Dandandan opened a new issue #566:
URL: https://github.com/apache/arrow-datafusion/issues/566
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
I like to support eliminating common subexpressions from a query plan.
For example, this example from TCP-H query 1:
```sql
select *
sum(l_extendedprice * (1 - l_discount)) as sum_disc_price,
sum(l_extendedprice * (1 - l_discount) * (1 + l_tax)) as sum_charge
from T
```
could be rewritten to reuse the result of `l_extendedprice * (1 -
l_discount)` by moving it into another projection.
This looks in SQL as something like:
```sql
with temp as (
select
l_extendedprice * (1 - l_discount) as cs
from t
)
select
sum(cs) as sum_disc_price,
sum(cs * (1 + l_tax)) as sum_charge
from temp
```
**Describe the solution you'd like**
Detect common subexpression in projections / group by, join conditions etc.
and remove them by moving doing them once in a projection step. Note: this only
makes sense when the expression includes any calculation (e.g. not a column
reference with alias).
**Describe alternatives you've considered**
n/a
**Additional context**
n/a
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