Dear Members!
I have a query which I extended with an extra calculated column.
I need to list the request, and the last date when they scheduled.
Example:
select
request.*,
(
select max(s_date) as s_date from schedule
where schedule.product_id = request.product_id and schedule.ok =
True
) as max_s_date,
...
from request
...
This can find the last scheduled timestamp for that product.
It's working, but now they wanted to search for this max_s_date column).
I have two ways:
1.) With query / subquery:
select * from
(
select
request.*,
(
select max(s_date) as s_date from schedule
where schedule.product_id = request.product_id and schedule.ok =
True
) as max_s_date,
...
from request
...
) t where t.max_s_date between ...
2.) I may relocate this section as join...
select
request.*, s.max_s_date
from request
left join
(
select schedule.product_id, max(s_date) as max_s_date from schedule
where schedule.ok = True
group by schedule.product_id
) s on (s.product_id = request.product_id)
...
But I really don't know what the hell will happen in query optimizer with
this method.
a.)
Optimizer is clever, and it calculates the aggregates only in the needed
rows.
So it find the request.product_id-s, and execute the "s" query only in
these rows.
b.)
Or it isn't enough wise, it executes the "s" subquery on whole schedule,
and later joins to main table.
The schedule table is big in customers' database, so this is worst case. :-(
---
I asked this because sometimes I need to get more result columns in the
select, but I couldn't retreive more...
(
select max(s_date) as s_date from schedule
where schedule.product_id = request.product_id and schedule.ok =
True
) as max_s_date, <=== only one column
So sometimes the join is better.
But if the optimizer isn't enough wise, I can get these values only "WITH"
queries (select the main rows to temp, run subselects with only these
records, return the mix of main and subselects in one query).
What is your experience with these kind of problems?
Can I use this join or I need to avoid because of very slow (and
slower-slower) running time.
Thank your for any help!
Best regards
dd
