Re: [HACKERS] WITH clause in CREATE STATISTICS

Tom Lane wrote:

> Although I've not done anything about it here, I'm not happy about the
> handling of dependencies for stats objects.  I do not think that cloning
> RemoveStatistics as RemoveStatisticsExt is sane at all.  The former is
> meant to deal with cleaning up pg_statistic rows that we know will be
> there, so there's no real need to expend pg_depend overhead to track them.
> For objects that are only loosely connected, the right thing is to use
> the dependency system; in particular, this design has no real chance of
> working well with cross-table stats.  Also, it's really silly to have
> *both* this hard-wired mechanism and a pg_depend entry; the latter is
> surely redundant if you have the former.  IMO we should revert
> RemoveStatisticsExt and instead handle things by making stats objects
> auto-dependent on the individual column(s) they reference (not the whole
> table).
> 
> I'm also of the opinion that having an AUTO dependency, rather than
> a NORMAL dependency, on the stats object's schema is the wrong semantics.
> There isn't any other case where you can drop a non-empty schema without
> saying CASCADE, and I'm mystified why this case should act that way.

Here are two patches regarding handling of dependencies.  The first one
implements your first suggestion: add a NORMAL dependency on each
column, and do away with RemoveStatisticsExt.  This works well and
should uncontroversial.

If we only do this, then DROP TABLE needs to say CASCADE if there's a
statistics object in the table.  This seems pointless to me, so the
second patch throws in an additional dependency on the table as a whole,
AUTO this time, so that if the table is dropped, the statistics goes
away without requiring cascade.  There is no point in forcing CASCADE
for this case, ISTM.  This works well too, but I admit there might be
resistance to doing it.  OTOH this is how CREATE INDEX works.

(I considered what would happen with a stats object covering multiple
tables.  I think it's reasonable to drop the stats too in that case,
under the rationale that the object is no longer useful.  Not really
sure about this.)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers