Re: Removing unneeded self joins

Zhihong Yu <zyu@yugabyte.com>

From: Zhihong Yu <zyu@yugabyte.com>

To: Andrey Lepikhov <a.lepikhov@postgrespro.ru>

Cc: Hywel Carver <hywel@skillerwhale.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>

Date: 2021-07-15T19:28:13Z

Lists: pgsql-hackers

Commits

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Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination
- 717d0e8dd945 18.0 landed
Put enable_self_join_elimination into postgresql.conf.sample
- c2d329260cd8 18.0 landed
Get rid of ojrelid local variable in remove_rel_from_query()
- e167191dc146 18.0 landed
Implement Self-Join Elimination
- fc069a3a6319 18.0 cited
Revert: Remove useless self-joins
- d1d286d83c0e 17.0 landed
Replace lateral references to removed rels in subqueries
- 466979ef031a 17.0 landed
Replace relids in lateral subquery parse tree during SJE
- 489072ab7a9e 17.0 landed
Forbid SJE with result relation
- 8c441c082797 17.0 landed
Fix misuse of RelOptInfo.unique_for_rels cache by SJE
- 30b4955a4668 17.0 landed
Replace the relid in some missing fields during SJE
- a7928a57b9f0 17.0 landed
Revert 56-bit relfilenode change and follow-up commits.
- a448e49bcbe4 16.0 cited
Stabilize timetz test across DST transitions.
- 4a071afbd056 14.0 cited
Speed up finding EquivalenceClasses for a given set of rels
- 3373c7155350 13.0 cited
Fix mark-and-restore-skipping test case to not be a self-join.
- 24d08f3c0a1f 12.0 landed

On Thu, Jul 15, 2021 at 8:25 AM Zhihong Yu <zyu@yugabyte.com> wrote:

>
>
> On Thu, Jul 15, 2021 at 7:49 AM Andrey Lepikhov <a.lepikhov@postgrespro.ru>
> wrote:
>
>> On 6/7/21 13:49, Hywel Carver wrote:
>> > On Mon, Jul 5, 2021 at 2:20 PM Andrey Lepikhov
>> > <a.lepikhov@postgrespro.ru <mailto:a.lepikhov@postgrespro.ru>> wrote:
>> > 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?
>> I think these tests weren't full coverage of possible use cases. It will
>> depend on a number of relations in the query. For the JOIN of
>> partitioned tables, for example, the overhead could grow. But in the
>> context of overall planning time this overhead will be small till the
>> large number of relations.
>> Also, we made this feature optional to solve possible problems.
>> Rebased on 768ea9bcf9
>>
>> --
>> regards,
>> Andrey Lepikhov
>> Postgres Professional
>>
> Hi,
>
> bq. We can proof the uniqueness
>
> proof -> prove
>
> 1. Collect all mergejoinable join quals looks like a.x = b.x
>
>  quals looks like -> quals which look like
>
> For update_ec_sources(), the variable cc is not used.
>
> Cheers
>
Hi,

+   *otherjoinquals = rjoinquals;

Maybe rename rjoinquals as ojoinquals to align with the target variable
name.

+   int k; /* Index of kept relation */
+   int r = -1; /* Index of removed relation */

Naming k as kept, r as removed would make the code more readable (remain
starts with r but has opposite meaning).

+               if (bms_is_member(r, info->syn_righthand) &&
+                   !bms_is_member(k, info->syn_righthand))
+                   jinfo_check = false;
+
+               if (!jinfo_check)
+                   break;

There are 4 if statements where jinfo_check is assigned false. Once
jinfo_check is assigned, we can break out of the loop - instead of checking
the remaining conditions.

+           else if (!innerrel_is_unique(root, joinrelids, outer->relids,

nit: the 'else' is not needed since the if block above it goes to next
iteration of the loop.

+           /* See for row marks. */
+           foreach (lc, root->rowMarks)

It seems once imark and omark are set, we can come out of the loop.

Cheers