Horton

Re: Removing unneeded self joins

Hywel Carver <hywel@skillerwhale.com>

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From: Hywel Carver <hywel@skillerwhale.com>
To: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
Cc: pgsql-hackers@lists.postgresql.org, Konstantin Knizhnik <knizhnik@garret.ru>
Date: 2021-07-06T10:49:54Z
Lists: pgsql-hackers

Commits

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  1. Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination

  2. Put enable_self_join_elimination into postgresql.conf.sample

  3. Get rid of ojrelid local variable in remove_rel_from_query()

  4. Implement Self-Join Elimination

  5. Revert: Remove useless self-joins

  6. Replace lateral references to removed rels in subqueries

  7. Replace relids in lateral subquery parse tree during SJE

  8. Forbid SJE with result relation

  9. Fix misuse of RelOptInfo.unique_for_rels cache by SJE

  10. Replace the relid in some missing fields during SJE

  11. Revert 56-bit relfilenode change and follow-up commits.

  12. Stabilize timetz test across DST transitions.

  13. Speed up finding EquivalenceClasses for a given set of rels

  14. Fix mark-and-restore-skipping test case to not be a self-join.

On Mon, Jul 5, 2021 at 2:20 PM Andrey Lepikhov <a.lepikhov@postgrespro.ru>
wrote:

> On 2/7/21 01:56, Hywel Carver wrote:
> > On Wed, Jun 30, 2021 at 12:21 PM Andrey Lepikhov
> > <a.lepikhov@postgrespro.ru <mailto:a.lepikhov@postgrespro.ru>> wrote:
> >     I think, here we could ask more general question: do we want to
> >     remove a
> >     'IS NOT NULL' clause from the clause list if the rest of the list
> >     implicitly implies it?
> >
> >
> > My suggestion was not to remove it, but to avoid adding it in the first
> > place. When your optimisation has found a join on a group of columns
> > under a uniqueness constraint, you would do something like this (forgive
> > the pseudo-code)
> >
> > foreach(column, join_clause) {
> >    if(column.nullable) { // This condition is what I'm suggesting is
> added
> >      add_null_test(column, IS_NOT_NULL);
> >    }
> > }
> >
> > But it may be that that's not possible or practical at this point in the
> > code.
> I think, such option will require to implement a new machinery to prove
> that arbitrary column couldn't produce NULL value.
>

Got it, and it makes sense to me that this would be out of scope for this
change.

I remember in the previous conversation about this, Tomas acknowledged that
while there are some silly queries that would benefit from this change,
there are also some well-written ones (e.g. properly denormalised table
structures, with decomposed views that need joining together in some
queries). So the optimization needs to be essentially free to run to
minimise impact on other queries.

Looking through the email chain, a previous version of this patch added
~0.6% to planning time in the worst case tested - does that meet the
"essentially free" requirement?