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: 2021-07-01T22:56:37Z
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 Wed, Jun 30, 2021 at 12:21 PM Andrey Lepikhov <a.lepikhov@postgrespro.ru>
wrote:

> On 12/3/21 12:05, Hywel Carver wrote:
> > I've built and tested this, and it seems to function correctly to me.
> One question I have is whether the added "IS NOT NULL" filters can be
> omitted when they're unnecessary. Some of the resulting plans included an
> "IS NOT NULL" filter on a non-nullable column. To be clear, this is still
> an improvement (to me) without that.

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.