SQL-Query (with subquery, group and order by) optimization - sql

Question

coud you help me optimizing the following statement. It has a bad prerformance when dealing with huge amount of data (in my case 3Mio Messages and 25Mio MessageWorkItems).
Does anybody have any suggestions? Thank you in advance.
select distinct msg.id, msgWorkItem_1.description
from message msg
left outer join message_work_item msgWorkItem_1 on msg.id=msgWorkItem_1.message_id
and ( msgWorkItem_1.id in (
select max(msgWorkItem_2.id)
from message_work_item msgWorkItem_2
inner join message_work_item_type msgWorkItem_Type on msgWorkItem_2.message_work_item_type_id=msgWorkItem_Type.id
where
msgWorkItem_2.creation_type= 'mobile'
and msgWorkItem_2.description is not null
and msgWorkItem_Type.code <> 'sent-to-app-manually'
-- Is it possible to avoid this correlation to the outer query ? )
and msgWorkItem_2.message_id = msg.id)
)
where msg.deactivation_time > ?
order by msgWorkItem_1.description asc

Answer 1

STEP 1 : Lay out the query so I have any hope of reading it
SELECT
DISTINCT
msg.id,
msgWorkItem_1.description
FROM
message msg
LEFT OUTER JOIN
message_work_item AS msgWorkItem_1
ON msgWorkItem_1.message_id = msg.id
AND msgWorkItem_1.id =
(
SELECT
MAX(msgWorkItem_2.id)
FROM
message_work_item AS msgWorkItem_2
INNER JOIN
message_work_item_type AS msgWorkItem_Type
ON msgWorkItem_2.message_work_item_type_id=msgWorkItem_Type.id
WHERE
msgWorkItem_2.creation_type= 'mobile'
AND msgWorkItem_2.description IS NOT NULL
AND msgWorkItem_Type.code <> 'sent-to-app-manually'
-- Is it possible to avoid this correlation to the outer query ?
AND msgWorkItem_2.message_id = msg.id
)
WHERE
msg.deactivation_time > ?
ORDER BY
msgWorkItem_1.description ASC
STEP 2 : rewrite using analytic functions instead of MAX()
SELECT
DISTINCT
message.id,
message_work_item_sorted.description
FROM
message
LEFT OUTER JOIN
(
SELECT
message_work_item.message_id,
message_work_item.description,
ROW_NUMBER() OVER (PARTITION BY message_work_item.message_id
ORDER BY message_work_item.id DESC
)
AS row_ordinal
FROM
message_work_item
INNER JOIN
message_work_item_type
ON message_work_item.message_work_item_type_id = message_work_item_type.id
WHERE
message_work_item.creation_type= 'mobile'
AND message_work_item.description IS NOT NULL
AND message_work_item_type.code <> 'sent-to-app-manually'
)
message_work_item_sorted
ON message_work_item_sorted.message_id = message.id
AND message_work_item_sorted.row_ordinal = 1
WHERE
message.deactivation_time > ?
ORDER BY
message_work_item_sorted.description ASC
With more information we could probably help further, but as you gave no definition of the tables, constraints, or business logic, this is just a re-write of what you're already implemented.
For example, I strongly doubt you need the DISTINCT (provided that the id columns in your tables are unique).