[jira] [Created] (CALCITE-1317) Rewrite "MAX" as "ORDER BY ... LIMIT ..."

["Julian Hyde (JIRA)" <ji...@apache.org>]

Julian Hyde created CALCITE-1317:
------------------------------------

             Summary: Rewrite "MAX" as "ORDER BY ... LIMIT ..."
                 Key: CALCITE-1317
                 URL: https://issues.apache.org/jira/browse/CALCITE-1317
             Project: Calcite
          Issue Type: Bug
            Reporter: Julian Hyde
            Assignee: Julian Hyde


We could optimize 

{code}
SELECT *
FROM emp
WHERE empno = (SELECT max(empno) FROM emp)
{code}

to

{code}
SELECT *
FROM emp
ORDER BY empno DESC LIMIT 1
{code}

(using the fact that {{empno}} is unique and non-NULL). Similarly, we can rewrite

{code}
SELECT max(sal)
FROM emp
{code}

to

{code}
SELECT sal
FROM emp
ORDER BY sal DESC LIMIT 1
{code}

(not making any assumptions about whether {{sal}} is unique or allows NULL values) and we can rewrite a query to find the highest paid employee(s) in each department

{code}
SELECT *
FROM emp AS e
WHERE sal = (
  SELECT max(sal)
  FROM emp AS e2
  WHERE e2.deptno = e.deptno)
{code}

as

{code}
SELECT deptno, empno, sal
FROM (
  SELECT deptno, empno, sal, FIRST_VALUE(sal) OVER w AS topSal
  FROM emp
  WINDOW w AS (PARTITION BY deptno ORDER BY sal DESC))
WHERE sal = topSal
{code}

We might benefit from a generalized {{Sort(limit)}} operator that can find the top N within any prefix of the sort key, not just the top N overall.



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