Horton

Re: Removing unneeded self joins

David Rowley <david.rowley@2ndquadrant.com>

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From: David Rowley <david.rowley@2ndquadrant.com>
To: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-11-08T05:59:19Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  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 19 October 2018 at 01:47, Alexander Kuzmenkov
<a.kuzmenkov@postgrespro.ru> wrote:
> Here is a version that compiles.

I had a quick read through this and I think its missing about a 1-page
comment section detailing when we can and when we cannot remove these
self joins, and what measures we must take when we do remove them.

Apart from that, I noted the following during my read:

1. I don't think this is the right way to do this. There are other
places where we alter the varnoold. For example:
search_indexed_tlist_for_var(). So you should likely be doing that too
rather than working around it.

@@ -166,10 +166,13 @@ _equalVar(const Var *a, const Var *b)
  COMPARE_SCALAR_FIELD(vartypmod);
  COMPARE_SCALAR_FIELD(varcollid);
  COMPARE_SCALAR_FIELD(varlevelsup);
- COMPARE_SCALAR_FIELD(varnoold);
- COMPARE_SCALAR_FIELD(varoattno);
  COMPARE_LOCATION_FIELD(location);

+ /*
+ * varnoold/varoattno are used only for debugging and may differ even
+ * when the variables are logically the same.
+ */
+

2. Surely the following loop is incorrect:

for (i = toKeep->min_attr; i <= toKeep->max_attr; i++)
{
int attno = i - toKeep->min_attr;
toKeep->attr_needed[attno] = bms_add_members(toKeep->attr_needed[attno],
toRemove->attr_needed[attno]);
}

What if toRemove has a lower min_attr or higher max_attr?

3. "wind" -> "find"

+ * When we wind such a join, we mark one of the participating relation as

4. I think the following shouldn't be happening:

+------------------------------------------------
  Result
    One-Time Filter: false
-(2 rows)
+   ->  Index Scan using parent_pkey on parent x
+         Index Cond: (k = 1)
+(4 rows)

5. I'd have thought the opposite. Surely there are more chances of
this being useful with more joins?

+ /* Limit the number of joins we process to control the quadratic behavior. */
+ if (n > join_collapse_limit)
+ break;

6. In remove_self_joins_one_level() I think you should collect the
removed relations in a Relids rather than a list.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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