Fix partition pruning to treat stable comparison operators properly.

Cross-type comparison operators in a btree or hash opclass might be
only stable not immutable (this is true of timestamp vs. timestamptz
for example).  partprune.c ignored this possibility and would perform
plan-time pruning with them anyway, possibly leading to wrong answers
if the environment changed between planning and execution.

To fix, teach gen_partprune_steps() to do things differently when
creating plan-time pruning steps vs. run-time pruning steps.
analyze_partkey_exprs() also needs an extra check, which is rather
annoying but now is not the time to restructure things enough to
avoid that.

While at it, simplify the logic for the plan-time case a little
by insisting that the comparison value be a Const and nothing else.
This relies on the assumption that eval_const_expressions will have
reduced any immutable expression to a Const; which is not quite
100% true, but certainly any case that comes up often enough to be
interesting should have simplification logic there.

Also improve a bunch of inadequate/obsolete/wrong comments.

Per discussion of a report from Alan Jackson (though this fixes only one
aspect of that problem).  Back-patch to v11 where this code came in.

David Rowley, with some further hacking by me

Discussion: https://postgr.es/m/FAD28A83-AC73-489E-A058-2681FA31D648@tvsquared.com

src/test/regress/expected/partition_prune.out

@@ -3036,6 +3036,93 @@ select * from listp where a = (select null::int);
 (7 rows)
 
 drop table listp;
+--
+-- check that stable query clauses are only used in run-time pruning
+--
+create table stable_qual_pruning (a timestamp) partition by range (a);
+create table stable_qual_pruning1 partition of stable_qual_pruning
+  for values from ('2000-01-01') to ('2000-02-01');
+create table stable_qual_pruning2 partition of stable_qual_pruning
+  for values from ('2000-02-01') to ('2000-03-01');
+create table stable_qual_pruning3 partition of stable_qual_pruning
+  for values from ('3000-02-01') to ('3000-03-01');
+-- comparison against a stable value requires run-time pruning
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning where a < localtimestamp;
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ Append (actual rows=0 loops=1)
+   Subplans Removed: 1
+   ->  Seq Scan on stable_qual_pruning1 (actual rows=0 loops=1)
+         Filter: (a < LOCALTIMESTAMP)
+   ->  Seq Scan on stable_qual_pruning2 (actual rows=0 loops=1)
+         Filter: (a < LOCALTIMESTAMP)
+(6 rows)
+
+-- timestamp < timestamptz comparison is only stable, not immutable
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning where a < '2000-02-01'::timestamptz;
+                                   QUERY PLAN                                   
+--------------------------------------------------------------------------------
+ Append (actual rows=0 loops=1)
+   Subplans Removed: 2
+   ->  Seq Scan on stable_qual_pruning1 (actual rows=0 loops=1)
+         Filter: (a < 'Tue Feb 01 00:00:00 2000 PST'::timestamp with time zone)
+(4 rows)
+
+-- check ScalarArrayOp cases
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2010-02-01', '2020-01-01']::timestamp[]);
+           QUERY PLAN           
+--------------------------------
+ Result (actual rows=0 loops=1)
+   One-Time Filter: false
+(2 rows)
+
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2000-02-01', '2010-01-01']::timestamp[]);
+                                                   QUERY PLAN                                                   
+----------------------------------------------------------------------------------------------------------------
+ Seq Scan on stable_qual_pruning2 (actual rows=0 loops=1)
+   Filter: (a = ANY ('{"Tue Feb 01 00:00:00 2000","Fri Jan 01 00:00:00 2010"}'::timestamp without time zone[]))
+(2 rows)
+
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2000-02-01', localtimestamp]::timestamp[]);
+                                                 QUERY PLAN                                                 
+------------------------------------------------------------------------------------------------------------
+ Append (actual rows=0 loops=1)
+   Subplans Removed: 2
+   ->  Seq Scan on stable_qual_pruning2 (actual rows=0 loops=1)
+         Filter: (a = ANY (ARRAY['Tue Feb 01 00:00:00 2000'::timestamp without time zone, LOCALTIMESTAMP]))
+(4 rows)
+
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2010-02-01', '2020-01-01']::timestamptz[]);
+                                                        QUERY PLAN                                                         
+---------------------------------------------------------------------------------------------------------------------------
+ Append (actual rows=0 loops=1)
+   Subplans Removed: 2
+   ->  Seq Scan on stable_qual_pruning1 (never executed)
+         Filter: (a = ANY ('{"Mon Feb 01 00:00:00 2010 PST","Wed Jan 01 00:00:00 2020 PST"}'::timestamp with time zone[]))
+(4 rows)
+
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2000-02-01', '2010-01-01']::timestamptz[]);
+                                                        QUERY PLAN                                                         
+---------------------------------------------------------------------------------------------------------------------------
+ Append (actual rows=0 loops=1)
+   Subplans Removed: 2
+   ->  Seq Scan on stable_qual_pruning2 (actual rows=0 loops=1)
+         Filter: (a = ANY ('{"Tue Feb 01 00:00:00 2000 PST","Fri Jan 01 00:00:00 2010 PST"}'::timestamp with time zone[]))
+(4 rows)
+
+drop table stable_qual_pruning;
 -- Ensure runtime pruning works with initplans params with boolean types
 create table boolvalues (value bool not null);
 insert into boolvalues values('t'),('f');

src/test/regress/sql/partition_prune.sql

@@ -754,6 +754,44 @@ select * from listp where a = (select null::int);
 
 drop table listp;
 
+--
+-- check that stable query clauses are only used in run-time pruning
+--
+create table stable_qual_pruning (a timestamp) partition by range (a);
+create table stable_qual_pruning1 partition of stable_qual_pruning
+  for values from ('2000-01-01') to ('2000-02-01');
+create table stable_qual_pruning2 partition of stable_qual_pruning
+  for values from ('2000-02-01') to ('2000-03-01');
+create table stable_qual_pruning3 partition of stable_qual_pruning
+  for values from ('3000-02-01') to ('3000-03-01');
+
+-- comparison against a stable value requires run-time pruning
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning where a < localtimestamp;
+
+-- timestamp < timestamptz comparison is only stable, not immutable
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning where a < '2000-02-01'::timestamptz;
+
+-- check ScalarArrayOp cases
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2010-02-01', '2020-01-01']::timestamp[]);
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2000-02-01', '2010-01-01']::timestamp[]);
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2000-02-01', localtimestamp]::timestamp[]);
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2010-02-01', '2020-01-01']::timestamptz[]);
+explain (analyze, costs off, summary off, timing off)
+select * from stable_qual_pruning
+  where a = any(array['2000-02-01', '2010-01-01']::timestamptz[]);
+
+drop table stable_qual_pruning;
+
 -- Ensure runtime pruning works with initplans params with boolean types
 create table boolvalues (value bool not null);
 insert into boolvalues values('t'),('f');