SQL IN operator in update query causes a lot of time - sql

Question

Below is a update query which is to update a table with about 40000 records:
UPDATE tableName
SET colA = val, colB = val
WHERE ID IN (select RecordIDs from tableB where needUpdate = 'Y')
When the above query is executed, I found out that the below query taken ~ 15 seconds
SELECT RecordIDs
FROM tableB
WHERE needUpdate = 'Y'
But when I take away the where clause (i.e. update tableName set colA = val, colB = val) The query runs smoothly.
Why this happens? are there any ways to shorten the time of execution?
Edited:
Below is the structure of both tables:
tableName:
ID int,
VehicleBrandID int,
VehicleLicenseExpiryDate nvarchar(25),
LicensePlateNo nvarchar(MAX),
ContactPerson nvarchar(MAX),
ContactPersonID nvarchar(MAX),
ContactPersonPhoneNumber nvarchar(MAX),
ContactPersonAddress nvarchar(MAX),
CreatedDate nvarchar(MAX),
CreatedBy nvarchar(MAX)
PRIMARY KEY (ID)
tableB:
RowNumber int
RecordIDs int
NeedUpdate char(1)
PRIMARY KEY (RowNumber)
Edited
Below screenshot is the execution plan for the update query

Answer 1

The execution plan shows you are using table variables and are missing a useful index.
Keep the existing PK on #output
DECLARE #output TABLE (
ID INT PRIMARY KEY,
VehicleBrandID INT,
VehicleLicenseExpiryDate NVARCHAR(25),
LicensePlateNo NVARCHAR(MAX),
ContactPerson NVARCHAR(MAX),
ContactPersonID NVARCHAR(MAX),
ContactPersonPhoneNumber NVARCHAR(MAX),
ContactPersonAddress NVARCHAR(MAX),
CreatedDate NVARCHAR(MAX), /*<-- Don't store dates as strings*/
CreatedBy NVARCHAR(MAX))
And add a new index to #tenancyEditable
DECLARE #tenancyEditable TABLE (
RowNumber INT PRIMARY KEY,
RecordIDs INT,
NeedUpdate CHAR(1),
UNIQUE (NeedUpdate, RecordIDs, RowNumber))
With these indexes in place the following query
UPDATE #output
SET LicensePlateNo = ''
WHERE ID IN (SELECT RecordIDs
FROM #tenancyEditable
WHERE NeedUpdate = 'Y')
OPTION (RECOMPILE)
Can generate the more efficient looking
Also you should use appropriate datatypes rather than storing everything as NVARCHAR(MAX). A person name isn't going to need more than nvarchar(100) at most and CreatedDate should be stored as date[time2] for example.

Answer 2

I suppose you are in one of the 2 cases below:
1/ STATISTICS are not updated due to a some recently modification of in your table. In this case you should execute this:
UPDATE STATISTICS tableB
2/ I suppose a wrong query plan is used, case when I recommend to execute this in order to force recompilation of the query:
SELECT RecordIDs
FROM tableB
WHERE needUpdate = 'Y'
OPTION (RECOMPILE)
Tell us the result and we'll come with more details about.

Answer 3

This is an alternative. It is worth it to try in your environment as it has been demonstrated for others to be faster.
MERGE INTO tableName tn
USING (
SELECT recordIDs
FROM tableB
WHERE needUpdate = 'Y'
) tb
ON tn.ID = tb.recordID
WHEN MATCHED THEN
UPDATE
SET colA = tb.val,
colB = tb.val;
EDIT:
I am not claiming this to be faster in every case or in every setup/environment - just that it is worth a try as it has worked for me and others I have worked with or read about.

Answer 4

you can use inner join instead of IN clause.
update t
set
t.colA = val, t.colB = val
From tablename
inner join tableb x on
t.id = x.recordid
where x.needUpdate = 'Y'
Although the UPDATE...FROM
syntax is essential in some
circumstances, I prefer to use
subqueries (by using IN clause) whenever
possible.