UPDATE performance for empty join

Question

I have this bit of sql

DECLARE @tmpTable TABLE(idTransaction int PRIMARY KEY, Value money)
DECLARE @idStatement int = 0

UPDATE T 
       SET idStatement = @idStatement
FROM tbl_Transaction T
INNER JOIN @tmpTable ST on T.id = ST.idTransaction 

The table tbl_Transaction has about 250,000 rows in it (which i think is gunna be the cause of this), but when I run the query as it is it takes about 650ms to execute, even though the tmpTable is completely empty and the join will update no rows.

I am assuming this is something to do with how UPDATE works but can anyone shed any light on why? I know for an empty table I can check the row count but I am wanting to find out if this will affect the performance of my query when the tmpTable has rows in?

Any advice is greatly appreciated.

Answer 1

This is because it won't match indexes on the table variable the same way it will with a temp table. YOu could convert it to a temp table and that will most likely solve the problem. Alternatively, you could try this:

UPDATE T 
       SET idStatement = @idStatement
FROM tbl_Transaction T
WHERE T.id n IN (SELECT ST.idTransaction from @tmptable)

This will probably be slower than a temp table but faster than a join with a table variable

Answer 2

A trigger will fire even if you update to the same value

UPDATE T 
       SET idStatement = @idStatement
 FROM tbl_Transaction T
INNER JOIN @tmpTable ST 
   on T.id = ST.idTransaction 
  and idStatement <> @idStatement

if you need to also update to null

or (@idStatement == null and idStatement is not null)

And as stated in a comment try a #temp
Query optimization on Table is limited