Re: PoC/WIP: Extended statistics on expressions

I got this crash running sqlsmith:

#1  0x00007f907574b801 in __GI_abort () at abort.c:79
#2  0x00005646b95a35f8 in ExceptionalCondition (conditionName=conditionName@entry=0x5646b97411db "bms_num_members(varnos) == 1", errorType=errorType@entry=0x5646b95fa00b "FailedAssertion", 
    fileName=fileName@entry=0x5646b9739dbe "selfuncs.c", lineNumber=lineNumber@entry=3332) at assert.c:69
#3  0x00005646b955c9a1 in add_unique_group_expr (vars=0x5646bbd9e200, expr=0x5646b9eb0c30, exprinfos=0x5646bbd9e100, root=0x5646ba9a0cb0) at selfuncs.c:3332
#4  add_unique_group_expr (root=0x5646ba9a0cb0, exprinfos=0x5646bbd9e100, expr=0x5646b9eb0c30, vars=0x5646bbd9e200) at selfuncs.c:3307
#5  0x00005646b955d560 in estimate_num_groups () at selfuncs.c:3558
#6  0x00005646b93ad004 in create_distinct_paths (input_rel=<optimized out>, root=0x5646ba9a0cb0) at planner.c:4808
#7  grouping_planner () at planner.c:2238
#8  0x00005646b93ae0ef in subquery_planner (glob=glob@entry=0x5646ba9a0b98, parse=parse@entry=0x5646ba905d80, parent_root=parent_root@entry=0x0, hasRecursion=hasRecursion@entry=false, tuple_fraction=tuple_fraction@entry=0)
    at planner.c:1024
#9  0x00005646b93af543 in standard_planner (parse=0x5646ba905d80, query_string=<optimized out>, cursorOptions=256, boundParams=<optimized out>) at planner.c:404
#10 0x00005646b94873ac in pg_plan_query (querytree=0x5646ba905d80, 
    query_string=0x5646b9cd87e0 "select distinct \n  \n    pg_catalog.variance(\n      cast(pg_catalog.pg_stat_get_bgwriter_timed_checkpoints() as int8)) over (partition by subq_0.c2 order by subq_0.c0) as c0, \n  subq_0.c2 as c1, \n  sub"..., cursorOptions=256, boundParams=0x0) at postgres.c:821
#11 0x00005646b94874a1 in pg_plan_queries (querytrees=0x5646baaba250, 
    query_string=query_string@entry=0x5646b9cd87e0 "select distinct \n  \n    pg_catalog.variance(\n      cast(pg_catalog.pg_stat_get_bgwriter_timed_checkpoints() as int8)) over (partition by subq_0.c2 order by subq_0.c0) as c0, \n  subq_0.c2 as c1, \n  sub"..., cursorOptions=cursorOptions@entry=256, boundParams=boundParams@entry=0x0) at postgres.c:912
#12 0x00005646b9487888 in exec_simple_query () at postgres.c:1104

2021-03-24 03:06:12.489 CDT postmaster[11653] LOG:  server process (PID 11696) was terminated by signal 6: Aborted
2021-03-24 03:06:12.489 CDT postmaster[11653] DETAIL:  Failed process was running: select distinct 
          
            pg_catalog.variance(
              cast(pg_catalog.pg_stat_get_bgwriter_timed_checkpoints() as int8)) over (partition by subq_0.c2 order by subq_0.c0) as c0, 
          subq_0.c2 as c1, 
          subq_0.c0 as c2, 
          subq_0.c2 as c3, 
          subq_0.c1 as c4, 
          subq_0.c1 as c5, 
          subq_0.c0 as c6
        from 
          (select  
                ref_1.foreign_server_catalog as c0, 
                ref_1.authorization_identifier as c1, 
                sample_2.tgname as c2, 
                ref_1.foreign_server_catalog as c3
              from 
                pg_catalog.pg_stat_database_conflicts as ref_0
                        left join information_schema._pg_user_mappings as ref_1
                        on (ref_0.datname < ref_0.datname)
                      inner join pg_catalog.pg_amproc as sample_0 tablesample system (5) 
                      on (cast(null as uuid) < cast(null as uuid))
                    left join pg_catalog.pg_aggregate as sample_1 tablesample system (2.9) 
                    on (sample_0.amprocnum = sample_1.aggnumdirectargs )
                  inner join pg_catalog.pg_trigger as sample_2 tablesampl