Horton

Re: PoC/WIP: Extended statistics on expressions

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2021-03-24T16:48:17Z
Lists: pgsql-hackers 

On 3/24/21 5:28 PM, Dean Rasheed wrote:
> 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.
> 

Yeah, I think that's true. I'll do a bit more research / experiments.

As for the changes proposed in the create_statistics, do we really want
to use univariate / multivariate there? Yes, the terms are correct, but
I'm not sure how many people looking at CREATE STATISTICS will
understand them.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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