Re: Statistics Import and Export

On Thu, 2024-03-21 at 03:27 -0400, Corey Huinker wrote:
>
> They can, but part of what I wanted to show was that the values that
> aren't directly passed in as parameters (staopN, stacollN) get set to
> the correct values, and those values aren't guaranteed to match
> across databases, hence testing them in the regression test rather
> than in a TAP test. I'd still like to be able to test that.

OK, that's fine.

> > The function signature for pg_set_attribute_stats could be more
> > friendly
...
> 1. We'd have to compare the stats provided against the stats that are
> already there, make that list in-memory, and then re-order what
> remains
> 2. There would be no way to un-set statistics of a given stakind,
> unless we added an "actually set it null" boolean for each parameter
> that can be null. 
> 3. I tried that with the JSON formats, it made the code even messier
> than it already was.

How about just some defaults then? Many of them have a reasonable
default, like NULL or an empty array. Some are parallel arrays and
either both should be specified or neither (e.g.
most_common_vals+most_common_freqs), but you can check for that.

> > Why are you calling checkCanModifyRelation() twice?
>
> Once for the relation itself, and once for pg_statistic.

Nobody has the privileges to modify pg_statistic except superuser,
right? I thought the point of a privilege check is that users could
modify statistics for their own tables, or the tables they maintain.

>
> I can see making it void and returning an error for everything that
> we currently return false for, but if we do that, then a statement
> with one pg_set_relation_stats, and N pg_set_attribute_stats (which
> we lump together in one command for the locking benefits and atomic
> transaction) would fail entirely if one of the set_attributes named a
> column that we had dropped. It's up for debate whether that's the
> right behavior or not.

I'd probably make the dropped column a WARNING with a message like
"skipping dropped column whatever". Regardless, have some kind of
explanatory comment.

>
> I pulled most of the hardcoded values from pg_stats itself. The
> sample set is trivially small, and the values inserted were in-order-
> ish. So maybe that's why.

In my simple test, most_common_freqs is descending:

CREATE TABLE a(i int);
INSERT INTO a VALUES(1);
INSERT INTO a VALUES(2);
INSERT INTO a VALUES(2);
INSERT INTO a VALUES(3);
INSERT INTO a VALUES(3);
INSERT INTO a VALUES(3);
INSERT INTO a VALUES(4);
INSERT INTO a VALUES(4);
INSERT INTO a VALUES(4);
INSERT INTO a VALUES(4);
ANALYZE a;
SELECT most_common_vals, most_common_freqs
FROM pg_stats WHERE tablename='a';
most_common_vals | most_common_freqs
------------------+-------------------
{4,3,2} | {0.4,0.3,0.2}
(1 row)

Can you show an example where it's not?

>
> Maybe we could have the functions restricted to a role or roles:
>
> 1. pg_write_all_stats (can modify stats on ANY table)
> 2. pg_write_own_stats (can modify stats on tables owned by user)

If we go that route, we are giving up on the ability for users to
restore stats on their own tables. Let's just be careful about
validating data to mitigate this risk.

Regards,
Jeff Davis