Update with join takes long time - sql

Question

I have a problem updating a table. I'm running the following query:
UPDATE Table1 SEt entrena = c.Count
FROM Table1 AS p INNER JOIN (
SELECT e.EmplID, COUNT(e.EmplID ) as Count
FROM Table2 AS e WHERE e.Start >= #Start AND e.Start <=#End
GROUP BY e.EmplID ) AS c ON p.EmplID = c.EmplID
WHERE P.Date = '2050-12-31'
The Table1 has 12000 rows and the select inside the join get only 51 rows but the update takes around 2 minutes, but if I delete the where clase p.date = '2050-12-31' the update takes less than a second. And I can't figure out how to solve it. I'm using SQL Server 2008.
Both tables don't have indexes.

Answer 1

This is your query:
UPDATE Table1
SET entrena = c.Count
FROM Table1 p INNER JOIN
(SELECT e.EmplID, COUNT(e.EmplID ) as Count
FROM Table2 e
WHERE e.Start >= #Start AND e.Start <=#End
GROUP BY e.EmplID
) c
ON p.EmplID = c.EmplID
WHERE P.Date = '2050-12-31';
First, you need to change the first line to:
UPDATE p
when you define an alias in the from clause, you need to use that in the update for the right thing to happen. (I wish your query generated an error in SQL Server, but it does not.)
To optimize this query, you want to add indexes. I would suggest these two:
Table1(date, EmplID)
Table2(EmplId, Start)
You may then find that the correlated subquery version is faster, particularly if the where clause is highly selective:
UPDATE p
SET entrena = (SELECT COUNT(e.EmplID ) as Count
FROM Table2 e
WHERE e.Start >= #Start AND e.Start <=#End AND
p.EmplID = e.EmplID
)
FROM Table1 p
WHERE P.Date = '2050-12-31';
However, I suspect that the p versus table1 is the root of your performance problem.