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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