This is relatively trivial to do with a correlated subquery. You can't use the COALESCE method highlighted in the blog post you mention unless you extract that to a user-defined function (or unless you only want to return one row at a time). Here is how I typically do this:
DECLARE @x TABLE
(
id INT,
row_num INT,
customer_code VARCHAR(32),
comments VARCHAR(32)
);
INSERT @x SELECT 1,1,'Dilbert','Hard'
UNION ALL SELECT 1,2,'Dilbert','Worker'
UNION ALL SELECT 2,1,'Wally','Lazy';
SELECT id, customer_code, comments = STUFF((SELECT ' ' + comments
FROM @x AS x2 WHERE id = x.id
ORDER BY row_num
FOR XML PATH('')), 1, 1, '')
FROM @x AS x
GROUP BY id, customer_code
ORDER BY id;
If you have a case where the data in comments could contain unsafe-for-XML characters (>
, <
, &
), you should change this:
FOR XML PATH('')), 1, 1, '')
To this more elaborate approach:
FOR XML PATH(''), TYPE).value(N'(./text())[1]', N'varchar(max)'), 1, 1, '')
(Be sure to use the right destination data type, varchar
or nvarchar
, and the right length, and prefix all string literals with N
if using nvarchar
.)
First things first, I notice that your 'what I do now' query:
SELECT TOP (1)
ca.SensorValue,
ca.Date
FROM sys.partitions AS p
CROSS APPLY
(
SELECT TOP (1)
v.Date,
v.SensorValue
FROM SensorValues AS v
WHERE
$PARTITION.SensorValues_Date_PF(v.Date) = p.[partition_number]
AND v.DeviceId = @fDeviceId
AND v.SensorId = @fSensorId
AND v.Date <= @fDate
ORDER BY
v.Date DESC
) AS ca
WHERE
p.[partition_number] <= $PARTITION.SensorValues_Date_PF(@fDate)
AND p.[object_id] = OBJECT_ID(N'dbo.SensorValues', N'U')
AND p.index_id = 1
ORDER BY
p.[partition_number] DESC,
ca.Date DESC;
...produces an execution plan like this:
This execution plan has an estimated total cost of 0.02 units. Over 50% of this estimated cost is the final Sort, running in Top-N mode. Now estimates are just that, but sorts can be expensive in general, so let's remove it without changing the semantics:
SELECT TOP (1)
ca.SensorId,
ca.SensorValue,
ca.Date
FROM
(
-- Partition numbers
SELECT DISTINCT
partition_number = prv.boundary_id
FROM
sys.partition_functions AS pf
JOIN sys.partition_range_values AS prv ON
prv.function_id = pf.function_id
WHERE
pf.name = N'SensorValues_Date_PF'
AND prv.boundary_id <= $PARTITION.SensorValues_Date_PF(@fDate)
) AS p
CROSS APPLY
(
SELECT TOP (1)
v.Date,
v.SensorValue,
v.SensorId
FROM dbo.SensorValues AS v
WHERE
$PARTITION.SensorValues_Date_PF(v.Date) = p.partition_number
AND v.DeviceId = @fDeviceId
AND v.SensorId = @fSensorId
AND v.Date <= @fDate
ORDER BY
v.Date DESC
) AS ca
ORDER BY
p.partition_number DESC,
ca.Date DESC
Now the execution plan has no blocking operators, and no sorts in particular. The estimated cost of the new query plan below is 0.01 units and the total cost is distributed evenly over the data access methods:
With the improvement in place, all we need to produce a result for each Sensor ID is to make a list of Sensor IDs and APPLY
the previous code to each one:
SELECT
PerSensor.SensorId,
PerSensor.SensorValue,
PerSensor.Date
FROM
(
-- Sensor ID list
VALUES
(@fSensorId1),
(@FSensorId2),
(@FSensorId3)
) AS Sensor (Id)
CROSS APPLY
(
-- Optimized code applied to each sensor
SELECT TOP (1)
ca.SensorId,
ca.SensorValue,
ca.Date
FROM
(
-- Partition numbers
SELECT DISTINCT
partition_number = prv.boundary_id
FROM
sys.partition_functions AS pf
JOIN sys.partition_range_values AS prv ON
prv.function_id = pf.function_id
WHERE
pf.name = N'SensorValues_Date_PF'
AND prv.boundary_id <= $PARTITION.SensorValues_Date_PF(@fDate)
) AS p
CROSS APPLY
(
SELECT TOP (1)
v.Date,
v.SensorValue,
v.SensorId
FROM dbo.SensorValues AS v
WHERE
$PARTITION.SensorValues_Date_PF(v.Date) = p.partition_number
AND v.DeviceId = @fDeviceId
AND v.SensorId = Sensor.Id--@fSensorId1
AND v.Date <= @fDate
ORDER BY
v.Date DESC
) AS ca
ORDER BY
p.partition_number DESC,
ca.Date DESC
) AS PerSensor;
The query plan is:
Estimated query plan cost for three Sensor IDs is 0.011 - half that of the original single-sensor plan.
Best Answer
Main idea:
Modify as you need - extract date and/or time parts, check that date parts are equal, etc.