>>
>> The table you are inserting into has 7 indexes, all of which have to
>> be maintained.  The index on the sequence column should be efficient
>> to maintain.  But for the rest, if the inserted rows are not naturally
>> ordered by any of the indexed columns then it would end up reading 6
>> random scattered leaf pages in order to insert row pointers.  If none
>> those pages are in memory, that is going to be slow to read off from
>> hdd in single-file.  Also, you are going dirty all of those scattered
>> pages, and they will be slow to write back to hdd because there
>> probably won't be much opportunity for write-combining.
>>
>> Do you really need all of those indexes?
>>
>> Won't the index on (jobid, pathid, filenameid) service any query that
>> (jobid) does, so you can get rid of the latter?
>>
>> And unless you have range queries on fileindex, like "where jobid = 12
>> and fileindex between 4 and 24" then you should be able to replace
>> (jobid, fileindex) with (fileindex,jobid) and then get rid of the
>> stand-alone index on (fileindex).
>>
>> If you add an "order by" to the select statement which order by the
>> fields of one of the remaining indexes, than you could make the
>> maintenance of that index become much cheaper.
>
> I will make these changes one-by-one and test each.  This will be interesting.

On a test server, the original insert takes about 45 minutes.  I removed all indexes.  25 minutes.

Thank you.

--
Dan Langille - BSDCan / PGCon
dan@langille.org