Thread: possible bug

Hello,

We had a support request today, and we have narrowed down the problem to a query that behaves very strangely. The actual query was much more complicated, but I came up with this minimal example.

This is what we have seen inside our application:

select * from test where id in (26643094740, 26437091668);

id         |code|regno|col_3|
-----------+----+-----+-----+
26437091668|TA-T| 2632|    1|

26643094740|PEG | 2905|    1|

select * from test where id = 26643094740;

id         |code|regno|col_3|
-----------+----+-----+-----+

26643094740|PEG | 2905|    0| 

The problem: value of col_3 changes for id=26643094740 if I query two rows vs. one row. This is without changing any data. The problem is 100% repeatable, if I query two rows from the same view, then I get different data for one of the rows.

I suspect that this is a bug. But I might be wrong. Please help me!

The actual test view looks like this:

create view test as
SELECT
    c.id,
    tt.code,
    c.regno,
    (
        select count(*)
        FROM kap.course_user cu
        JOIN wf.workflow w_1 ON w_1.rec_id = cu.id AND w_1.head_table_id = 25408438504
        where cu.is_active AND cu.course_id = c.id AND w_1.station_id = 25406740434
    ) AS col_3
FROM
    kap.course c
    INNER JOIN kap.training_type tt ON tt.id = c.training_type_id;

Below are some DDL s (simplified, the actual tables contain much more fields).

Do you think that this might be a bug? If not, then can somebody please explain how this can happen?

   Laszlo

explain analyze select * from test where id in (26643094740, 26437091668);

QUERY PLAN                                                                                                                                             |
-------------------------------------------------------------------------------------------------------------------------------------------------------+
Hash Join  (cost=16.08..124.99 rows=2 width=29) (actual time=0.067..0.107 rows=2 loops=1)                                                              |
  Hash Cond: (tt.id = c.training_type_id)                                                                                                              |
  ->  Seq Scan on training_type tt  (cost=0.00..12.71 rows=71 width=13) (actual time=0.004..0.033 rows=71 loops=1)                                     |
  ->  Hash  (cost=16.05..16.05 rows=2 width=24) (actual time=0.024..0.025 rows=2 loops=1)                                                              |
        Buckets: 1024  Batches: 1  Memory Usage: 9kB                                                                                                   |
        ->  Bitmap Heap Scan on course c  (cost=8.58..16.05 rows=2 width=24) (actual time=0.018..0.021 rows=2 loops=1)                                 |
              Recheck Cond: (id = ANY ('{26643094740,26437091668}'::bigint[]))                                                                         |
              Heap Blocks: exact=2                                                                                                                     |
              ->  Bitmap Index Scan on pk_course  (cost=0.00..8.58 rows=2 width=0) (actual time=0.012..0.012 rows=2 loops=1)                           |
                    Index Cond: (id = ANY ('{26643094740,26437091668}'::bigint[]))                                                                     |
  SubPlan 1                                                                                                                                            |
    ->  Aggregate  (cost=47.91..47.92 rows=1 width=8) (actual time=0.014..0.014 rows=1 loops=2)                                                        |
          ->  Nested Loop  (cost=0.59..47.90 rows=1 width=0) (actual time=0.010..0.012 rows=1 loops=2)                                                 |
                ->  Index Scan using workflow_idx_station on workflow w_1  (cost=0.29..22.90 rows=3 width=8) (actual time=0.005..0.007 rows=1 loops=2) |
                      Index Cond: (station_id = '25406740434'::bigint)                                                                                 |
                      Filter: (head_table_id = '25408438504'::bigint)                                                                                  |
                ->  Memoize  (cost=0.30..8.32 rows=1 width=8) (actual time=0.004..0.004 rows=1 loops=2)                                                |
                      Cache Key: w_1.rec_id                                                                                                            |
                      Hits: 1  Misses: 1  Evictions: 0  Overflows: 0  Memory Usage: 1kB                                                                |
                      ->  Index Scan using pk_course_user on course_user cu  (cost=0.29..8.31 rows=1 width=8) (actual time=0.005..0.005 rows=1 loops=1)|
                            Index Cond: (id = w_1.rec_id)                                                                                              |
                            Filter: (is_active AND (course_id = c.id))                                                                                 |
Planning Time: 0.527 ms                                                                                                                                |
Execution Time: 0.175 ms                                                                                                                               |

explain analyze select * from test where id in (26643094740)

QUERY PLAN                                                                                                                                             |
-------------------------------------------------------------------------------------------------------------------------------------------------------+
Nested Loop  (cost=0.42..64.60 rows=1 width=29) (actual time=0.033..0.035 rows=1 loops=1)                                                              |
  ->  Index Scan using pk_course on course c  (cost=0.28..8.30 rows=1 width=24) (actual time=0.007..0.008 rows=1 loops=1)                              |
        Index Cond: (id = '26643094740'::bigint)                                                                                                       |
  ->  Index Scan using pk_training_type on training_type tt  (cost=0.14..8.16 rows=1 width=13) (actual time=0.002..0.002 rows=1 loops=1)               |
        Index Cond: (id = c.training_type_id)                                                                                                          |
  SubPlan 1                                                                                                                                            |
    ->  Aggregate  (cost=47.91..47.92 rows=1 width=8) (actual time=0.018..0.019 rows=1 loops=1)                                                        |
          ->  Nested Loop  (cost=0.59..47.90 rows=1 width=0) (actual time=0.017..0.017 rows=0 loops=1)                                                 |
                ->  Index Scan using workflow_idx_station on workflow w_1  (cost=0.29..22.90 rows=3 width=8) (actual time=0.007..0.009 rows=1 loops=1) |
                      Index Cond: (station_id = '25406740434'::bigint)                                                                                 |
                      Filter: (head_table_id = '25408438504'::bigint)                                                                                  |
                ->  Memoize  (cost=0.30..8.32 rows=1 width=8) (actual time=0.007..0.007 rows=0 loops=1)                                                |
                      Cache Key: w_1.rec_id                                                                                                            |
                      Hits: 0  Misses: 1  Evictions: 0  Overflows: 0  Memory Usage: 1kB                                                                |
                      ->  Index Scan using pk_course_user on course_user cu  (cost=0.29..8.31 rows=1 width=8) (actual time=0.005..0.005 rows=0 loops=1)|
                            Index Cond: (id = w_1.rec_id)                                                                                              |
                            Filter: (is_active AND (course_id = c.id))                                                                                 |
                            Rows Removed by Filter: 1                                                                                                  |
Planning Time: 0.528 ms                                                                                                                                |
Execution Time: 0.096 ms                                                                                                                               |

CREATE TABLE kap.course (
  id int8 NOT NULL,
  training_type_id int8 NOT NULL,
  regno int8 NOT NULL,

  -- many more fields here...
  CONSTRAINT pk_course PRIMARY KEY (id),
  CONSTRAINT fk_course_training_type_id FOREIGN KEY (training_type_id) REFERENCES kap.training_type(id) DEFERRABLE

  -- many more constraints here...
);

CREATE TABLE kap.training_type (
  id int8 NOT NULL,
  code text NOT NULL,
  -- many more fields here 

 CONSTRAINT pk_training_type PRIMARY KEY (id)

  -- many more constraints here
);
CREATE UNIQUE INDEX training_type_uidx_code ON kap.training_type USING btree (code);

CREATE TABLE kap.course_user (
  id int8 NOT NULL,
  is_active bool NOT NULL DEFAULT true,
  course_id int8 NOT NULL,
  sec_user_id int8 NOT NULL,
  CONSTRAINT pk_course_user PRIMARY KEY (id),
  CONSTRAINT fk_course_user_course_id FOREIGN KEY (course_id) REFERENCES kap.course(id) DEFERRABLE,
  CONSTRAINT fk_course_user_sec_user_id FOREIGN KEY (sec_user_id) REFERENCES sys.sec_user(id) DEFERRABLE

  -- many more constraints here...
);
CREATE UNIQUE INDEX course_user_uidx ON kap.course_user USING btree (course_id, sec_user_id);

CREATE TABLE wf.workflow (
  id int8 NOT NULL,
wf_type_id int8 NOT NULL,
head_table_id int8 NOT NULL,
table_info_id int8 NOT NULL,
rec_id int8 NOT NULL,
station_id int8 NOT NULL,

-- many more fields here...
CONSTRAINT pk_workflow PRIMARY KEY (id),
CONSTRAINT fk_workflow_station_id FOREIGN KEY (station_id) REFERENCES wf.station(id) DEFERRABLE,
CONSTRAINT fk_workflow_table_info_id FOREIGN KEY (table_info_id) REFERENCES meta.table_info(id) DEFERRABLE,
CONSTRAINT fk_workflow_wf_type_id FOREIGN KEY (wf_type_id) REFERENCES wf.wf_type(id) DEFERRABLE

-- many more constraints here
);
CREATE UNIQUE INDEX uidx_ht ON wf.workflow USING btree (head_table_id, rec_id);
CREATE INDEX workflow_idx_station ON wf.workflow USING btree (station_id);
CREATE UNIQUE INDEX workflow_uidx_ht ON wf.workflow USING btree (head_table_id, rec_id);

On 10/21/22 10:50 AM, Les wrote:
> Hello,
>
> We had a support request today, and we have narrowed down the problem to
> a query that behaves very strangely. The actual query was much more
> complicated, but I came up with this minimal example.
>
> This is what we have seen inside our application:
>
> select * from test where id in (26643094740, 26437091668);
>
> id         |code|regno|col_3|
> -----------+----+-----+-----+
> 26437091668|TA-T| 2632|    1|
> 26643094740|PEG | 2905|    1|
>
> select * from test where id = 26643094740;
>
> id         |code|regno|col_3|
> -----------+----+-----+-----+
> 26643094740|PEG | 2905|    0|
>
> The problem: value of col_3 changes for id=26643094740 if I query two
> rows vs. one row. This is without changing any data. The problem is 100%
> repeatable, if I query two rows from the same view, then I get different
> data for one of the rows.
>
> I suspect that this is a bug. But I might be wrong. Please help me!

I suspect an index problem. Have you tried reindexing the source table,
kap.course if I am following correctly.

Have there been any issues with the database lately, e.g. crash or other
significant event?

>
> The actual test view looks like this:
>


--
Adrian Klaver
adrian.klaver@aklaver.com