大表上的简单 UPDATE 查询性能不佳

Question

I need to do the following update query through a stored procedure:

UPDATE table1
SET name = @name (this is the stored procedure inputparameter)
WHERE name IS NULL

Table1 has no indexes or keys, 5 columns which are 4 integers and 1 varchar (updatable column 'name' is the varchar column)

The NULL records are about 15.000.000 rows that need updating. This takes about 50 minutes, which I think is too long.

I'm running an Azure SQL DB Standard S6 (400DTU's).

Can anyone give me an advise to improve performance?

Answer 1

As you don't have any keys, or indexes, I can suggest following approach.

1- Create a new table using INTO (which will copy the data) like following query.

  SELECT 
       CASE 
              WHEN NAME IS NULL THEN @name 
              ELSE NAME 
       END AS NAME, 
       <other columns > 
INTO   dbo.newtable
FROM table1

2- Drop the old table

drop table table1

3- Rename the new table to table1

exec sp_rename 'dbo.newtable', 'table1'

Another approach can be using batch update, sometime you get better performance compared to bulk update (You need to test by adjusting the batch size).

WHILE EXISTS (SELECT 1 FROM table1 WHERE name is null)
BEGIN
    UPDATE TOP (10000) table1
    SET name = @name
    WHERE n ame is null
END

Answer 2

All you need is an INDEX via

       Create INDEX I1 ON table1(P_key)

And, then run your update query. This will help in fast data retrieval and updation of records on basis of some unique/key column made as an index in your existing table.

Answer 3

can you do with following method ?

UPDATE table1
SET name = ISNULL(name,@name)

for null values it will update with @name and rest will be updated with same value.

Answer 4

No. You are updating 15,000,000 rows which is going to take a long time. Each update has overhead for finding the row and logging the value.

With so many rows to update, it is unlikely that the overhead is finding the rows. If you add an index on name , the update is going to actually have to update the index as well as updating the original values.

If your concern is locking the database, you can set up a loop where you do something like this over and over:

UPDATE TOP (100000) table1
    SET name = @name (this is the stored procedure inputparameter)
    WHERE name IS NULL;

100,000 rows should be about 30 seconds or so.

In this case, an index on name does help. Otherwise, each iteration of the loop would in essence be reading the entire table.