Re: Removing unneeded self joins

On Thu, Jul 15, 2021 at 8:25 AM Zhihong Yu <zyu(at)yugabyte(dot)com> wrote:

>
>
> On Thu, Jul 15, 2021 at 7:49 AM Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
> wrote:
>
>> On 6/7/21 13:49, Hywel Carver wrote:
>> > On Mon, Jul 5, 2021 at 2:20 PM Andrey Lepikhov
>> > <a(dot)lepikhov(at)postgrespro(dot)ru <mailto:a(dot)lepikhov(at)postgrespro(dot)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