Thread: Handling of mutable functions in subqueries?

In attempting to test the randomness of the random() function (because someone
was complaining on pgsql-general) I found the following strange behaviour.

Shouldn't Postgres be noticing the non-immutable random() function and not
making the subquery an InitPlan? 

test=> explain select (select * from test order by random() limit 1) as b from b limit 1000;
    QUERY PLAN
----------------------------------------------------------------------------Limit  (cost=72.33..86.74 rows=1000
width=0)  InitPlan     ->  Limit  (cost=72.33..72.33 rows=1 width=4)           ->  Sort  (cost=72.33..74.83 rows=1000
width=4)                Sort Key: random()                 ->  Seq Scan on test  (cost=0.00..22.50 rows=1000 width=4)
-> Seq Scan on b  (cost=0.00..2972.00 rows=206300 width=0)(7 rows)
 


I tried putting random() in more places:

test=> explain select (select random() from test where random() < 0.5 order by random() limit 1) as b from b limit
1000;                               QUERY PLAN
---------------------------------------------------------------------------Limit  (cost=39.84..54.24 rows=1000 width=0)
 InitPlan     ->  Limit  (cost=39.84..39.84 rows=1 width=0)           ->  Sort  (cost=39.84..40.67 rows=334 width=0)
            Sort Key: random()                 ->  Seq Scan on test  (cost=0.00..25.84 rows=334 width=0)
      Filter: (random() < 0.5::double precision)   ->  Seq Scan on b  (cost=0.00..2972.00 rows=206300 width=0)(8 rows)
 



The only way I got it to work properly was by making sure some columns from
the outer table were present in the subquery

test=> explain select (select case when b.b then a else a end from test order by random() limit 1) as b from b limit
1000;                                   QUERY PLAN
----------------------------------------------------------------------------------Limit  (cost=0.00..72345.83 rows=1000
width=1)  ->  Seq Scan on b  (cost=0.00..14924944.24 rows=206300 width=1)         SubPlan           ->  Limit
(cost=72.33..72.33rows=1 width=4)                 ->  Sort  (cost=72.33..74.83 rows=1000 width=4)
SortKey: random()                       ->  Seq Scan on test  (cost=0.00..22.50 rows=1000 width=4)(7 rows)
 



This was with 7.4.3 but I see the same behaviour with a CVS build near 8.0beta2:

test=# explain select (select * from test order by random() limit 1) as b from b limit 1000;
  QUERY PLAN                                -------------------------------------------------------------------------
Limit (cost=1.06..21.06 rows=1000 width=0)   InitPlan     ->  Limit  (cost=1.06..1.06 rows=1 width=32)           ->
Sort (cost=1.06..1.07 rows=3 width=32)                 Sort Key: random()                 ->  Seq Scan on test
(cost=0.00..1.04rows=3 width=32)   ->  Seq Scan on b  (cost=0.00..20.00 rows=1000 width=0)(7 rows)
 
test=# explain select (select random() from test where random() < 0.5 order by random() limit 1) as b from b limit
1000;                              QUERY PLAN
------------------------------------------------------------------------Limit  (cost=1.06..21.06 rows=1000 width=0)
InitPlan    ->  Limit  (cost=1.06..1.06 rows=1 width=0)           ->  Sort  (cost=1.06..1.06 rows=1 width=0)
    Sort Key: random()                 ->  Seq Scan on test  (cost=0.00..1.05 rows=1 width=0)
Filter:(random() < 0.5::double precision)   ->  Seq Scan on b  (cost=0.00..20.00 rows=1000 width=0)(8 rows)
 



-- 
greg

Greg Stark <gsstark@mit.edu> writes:
> Shouldn't Postgres be noticing the non-immutable random() function and not
> making the subquery an InitPlan? 

Perhaps.  In the dim past, using a dummy subquery was the only way to
persuade the planner not to re-evaluate functions that you didn't want
evaluated more than once, so changing this could be expected to break
some existing applications.  Barring some non-invented cases where it
causes problems, I'm inclined to leave it alone.
        regards, tom lane