Horton

Re: PoC/WIP: Extended statistics on expressions

Dean Rasheed <dean.a.rasheed@gmail.com>

View thread View on postgr.es
From: Dean Rasheed <dean.a.rasheed@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2021-03-24T16:28:05Z
Lists: pgsql-hackers 
On Wed, 24 Mar 2021 at 14:48, Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> AFAIK the primary issue here is that the two places disagree. While
> estimate_num_groups does this
>
>     varnos = pull_varnos(root, (Node *) varshere);
>     if (bms_membership(varnos) == BMS_SINGLETON)
>     { ... }
>
> the add_unique_group_expr does this
>
>     varnos = pull_varnos(root, (Node *) groupexpr);
>
> That is, one looks at the group expression, while the other look at vars
> extracted from it by pull_var_clause(). Apparently for PlaceHolderVar
> this can differ, causing the crash.
>
> So we need to change one of those places - my fix tweaked the second
> place to also look at the vars, but maybe we should change the other
> place? Or maybe it's not the right fix for PlaceHolderVars ...
>

I think that it doesn't make any difference which place is changed.

This is a case of an expression with no stats. With your change,
you'll get a single GroupExprInfo containing a list of
VariableStatData's for each of it's Var's, whereas if you changed it
the other way, you'd get a separate GroupExprInfo for each Var. But I
think they'd both end up being treated the same by
estimate_multivariate_ndistinct(), since there wouldn't be any stats
matching the expression, only the individual Var's. Maybe changing the
first place would be the more bulletproof fix though.

Regards,
Dean

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