Question about double table scans for a table

Hi everyone,

Consider the query 11 in the TPC-H benchmark:
select
    ps_partkey,
    sum(ps_supplycost * ps_availqty) as value
from
    PARTSUPP,
    SUPPLIER,
    NATION
where
    ps_suppkey = s_suppkey
    and s_nationkey = n_nationkey
    and n_name = 'MOZAMBIQUE'
group by
    ps_partkey
having
    sum(ps_supplycost * ps_availqty) > (
        select
            sum(ps_supplycost * ps_availqty) * 0.0001000000
        from
            PARTSUPP,
            SUPPLIER,
            NATION
        where
            ps_suppkey = s_suppkey
            and s_nationkey = n_nationkey
            and n_name = 'MOZAMBIQUE'
    )
order by
    value desc;


PostgreSQL generates the following query plan:
 Sort  (cost=1798.52..1799.32 rows=320 width=36)
   Sort Key: (sum((partsupp.ps_supplycost * (partsupp.ps_availqty)::numeric))) DESC
   InitPlan 1 (returns $0)
     ->  Aggregate  (cost=884.20..884.21 rows=1 width=32)
           ->  Hash Join  (cost=12.40..877.00 rows=960 width=10)
                 Hash Cond: (partsupp_1.ps_suppkey = supplier_1.s_suppkey)
                 ->  Seq Scan on partsupp partsupp_1  (cost=0.00..765.00 rows=24000 width=14)
                 ->  Hash  (cost=12.25..12.25 rows=12 width=4)
                       ->  Hash Join  (cost=1.32..12.25 rows=12 width=4)
                             Hash Cond: (supplier_1.s_nationkey = nation_1.n_nationkey)
                             ->  Seq Scan on supplier supplier_1  (cost=0.00..10.00 rows=300 width=8)
                             ->  Hash  (cost=1.31..1.31 rows=1 width=4)
                                   ->  Seq Scan on nation nation_1  (cost=0.00..1.31 rows=1 width=4)
                                         Filter: (n_name = 'MOZAMBIQUE'::bpchar)
   ->  HashAggregate  (cost=886.60..901.00 rows=320 width=36)
         Group Key: partsupp.ps_partkey
         Filter: (sum((partsupp.ps_supplycost * (partsupp.ps_availqty)::numeric)) > $0)
         ->  Hash Join  (cost=12.40..877.00 rows=960 width=14)
               Hash Cond: (partsupp.ps_suppkey = supplier.s_suppkey)
               ->  Seq Scan on partsupp  (cost=0.00..765.00 rows=24000 width=18)
               ->  Hash  (cost=12.25..12.25 rows=12 width=4)
                     ->  Hash Join  (cost=1.32..12.25 rows=12 width=4)
                           Hash Cond: (supplier.s_nationkey = nation.n_nationkey)
                           ->  Seq Scan on supplier  (cost=0.00..10.00 rows=300 width=8)
                           ->  Hash  (cost=1.31..1.31 rows=1 width=4)
                                 ->  Seq Scan on nation  (cost=0.00..1.31 rows=1 width=4)
                                       Filter: (n_name = 'MOZAMBIQUE'::bpchar)

While TiDB has the following query plan:

Projection_63
 └─Sort_64
   └─Selection_66
     └─HashAgg_67
       └─Projection_94
         └─HashJoin_71
           ├─HashJoin_84(Build)
           │ ├─TableReader_89(Build)
           │ │ └─Selection_88
           │ │   └─TableFullScan_87
           │ └─TableReader_86(Probe)
           │   └─TableFullScan_85
           └─TableReader_91(Probe)
             └─TableFullScan_90
...

Both query plans include different numbers of table scans, as highlighted in red color. PostgreSQL uses six table scans, while TiDB has only three. I understand that the table scanning operation is expensive and query plans are typically more efficient with fewer table scans. My question is why PostgreSQL uses six table scans to scan each table twice? Is it a more efficient query plan, or does this indicate an optimization that is not performed by PostgreSQL?



Best regards,
Jinsheng Ba