84c6f47e4f
refer paper: WinMagic - Subquery Elimination Using Window Aggregation
SQL like TPC-H Q2 and Q17, which contains a correlated sub-query with only one aggregation function output, we can eliminate the sub-query and transform it to window function. For example, TPC-H Q17 is
```sql
select
sum(l_extendedprice) / 7.0 as avg_yearly
from
lineitem,
part
where
p_partkey = l_partkey
and p_brand = 'Brand#23'
and p_container = 'MED BOX'
and l_quantity < (
select
0.2 * avg(l_quantity)
from
lineitem
where
l_partkey = p_partkey
);
```
we rewrite it to
```sql
select
sum(l_extendedprice) / 7.0 as avg_yearly
from (
select
l_extendedprice, l_quantity, avg(l_quantity) over(partition by l_partkey) avg_l_quantity
from
lineitem,
part
where
p_partkey = l_partkey
and p_brand = 'Brand#23'
and p_container = 'MED BOX' )
where l_quantity < 0.2 * avg_l_quantity
```
now the rule can only handle: where conjuncts in outer scope contain one sub-query and the conjunct contain sub-query is a comparison-predicate, we will support compound-predicate and more than one conjuncts containing sub-query later.
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