BUG #17564: Planner bug in combination of generate_series(), unnest() and ORDER BY - Mailing list pgsql-bugs

The following bug has been logged on the website:

Bug reference:      17564
Logged by:          Martijn van Oosterhout
Email address:      kleptog@gmail.com
PostgreSQL version: 14.4
Operating system:   Debian Linux (Bullseye)
Description:

We ran into a strange planner issue on our production system on Friday.
Basically, a query would fail in the planning stage, depending one of the
parameters. This shouldn't happen (AIUI). The query is as follows
(simplified considerably from the original):

db=# explain SELECT generate_subscripts(ARRAY[]::integer[], 1) AS id, 
       unnest(ARRAY[]::jsonb[]) ->> 'timestamp'  AS timestamp 
FROM results 
JOIN groups ON groups.id = results.group_id 
WHERE results.search_id = 3336 
order by timestamp;

(Yes, I know the combination of unnest() and generate_subscripts() in this
way is evil, but it does work.)

The error is:

ERROR:  set-valued function called in context that cannot accept a set
LINE 2:        unnest(ARRAY[]::jsonb[]) ->> 'timestamp'  AS timestam...

However, if you disable nested loops, it works fine:

db=# set enable_nestloop =false;
SET
db=# explain SELECT generate_subscripts(ARRAY[]::integer[], 1) AS id, 
       unnest(ARRAY[]::jsonb[]) ->> 'timestamp'  AS timestamp 
FROM results 
JOIN groups ON groups.id = results.group_id 
WHERE results.search_id = 3336 
order by timestamp;
                                                        QUERY PLAN
                                             

--------------------------------------------------------------------------------------------------------------------------
 Sort  (cost=54523.19..55478.19 rows=382000 width=36)
   Sort Key: (((unnest('{}'::jsonb[])) ->> 'timestamp'::text))
   ->  Result  (cost=19.75..8658.15 rows=382000 width=36)
         ->  ProjectSet  (cost=19.75..1973.15 rows=382000 width=36)
               ->  Hash Join  (cost=19.75..59.33 rows=382 width=0)
                     Hash Cond: (groups.id = results.group_id)
                     ->  Seq Scan on groups  (cost=0.00..36.54 rows=1154
width=4)
                     ->  Hash  (cost=14.97..14.97 rows=382 width=4)
                           ->  Index Only Scan using results_pkey on results
 (cost=0.29..14.97 rows=382 width=4)
                                 Index Cond: (search_id = 3336)
(10 rows)

If you remove the ORDER BY, it works.

If you remove the generate_series(), it works.

If you remove the JOIN, it works.

If you remove the "->> 'timestamp'", it works.

If you wrap the query in a subquery without the ORDER BY, and then the put
the ORDER BY on that, it works. Like so:

explain SELECT * FROM (SELECT generate_subscripts(ARRAY[]::integer[], 1) AS
id, 
       unnest(ARRAY[]::jsonb[]) ->> 'timestamp'  AS timestamp 
FROM results 
JOIN groups ON groups.id = results.group_id 
WHERE results.search_id = 3336) x
order by timestamp;

This gives the same query plan as above after disabling the nested loops.

What appears to be happening is that the planner attempts a transformation
and places the unnest() in the ORDER BY statement replacing the reference to
the "timestamp" field with the actual expression. There unnest() is clearly
not allowed. Perhaps the fact that the unnest() is hidden beneath the
operator(->>) prevents the planner from noticing the transformation is not
permitted.

This is a pain to reproduce. Just dumping are restoring the tables elsewhere
did not work. We noticed that the 'results' table in production was quite
bloated and when we replicated that in the test environment, it finally
triggered. At least at 300% bloat it triggered, that probably triggers the
planner to try some other plans.

This is not a critical bug, since it is easily worked around and the
combination of conditions seems quite unusual.

Noticed in 13.4, reproduced in 13.7 and 14.4.

Have a nice day,
Martijn