Обсуждение: Retire has_multiple_baserels()

Hi,

> The function has_multiple_baserels() is used in set_subquery_pathlist()
> to check and see if there are more than 1 base rel, by looping through
> simple_rel_array[].  I think one simpler way to do that is to leverage
> root->all_baserels by
>
>     bms_membership(root->all_baserels) == BMS_MULTIPLE
>
> all_baserels is computed in deconstruct_jointree (v16) or in
> make_one_rel (v15 and earlier), both are before we generate access paths
> for subquery RTEs, and it contains all base rels (but not "other" rels).
> So it should be a suitable replacement.  I doubt that there would be any
> measurable performance gains.  So please consider it cosmetic.
>
> I've attached a patch to do that.  Any thoughts?

I used the following patch to double check that nothing was missed:

```
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2207,8 +2207,13 @@ has_multiple_baserels(PlannerInfo *root)
                /* ignore RTEs that are "other rels" */
                if (brel->reloptkind == RELOPT_BASEREL)
                        if (++num_base_rels > 1)
+                       {
+
Assert(bms_membership(root->all_baserels) == BMS_MULTIPLE);
                                return true;
+                       }
        }
+
+       Assert(bms_membership(root->all_baserels) != BMS_MULTIPLE);
        return false;
 }
```

It wasn't. The patch LGTM.

-- 
Best regards,
Aleksander Alekseev

Aleksander Alekseev <aleksander@timescale.com> writes:
>> The function has_multiple_baserels() is used in set_subquery_pathlist()
>> to check and see if there are more than 1 base rel, by looping through
>> simple_rel_array[].  I think one simpler way to do that is to leverage
>> root->all_baserels by
>>     bms_membership(root->all_baserels) == BMS_MULTIPLE

> I used the following patch to double check that nothing was missed:
> ...
> It wasn't. The patch LGTM.

I thought this test wasn't too complete, because has_multiple_baserels
isn't reached at all in many cases thanks to the way the calling if()
is coded.  I tried testing like this instead:

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index eea49cca7b..3f6fc51fb4 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2649,6 +2649,8 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
      */
     remove_unused_subquery_outputs(subquery, rel, run_cond_attrs);

+    Assert(has_multiple_baserels(root) == (bms_membership(root->all_baserels) == BMS_MULTIPLE));
+
     /*
      * We can safely pass the outer tuple_fraction down to the subquery if the
      * outer level has no joining, aggregation, or sorting to do. Otherwise

and came to the same conclusion: check-world finds no cases where
the assertion fails.  So it LGTM too.  Pushed.

            regards, tom lane