Horton

Re: PoC/WIP: Extended statistics on expressions (\d in old client)

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>, pgsql-hackers@postgresql.org
Date: 2021-06-03T02:39:20Z
Lists: pgsql-hackers 
On Fri, Jan 22, 2021 at 02:09:04PM +0100, Tomas Vondra wrote:
> On 1/22/21 5:01 AM, Justin Pryzby wrote:
> > On Fri, Jan 22, 2021 at 04:49:51AM +0100, Tomas Vondra wrote:
> > > > > | Statistics objects:
> > > > > |     "public"."s2" (ndistinct, dependencies, mcv) ON  FROM t
> > > 
> > > Umm, for me that prints:
> > 
> > >      "public"."s2" ON ((i + 1)), (((i + 1) + 0)) FROM t
> > > 
> > > which I think is OK. But maybe there's something else to trigger the
> > > problem?
> > 
> > Oh.  It's because I was using /usr/bin/psql and not ./src/bin/psql.
> > I think it's considered ok if old client's \d commands don't work on new
> > server, but it's not clear to me if it's ok if they misbehave.  It's almost
> > better it made an ERROR.
> > 
> 
> Well, how would the server know to throw an error? We can't quite patch the
> old psql (if we could, we could just tweak the query).

To refresh: stats objects on a v14 server which include expressions are shown
by pre-v14 psql client with the expressions elided (because the attnums don't
correspond to anything in pg_attribute).

I'm mentioning it again since, even though I knew about this earlier in the
year, it caused some confusion for me again just now while testing our
application.  I had the v14 server installed but the psql symlink still pointed
to the v13 client.

There may not be anything we can do about it.
And it may not be a significant issue outside the beta period: more typically,
the client version would match the server version.

-- 
Justin

Commits

  1. Disallow extended statistics on system columns

  2. Identify simple column references in extended statistics

  3. Don't print extra parens around expressions in extended stats

  4. Change position of field "transformed" in struct CreateStatsStmt.

  5. Add transformed flag to nodes/*funcs.c for CREATE STATISTICS

  6. Stabilize stats_ext test with other collations

  7. Extended statistics on expressions

  8. Reduce duration of stats_ext regression tests

  9. Allow composite types in catalog bootstrap

  10. Convert Typ from array to list in bootstrap

  11. Disallow CREATE STATISTICS on system catalogs