excessive sorting of expressions
--------------------------------
Key: CORE-5814
URL: http://tracker.firebirdsql.org/browse/CORE-5814
Project: Firebird Core
Issue Type: Bug
Components: Engine
Affects Versions: 3.0.3, 2.5.7
Reporter: kdv
Grouping and sorting on expression produces additional SORT in plan.
Example.
When we group and sort on non-indexed field
select first_name, max(first_name)
from employee
group by 1
order by 1
we see PLAN SORT (EMPLOYEE NATURAL)
i.e only one sort used for grouping and sorting.
But, if we convert field to expression
select first_name||'', max(first_name)
from employee
group by 1
order by 1
plan will have 2 sorts - for group by and order by
PLAN SORT (SORT (EMPLOYEE NATURAL))
I don't know is this a real bug or engine specific, but maybe it can be fixed.
In addition. If I create computed index
create index bfc on employee computed by (first_name||'')
plan for the last query will be
PLAN SORT (EMPLOYEE ORDER BFC)
So, index is used by group by, but order by still uses additional sorting.
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