Re: Statistics Import and Export

Greetings,

* Corey Huinker (corey(dot)huinker(at)gmail(dot)com) wrote:
> On Thu, Feb 15, 2024 at 4:09 AM Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
> wrote:
> > Posting v5 updates of pg_import_rel_stats() and pg_import_ext_stats(),
> > which address many of the concerns listed earlier.
> >
> > Leaving the export/import scripts off for the time being, as they haven't
> > changed and the next likely change is to fold them into pg_dump.

> v6 posted below.
>
> Changes:
>
> - Additional documentation about the overall process.
> - Rewording of SGML docs.
> - removed a fair number of columns from the transformation queries.
> - enabled require_match_oids in extended statistics, but I'm having my
> doubts about the value of that.
> - moved stats extraction functions to an fe_utils file stats_export.c that
> will be used by both pg_export_stats and pg_dump.
> - pg_export_stats now generates SQL statements rather than a tsv, and has
> boolean flags to set the validate and require_match_oids parameters in the
> calls to pg_import_(rel|ext)_stats.
> - pg_import_stats is gone, as importing can now be done with psql.

Having looked through this thread and discussed a bit with Corey
off-line, the approach that Tom laid out up-thread seems like it would
make the most sense overall- that is, eliminate the JSON bits and the
SPI and instead export the stats data by running queries from the new
version of pg_dump/server (in the FDW case) against the old server
with the intelligence of how to transform the data into the format
needed for the current pg_dump/server to accept, through function calls
where the function calls generally map up to the rows/information being
updated- a call to update the information in pg_class for each relation
and then a call for each attribute to update the information in
pg_statistic.

Part of this process would include mapping from OIDs/attrnum's to names
on the source side and then from those names to the appropriate
OIDs/attrnum's on the destination side.

As this code would be used by both pg_dump and the postgres_fdw, it
seems logical that it would go into the common library. Further, it
would make sense to have this code be able to handle multiple major
versions for the foreign side, such as how postgres_fdw and pg_dump
already do.

In terms of working to ensure that newer versions support loading from
older dumps (that is, that v18 would be able to load a dump file created
by a v17 pg_dump against a v17 server in the face of changes having been
made to the statistics system in v18), we could have the functions take
a version parameter (to handle cases where the data structure is the
same but the contents have to be handled differently), use overloaded
functions, or have version-specific names for the functions. I'm also
generally supportive of the idea that we, perhaps initially, only
support dumping/loading stats with pg_dump when in binary-upgrade mode,
which removes our need to be concerned with this (perhaps that would be
a good v1 of this feature?) as the version of pg_dump needs to match
that of pg_upgrade and the destination server for various other reasons.
Including a switch to exclude stats on restore might also be an
acceptable answer, or even simply excluding them by default when going
between major versions except in binary-upgrade mode.

Along those same lines when it comes to a 'v1', I'd say that we may wish
to consider excluding extended statistics, which I am fairly confident
Corey's heard a number of times previously already but thought I would
add my own support for that. To the extent that we do want to make
extended stats work down the road, we should probably have some
pre-patches to flush out the missing _in/_recv functions for those types
which don't have them today- and that would include modifying the _out
of those types to use names instead of OIDs/attrnums. In thinking about
this, I was reviewing specifically pg_dependencies. To the extent that
there are people who depend on the current output, I would think that
they'd actually appreciate this change.

I don't generally feel like we need to be checking that the OIDs between
the old server and the new server match- I appreciate that that should
be the case in a binary-upgrade situation but it still feels unnecessary
and complicated and clutters up the output and the function calls.

Overall, I definitely think this is a good project to work on as it's an
often, rightfully, complained about issue when it comes to pg_upgrade
and the amount of downtime required for it before the upgraded system
can be reasonably used again.

Thanks,

Stephen