pg_restore enhancements

On Wednesday, November 22, 2023, Efrain J. Berdecia <ejberdecia@yahoo.com>
wrote:

Thanks in advance for any suggestions or the green light to post this to
the PG-developer group :-)

If you aren’t offering up a patch for these it isn’t developer material and
belongs right here.

David J.

Thanks, I'm trying to gage the interest on such a feature enhancement. 
Up to now I have not actively contributed to the Postgres Project but this is itching my rusty programming fingers lol
Thanks,Efrain J. Berdecia

On Wednesday, November 22, 2023 at 08:28:18 AM EST, David G. Johnston <david.g.johnston@gmail.com> wrote:

On Wednesday, November 22, 2023, Efrain J. Berdecia <ejberdecia@yahoo.com> wrote:

Thanks in advance for any suggestions or the green light to post this to the PG-developer group :-)

If you aren’t offering up a patch for these it isn’t developer material and belongs right here.
David J.

On 11/22/23 05:25, Efrain J. Berdecia wrote:

After working for a site where we are constantly doing logical pg_dump
to refresh environments I've come to miss features available in other
RDBMS' refresh/restore utilities.

Someone could point me in the right direction otherwise, but pg_restore
seems to be lacking the ability to resume a restore upon failure, is all
or nothing with this guy. There also doesn't seem to be a way to control
batch size when doing the COPY phase, therefore preventing the WAL
directory from filling up and crashing the system.

The above needs more information on Postgres version(community or fork),
OS and version, the size of the data set, the storage type and size, the
Postgres conf, etc. Restores are being done all the time and this is the
first report, as far as I can remember, about an issue with COPY and
WAL in a restore.

pg_restore
https://www.postgresql.org/docs/current/app-pgrestore.html

Does have:

--section=sectionname

Only restore the named section. The section name can be pre-data,
data, or post-data. This option can be specified more than once to
select multiple sections. The default is to restore all sections.

The data section contains actual table data as well as large-object
definitions. Post-data items consist of definitions of indexes,
triggers, rules and constraints other than validated check constraints.
Pre-data items consist of all other data definition items.

AND

-l
--list

List the table of contents of the archive. The output of this
operation can be used as input to the -L option. Note that if filtering
switches such as -n or -t are used with -l, they will restrict the items
listed.

-L list-file
--use-list=list-file

Restore only those archive elements that are listed in list-file,
and restore them in the order they appear in the file. Note that if
filtering switches such as -n or -t are used with -L, they will further
restrict the items restored.

list-file is normally created by editing the output of a previous
-l operation. Lines can be moved or removed, and can also be commented
out by placing a semicolon (;) at the start of the line. See below for
examples.

IMHO, it would be nice to have a feature that would allow pg_restore to
resume based on which part of the restore and/or object have already
been restored.

When it comes to the COPY phase of the restore, it would be nice to be
able to control batch size and resume COPY of a particular object upon
failure.

COPY as it stands now is all or none, so that command would have to be
changed.

Thanks in advance for any suggestions or the green light to post this to
the PG-developer group :-)

Thanks,
Efrain J. Berdecia

--
Adrian Klaver
adrian.klaver@aklaver.com

Thanks, the issue we've run into, which I guess could be really a setup issue, with running a COPY command while executing pg_restore, is that if we are restoring a large table (bigger than 500GB) our WAL directory can grow to be very large.
I would think that if the pg_restore or COPY command was able to support a batch-size option, this should allow postgres to either archive or remove wal files and prevent having to re-size the WAL directory for a one time refresh operation.
I'm trying to gage how feasible would be to start looking at contributing to add such a feature to either the COPY command or pg_restore.
Thanks,Efrain J. Berdecia

On Wednesday, November 22, 2023 at 11:37:13 AM EST, Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 11/22/23 05:25, Efrain J. Berdecia wrote:

After working for a site where we are constantly doing logical pg_dump
to refresh environments I've come to miss features available in other
RDBMS' refresh/restore utilities.

Someone could point me in the right direction otherwise, but pg_restore
seems to be lacking the ability to resume a restore upon failure, is all
or nothing with this guy. There also doesn't seem to be a way to control
batch size when doing the COPY phase, therefore preventing the WAL
directory from filling up and crashing the system.

The above needs more information on Postgres version(community or fork),
OS and version, the size of the data set, the storage type and size, the
Postgres conf, etc. Restores are being done all the time and this is the
first report, as far as I can remember,  about an issue with COPY and
WAL in a restore.

pg_restore
https://www.postgresql.org/docs/current/app-pgrestore.html

Does have:

--section=sectionname

    Only restore the named section. The section name can be pre-data,
data, or post-data. This option can be specified more than once to
select multiple sections. The default is to restore all sections.

    The data section contains actual table data as well as large-object
definitions. Post-data items consist of definitions of indexes,
triggers, rules and constraints other than validated check constraints.
Pre-data items consist of all other data definition items.

AND

-l
--list

    List the table of contents of the archive. The output of this
operation can be used as input to the -L option. Note that if filtering
switches such as -n or -t are used with -l, they will restrict the items
listed.

-L list-file
--use-list=list-file

    Restore only those archive elements that are listed in list-file,
and restore them in the order they appear in the file. Note that if
filtering switches such as -n or -t are used with -L, they will further
restrict the items restored.

    list-file is normally created by editing the output of a previous
-l operation. Lines can be moved or removed, and can also be commented
out by placing a semicolon (;) at the start of the line. See below for
examples.

IMHO, it would be nice to have a feature that would allow pg_restore to
resume based on which part of the restore and/or object have already
been restored.

When it comes to the COPY phase of the restore, it would be nice to be
able to control batch size and resume COPY of a particular object upon
failure.

COPY as it stands now is all or none, so that command would have to be
changed.

Thanks in advance for any suggestions or the green light to post this to
the PG-developer group :-)

Thanks,
Efrain J. Berdecia

--
Adrian Klaver
adrian.klaver@aklaver.com

"Efrain J. Berdecia" <ejberdecia@yahoo.com> writes:

Thanks, the issue we've run into, which I guess could be really a setup issue, with running a COPY command while executing pg_restore, is that if we are restoring a large table (bigger than 500GB) our WAL directory can grow to be very large.
I would think that if the pg_restore or COPY command was able to support a batch-size option, this should allow postgres to either archive or remove wal files and prevent having to re-size the WAL directory for a one time refresh operation.
I'm trying to gage how feasible would be to start looking at contributing to add such a feature to either the COPY command or pg_restore.

Given the shortage of other complaints, I tend to agree with Adrian
that there's not likely to be much interest in adding complexity
to pg_restore (or COPY) to address this. You should probably look
harder at the idea that you have some configuration problem that's
triggering your WAL bloat. If COPY can run you out of WAL space,
then so could any future bulk insert or update.

regards, tom lane

On Wed, Nov 22, 2023 at 2:28 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

"Efrain J. Berdecia" <ejberdecia@yahoo.com> writes:

Thanks, the issue we've run into, which I guess could be really a setup

issue, with running a COPY command while executing pg_restore, is that if
we are restoring a large table (bigger than 500GB) our WAL directory can
grow to be very large.

I would think that if the pg_restore or COPY command was able to support

a batch-size option, this should allow postgres to either archive or remove
wal files and prevent having to re-size the WAL directory for a one time
refresh operation.

I'm trying to gage how feasible would be to start looking at

contributing to add such a feature to either the COPY command or pg_restore.

Given the shortage of other complaints, I tend to agree with Adrian
that there's not likely to be much interest in adding complexity
to pg_restore (or COPY) to address this. You should probably look
harder at the idea that you have some configuration problem that's
triggering your WAL bloat. If COPY can run you out of WAL space,
then so could any future bulk insert or update.

What OP needs, I think, since I'd use it, too, is "pg_bulkload without the
intrusive hacks and restrictions".

On Wed, 2023-11-22 at 16:55 +0000, Efrain J. Berdecia wrote:

Thanks, the issue we've run into, which I guess could be really a setup
issue, with running a COPY command while executing pg_restore, 
is that if we are restoring a large table (bigger than 500GB) our WAL directory can grow to be very large.

You can avoidwriting WAL if you set "wal_level = minimal", restart PostgreSQL
and restore the dump with the --single-transaction option.

Yours,
Laurenz Albe

On Thu, Nov 23, 2023 at 3:37 AM Laurenz Albe <laurenz.albe@cybertec.at>
wrote:
[snip]

You can avoidwriting WAL if you set "wal_level = minimal", restart
PostgreSQL
and restore the dump with the --single-transaction option.

Why does "--single-transaction" prevent WAL writes? I'd expect _more_
pg_wal growth from One Ginormous Transaction.

Ron Johnson <ronljohnsonjr@gmail.com> writes:

On Thu, Nov 23, 2023 at 3:37 AM Laurenz Albe <laurenz.albe@cybertec.at>
wrote:

You can avoidwriting WAL if you set "wal_level = minimal", restart
PostgreSQL
and restore the dump with the --single-transaction option.

Why does "--single-transaction" prevent WAL writes? I'd expect _more_
pg_wal growth from One Ginormous Transaction.

I don't recall all the details offhand, but there's some optimization
concerned with not writing WAL if COPY's target table was created in
the current transaction. WAL will still be made for the catalog
changes, but usually the bulk of the WAL for a pg_restore run comes
from loading data, and this recipe eliminates that. (Of course,
you cannot use it on a replication primary.)

regards, tom lane

Thanks for the explanation.

On Thu, Nov 23, 2023 at 10:55 AM Tom Lane <tgl@sss.pgh.pa.us> wrote: