Question

I have the following sql query:

SELECT (SELECT ...) AS X, (SELECT ...) AS Y from my_table

‘X’ is quite hard to compute, and it’s used as an input for computing Y. However, if I try to reference X within the query that computes Y or even within the main query I get the following error message:

Error: column "X" does not exist

Is there any way to reference X once it is computed? I really don’t want to compute it twice, as this seems very inefficient.

Answer 1

A subselect might do it for you, e.g.:

select *, hard_to_compute * 2 as using_hard_to_compute \
from (select *, (i * i) as hard_to_compute from foo) bar;

 i | hard_to_compute | using_hard_to_compute 
---+-----------------+-----------------------
 1 |               1 |                     2
 2 |               4 |                     8
 3 |               9 |                    18

Of course, i * i isn’t really hard to compute. I’m just pretending it is for this example.