MDEV-37510: crash when tracing with max_sel_arg_weight equal to 1

[bsrikanth-mariadb]

When the optimizer_max_sel_arg_weight is set to 1, a nested query crashed while tracing.

SEL_ARG object has a field named 'field', that is not set when the type is other than KEY_RANGE. But, the field was accessed to store its name, and weight to the trace. This resulted in a crash due to NULL pointer.

Added a check to access field if the type is KEY_RANGE, and if not, just trace the type.

[spetrunia]

I don't believe this is the minimal testcase.

Please try to reduce it as much as possible.

Tables need to have two records to avoid the "1-row table is const table" code path.

when one sees

CREATE TABLE t2 (c1 INT, c2 VARCHAR(10), c3 VARCHAR(10) DEFAULT NULL);

one wonders what is DEFAULT NULL there for? It's not needed in any case, please remove it.

[bsrikanth-mariadb]

sure

[spetrunia]

So I just took the above query and removed things that were irrelevant.

Please verify that the below still crashes and if yes , use it:

select * from
t1
JOIN
(
  SELECT c1, c2, c3 FROM t2
  UNION
  SELECT c1, c2, c3 FROM t2
) dt1
ON ((t1.c1 = dt1.c1 OR dt1.c1 IS NULL) AND
    (t1.c2 = dt1.c2 OR dt1.c3 IS NULL));

[bsrikanth-mariadb]

yes, it crashes. So, using it.

[spetrunia]

Please print key1_weight and key2_weight in any case.

[bsrikanth-mariadb]

ok

[spetrunia]

it is non-intuitive which condition the SEL_ARG with key2_type=X was made of...
But I see that SEL_ARG::part might not be set either. Ok, let's not bother and just print 'key2_type'.

[bsrikanth-mariadb]

I am thinking to print type and weight for both key1, and key2. Additionally if key1 type is RANGE then also print the field name. What do you think?

[spetrunia]

So I just took the above query and removed things that were irrelevant.

Please verify that the below still crashes and if yes , use it:

select * from
t1
JOIN
(
  SELECT c1, c2, c3 FROM t2
  UNION
  SELECT c1, c2, c3 FROM t2
) dt1
ON ((t1.c1 = dt1.c1 OR dt1.c1 IS NULL) AND
    (t1.c2 = dt1.c2 OR dt1.c3 IS NULL));

[spetrunia]

Hmm

                "key1_type": 2,
                "key2_type": 3,

looks very suspicious.

[spetrunia]

This is because

      if (key1->type == SEL_ARG::KEY_RANGE)
        obj.add("key1_field", key1->field->field_name);
      else
        obj.add("key1_type", (uint) key1->type);

      if (key1->type == SEL_ARG::KEY_RANGE) /// <<-- why key1->type?   key2->type !
        obj.add("key2_field", key2->field->field_name);
      else
        obj.add("key2_type", (uint) key2->type);
        

[spetrunia]

Looks ok now