Hi,
I am using Firebird Version: 4.0.2 (issue is there also with Firebird 3.0.7)
Due to performance issues I was facing with an sql query on a database, I generated a simple reproducible example, based on the "employee" sample database of firebird.
The issue in short: The query in question uses a view. When I replace the view, that is referenced in the query, with a common table expression (CTE), using the same SQL for the CTE as for the view, the query executes in fact very differently. See the comparison of the performance analysis. Interestingly enough, the execution plans of both queries are the same, though execute differently.
In the following example, Query A references the view "EMP_DEPT_TEAM". Query B uses the content of the view's query as CTE. Query A and B produce the same execution plans. I would expect query A and B performing just the same, but query A performs less good than query B.
How to reproduce (use the database "employee.fdb"):
Create a view:
create view EMP_DEPT_TEAM
as
select
EMP_NO,
DEPARTMENT.DEPT_NO,
iif(PROJECT.TEAM_LEADER is not null, 'yes', 'no') as IS_TEAMLEADER
from EMPLOYEE
inner join DEPARTMENT on EMPLOYEE.DEPT_NO = DEPARTMENT.DEPT_NO
left join PROJECT on EMPLOYEE.EMP_NO = PROJECT.TEAM_LEADER;
Query A (using the view):
select
EMP_NO,
DEPT_NO,
IS_TEAMLEADER
from EMP_DEPT_TEAM
where EMP_DEPT_TEAM.EMP_NO = (select max(EMP_NO) from EMP_DEPT_TEAM);
Query B (not using the view, but a CTE):
with EMP_DEPT_TEAM_ as (select
EMP_NO,
DEPARTMENT.DEPT_NO,
iif(PROJECT.TEAM_LEADER is not null, 'yes', 'no') as IS_TEAMLEADER
from EMPLOYEE
inner join DEPARTMENT on EMPLOYEE.DEPT_NO = DEPARTMENT.DEPT_NO
left join PROJECT on EMPLOYEE.EMP_NO = PROJECT.TEAM_LEADER
)
select
EMP_NO,
DEPT_NO,
IS_TEAMLEADER
from EMP_DEPT_TEAM_
where EMP_NO = (select max(EMP_NO) from EMP_DEPT_TEAM_);
Execution plan for Query A (see also attached screenshots):
PLAN JOIN (JOIN (EMP_DEPT_TEAM DEPARTMENT NATURAL, EMP_DEPT_TEAM EMPLOYEE INDEX (RDB$FOREIGN8)), EMP_DEPT_TEAM PROJECT INDEX (RDB$FOREIGN13))
PLAN JOIN (JOIN (EMP_DEPT_TEAM EMPLOYEE INDEX (RDB$PRIMARY7), EMP_DEPT_TEAM DEPARTMENT INDEX (RDB$PRIMARY5)), EMP_DEPT_TEAM PROJECT INDEX (RDB$FOREIGN13))
Details:
Execution plan for Query B (see also attached screenshots):
PLAN JOIN (JOIN (EMP_DEPT_TEAM_ DEPARTMENT NATURAL, EMP_DEPT_TEAM_ EMPLOYEE INDEX (RDB$FOREIGN8)), EMP_DEPT_TEAM_ PROJECT INDEX (RDB$FOREIGN13))
PLAN JOIN (JOIN (EMP_DEPT_TEAM_ EMPLOYEE INDEX (RDB$PRIMARY7), EMP_DEPT_TEAM_ DEPARTMENT INDEX (RDB$PRIMARY5)), EMP_DEPT_TEAM_ PROJECT INDEX (RDB$FOREIGN13))
Details:
Performance Analysis:
Compare number of reads!
Query A (View):
Query B (CTE):
Hi,
I am using Firebird Version: 4.0.2 (issue is there also with Firebird 3.0.7)
Due to performance issues I was facing with an sql query on a database, I generated a simple reproducible example, based on the "employee" sample database of firebird.
The issue in short: The query in question uses a view. When I replace the view, that is referenced in the query, with a common table expression (CTE), using the same SQL for the CTE as for the view, the query executes in fact very differently. See the comparison of the performance analysis. Interestingly enough, the execution plans of both queries are the same, though execute differently.
In the following example, Query A references the view "EMP_DEPT_TEAM". Query B uses the content of the view's query as CTE. Query A and B produce the same execution plans. I would expect query A and B performing just the same, but query A performs less good than query B.
How to reproduce (use the database "employee.fdb"):
Create a view:
Query A (using the view):
Query B (not using the view, but a CTE):
Execution plan for Query A (see also attached screenshots):
Details:
Execution plan for Query B (see also attached screenshots):
Details:
Performance Analysis:
Compare number of reads!
Query A (View):
Query B (CTE):
