Don't presort ORDER BY/DISTINCT Aggrefs with volatile functions

In 1349d2790, we gave the planner the ability to provide ORDER BY/DISTINCT
Aggrefs with presorted input so that nodeAgg would not have to perform
sorts during execution.  That commit failed to properly consider the
implications of if the Aggref had a volatile function in its ORDER
BY/DISTINCT clause.  As it happened, this resulted in an ERROR about the
volatile function being missing from the targetlist.

Here, instead of adding the volatile function to the targetlist, we just
never consider an Aggref with a volatile function in its ORDER BY/DISTINCT
clause when choosing which Aggrefs we should sort by.  We do this as if we
were to choose a plan which provided these aggregates with presorted
input, then if there were many such aggregates which could all share the
same sort order, then it may be surprising if they all shared the same
sort sometimes and didn't at other times when some other set of aggregates
were given presorted results.  We can avoid this inconsistency by just
never providing these volatile function aggregates with presorted input.

Reported-by: Dean Rasheed
Discussion: https://postgr.es/m/CAEZATCWETioXs5kY8vT6BVguY41_wD962VDk=u_Nvd7S1UXzuQ@mail.gmail.com

1 files changed, 19 insertions, 0 deletions

diff --git a/src/test/regress/expected/aggregates.out b/src/test/regress/expected/aggregates.out
index 309c2dc8654..ae3b9053318 100644
--- a/src/test/regress/expected/aggregates.out
+++ b/src/test/regress/expected/aggregates.out
@@ -1471,6 +1471,25 @@ group by ten;
          ->  Seq Scan on tenk1
 (5 rows)
 
+-- Ensure that we never choose to provide presorted input to an Aggref with
+-- a volatile function in the ORDER BY / DISTINCT clause.  We want to ensure
+-- these sorts are performed individually rather than at the query level.
+explain (costs off)
+select
+  sum(unique1 order by two), sum(unique1 order by four),
+  sum(unique1 order by four, two), sum(unique1 order by two, random()),
+  sum(unique1 order by two, random(), random() + 1)
+from tenk1
+group by ten;
+            QUERY PLAN            
+----------------------------------
+ GroupAggregate
+   Group Key: ten
+   ->  Sort
+         Sort Key: ten, four, two
+         ->  Seq Scan on tenk1
+(5 rows)
+
 -- Ensure no ordering is requested when enable_presorted_aggregate is off
 set enable_presorted_aggregate to off;
 explain (costs off)