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[FB-Tracker] Created: (CORE-5497) Remove unnescesary sorting on fields different then first when do union

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[FB-Tracker] Created: (CORE-5497) Remove unnescesary sorting on fields different then first when do union

JIRA tracker@firebirdsql.org
Remove unnescesary sorting on fields different then first when do union
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                 Key: CORE-5497
                 URL: http://tracker.firebirdsql.org/browse/CORE-5497
             Project: Firebird Core
          Issue Type: Improvement
            Reporter: Karol Bieniaszewski


For query like this Firebird remove unnescessary sorting but only if we order by first column
SELECT
        R.RDB$RELATION_NAME, R.RDB$DBKEY_LENGTH
FROM
RDB$RELATIONS R
UNION
SELECT
        R.RDB$RELATION_NAME, R.RDB$DBKEY_LENGTH
FROM
RDB$RELATIONS R
ORDER BY 1

Select Expression
    -> Unique Sort (record length: 150, key length: 108)
        -> Union
            -> Table "RDB$RELATIONS" as "R" Full Scan
            -> Table "RDB$RELATIONS" as "R" Full Scan


but when we change query to and sort on second field (same fields but swapped)


SELECT
        R.RDB$DBKEY_LENGTH, R.RDB$RELATION_NAME
FROM
RDB$RELATIONS R
UNION
SELECT
        R.RDB$DBKEY_LENGTH, R.RDB$RELATION_NAME
FROM
RDB$RELATIONS R
ORDER BY 2

Select Expression
    -> Sort (record length: 142, key length: 100)
        -> Unique Sort (record length: 150, key length: 108)
            -> Union
                -> Table "RDB$RELATIONS" as "R" Full Scan
                -> Table "RDB$RELATIONS" as "R" Full Scan

we have 2 sorts - one is unnescessary

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