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I'm new to SQL and so this is probably a stupid question but can I reduce the duplication in this query? My actual query involves more complex expressions. Right now I'm just doing string concatenation from the language I'm using to generate the long query but I think there must be a way to do this with plain SQL?

select
  x + y as a,
  x + z as b,
  (x + y) / (x + z) as c
from
  integers
order by
  z + (x + y) / (x + z) desc
limit
  10;

What I'd like:

select
  x + y as a,
  x + z as b,
  a / b as c
from
  integers
order by
  z + c desc
limit
  10;

-- Fails with: Query Error: error: column "a" does not exist

Sample table:

create table integers as
select x, y, z
from
  generate_series(1, 10) x,
  generate_series(x, 10) y,
  generate_series(y, 10) z;

Fiddle: https://www.db-fiddle.com/f/u3cVh7QsafWNE6ELWhvBc1/0

4

1 Answer 1

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Two levels of derived tables (subquery or CTE) to avoid all repeated calculations:

SELECT a, b, c
FROM  (
   SELECT z, a, b, a / b AS c
   FROM (
      SELECT z
           , x + y AS a
           , x + z AS b
      FROM   integers
      ) i
   ) i
ORDER  BY  z + c DESC
LIMIT  10;

With simple expressions, it won't matter for performance.
What might matter with repeating more expensive expressions is that all involved functions have (correct!) volatility STABLE or IMMUTABLE. See:

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  • Thanks I understand how this one would avoid all repeated calculations. For simple expressions, can this actually be slower than my original query? Is Postgres (and other databases) smart enough to optimize this? (If I write this kind of code in a normal programming language, creating intermediary data structures, it'll be terribly inefficient.)
    – user225326
    Commented Mar 7, 2021 at 12:53
  • @user225326: The Postgres query planner / optimizer rearranges things quite a bit, and verbose queries often end up in a very simple query plan. Compare EXPLAIN output to see resulting differences. And the ultimate judge will be effective execution times. Commented Mar 7, 2021 at 12:59

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