Horton

Re: PoC/WIP: Extended statistics on expressions

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: 2021-09-03T03:56:01Z
Lists: pgsql-hackers 
On Wed, Sep 01, 2021 at 06:45:29PM +0200, Tomas Vondra wrote:
> However while polishing the second patch, I realized we're allowing
> statistics on expressions referencing system attributes. So this fails;
> 
> CREATE STATISTICS s ON ctid, x FROM t;
> 
> but this passes:
> 
> CREATE STATISTICS s ON (ctid::text), x FROM t;
> 
> IMO we should reject such expressions, just like we reject direct references
> to system attributes - patch attached.

Right, same as indexes.  +1

> Furthermore, I wonder if we should reject expressions without any Vars? This
> works now:
> 
> CREATE STATISTICS s ON (11:text) FROM t;
> 
> but it seems rather silly / useless, so maybe we should reject it.

To my surprise, this is also allowed for indexes...

But (maybe this is what I was remembering) it's prohibited to have a constant
expression as a partition key.

Expressions without a var seem like a case where the user did something
deliberately silly, and dis-similar from the case of making a stats expression
on a simple column - that seemed like it could be a legitimate
mistake/confusion (it's not unreasonable to write an extra parenthesis, but
it's strange if that causes it to behave differently).

I think it's not worth too much effort to prohibit this: if they're determined,
they can still write an expresion with a var which is constant.  I'm not going
to say it's worth zero effort, though.

-- 
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