Looking for an alternative method to this query - sql

Question

There are 2 tables, a Component table and a Log table. The component table holds the actual(current) value description and a timestamp when it was last updated.
The Log table contains a component ID that references to wich component it belongs:
Component:
Id
Actual
LastUpdated
Log:
Id
ComponentId
Value
Timestamp
the query that used to work but currently lock the table looks like this.
update Component set Actual =
(select top 1 Value from Log
where Component.Id = ComponentId
order by Id desc),
LastUpdated =
(select top 1 TimeStamp from Log
where Component.Id = ComponentId
order by Id desc)
Both the log and component tables are growing and this query can't keep up like it used to be able to do. there are around 80 components now and a couple of million records.
Is it possible to work in a way like this and just improve the query or is the entire approach wrong?
ps the devices that send the data don't have an reliable system time and therefor having them update the component table leads to inconsistency. When inserting a log i take the system time on the SQL server(default value)
EDIT:
taking a suggestion from the awnsers im trying to create a trigger on log to automaticly update Component when a log is created.
CREATE TRIGGER trg_log_ins
ON Log
AFTER INSERT
AS
BEGIN
update Component
SET Actual = (SELECT i.value FROM inserted as i),
LastUpdated = (SELECT i.Timestamp FROM inserted as i);
END;
but for some reason the query doesn't finish and keeps executing.

Answer 1

I think you're going about this all wrong. A better solution would be a trigger on the Component table, that inserts into the Log table whenever a Component is inserted or updated.
CREATE TRIGGER trg_component_biu
ON Component
AFTER INSERT, UPDATE
AS
BEGIN
INSERT INTO Log(
ComponentId,
Value,
Timestamp
)
SELECT
Id,
Actual,
LastUpdated
FROM inserted;
END;

Answer 2

You can do it by using ROW_NUMBER() like this:
UPDATE t1
SET t1.Actual = t2.value,
t1.LastUpdated = t2.TimeStamp
FROM Component t1
INNER JOIN (SELECT log.*,ROW_NUMBER() OVER (PARTITION BY log.componentID order by log.ID DESC) as rnk
FROM log) t2
ON(t2.componentID = t1.id and t2.rnk = 1)

Answer 3

Based on TOP 1 in your query I guess you are using SQL SERVER. In SQL Server you can use OUTER APPLY
UPDATE c
SET c.Actual = cs.Value,
c.LastUpdated = cs.TimeStamp
FROM Component C
OUTER apply (SELECT TOP 1 TimeStamp,
ComponentId
FROM Log l
WHERE c.Id = l.ComponentId
ORDER BY Id DESC) cs
Adding a non-clustered index on Log table ID column and include TimeStamp,ComponentId will improve query performance
Another way is using ROW_NUMBER and LEFT OUTER JOIN
UPDATE c
SET c.Actual = cs.Value,
c.LastUpdated = cs.TimeStamp
FROM Component C
LEFT OUTER JOIN (SELECT Row_number()OVER(partition BY ComponentId
ORDER BY id DESC) rn,*
FROM Log) cs
ON cs.ComponentId = c.id
AND cs.rn = 1

Answer 4

All data in your Component table is coming from the Log table. Instead of making Component an actual table, you can make it a view, indexed if necessary.
CREATE VIEW Component
WITH SCHEMABINDING
AS
SELECT
ComponentId AS Id,
FIRST_VALUE(Value)
OVER(PARTITION BY ComponentId
ORDER BY Timestamp DESC)
AS Actual,
MAX(Timestamp) AS LastUpdated
FROM Log
GROUP BY ComponentId;

Answer 5

If you are going to use trigger on the Log table it has to work even if several rows are inserted. Here is one possible variant.
Also, this variant would not capture values for a new ComponentID that doesn't exist in the Component table yet.
If there is a possibility that such values would be inserted into the Log table, I'd use MERGE instead of simple UPDATE.
CREATE TRIGGER trg_log_ins
ON Log
AFTER INSERT
AS
BEGIN
WITH
CTE
AS
(
SELECT
Component.Actual AS OldValue
,Component.LastUpdated AS OldTimestamp
,inserted.Value AS NewValue
,inserted.Timestamp AS NewTimestamp
FROM
Component
INNER JOIN inserted ON inserted.ComponentID = Component.ID
)
UPDATE CTE
SET
OldValue = NewValue,
OldTimestamp = NewTimestamp
;
END
Also, if it is possible to insert into Log several rows with the same ComponentID in the same INSERT statement, you'd better choose explicitly which value to use for update. Likely, the one with the latest Timestamp.
So, the query becomes more complicated:
CREATE TRIGGER trg_log_ins
ON Log
AFTER INSERT
AS
BEGIN
WITH
CTE_InsertedRowNumbers
AS
(
SELECT
inserted.ComponentID
,inserted.Value AS NewValue
,inserted.Timestamp AS NewTimestamp
,ROW_NUMBER() OVER (
PARTITION BY inserted.ComponentID
ORDER BY inserted.Timestamp DESC, inserted.ID DESC) AS rn
FROM inserted
)
,CTE_LatestInsertedComponents
AS
(
SELECT
ComponentID
,NewValue
,NewTimestamp
FROM CTE_InsertedRowNumbers
WHERE rn = 1
)
,CTE
AS
(
SELECT
Component.Actual AS OldValue
,Component.LastUpdated AS OldTimestamp
,CTE_LatestInsertedComponents.NewValue
,CTE_LatestInsertedComponents.NewTimestamp
FROM
Component
INNER JOIN CTE_LatestInsertedComponents
ON CTE_LatestInsertedComponents.ComponentID = Component.ID
)
UPDATE CTE
SET
OldValue = NewValue,
OldTimestamp = NewTimestamp
;
END