This query was completed within 68 seconds. We added a non-clustered index on Persons table before to update and the added index involves the PersonCityName and PersonPostCode columns as the index key. The following execution plan is demonstrating an execution plan of the same query, but this query was completed within seconds because of the added index, unlike the first one.
We have seen this obvious performance difference between the same query because of index usage on the updated columns. As a result, if the updated columns are being used by the indexes, like this, for example, the query performance might be affected negatively. In particular, we should consider this problem if we will update a large number of rows. To overcome this issue, we can disable or remove the index before executing the update query. On the other hand, a warning sign is seen on the Sort operator, and it indicates something does not go well for this operator.
When we hover the mouse over this operator, we can see the warning details. During the execution of the query, the query optimizer calculates a required memory consumption for the query based on the estimated row numbers and row size.
However, this consumption estimation can be wrong for a variety of reasons, and if the query requires more memory than the estimation, it uses the tempdb data. This mechanism is called a tempdb spill and causes performance loss. The reason for this: the memory always faster than the tempdb database because the tempdb database uses the disk resources. Now, if we go back to our position, the MERGE statement can be used as an alternative method for updating data in a table with those in another table.
In this method, the reference table can be thought of as a source table and the target table will be the table to be updated. The following query can be an example of this usage method. We have typed the Persons table after the MERGE statement because it is our target table, which we want to update, and we gave Per alias to it in order to use the rest of the query. With the help of this syntax, the join condition is defined between the target and source table.
In this last line of the query, we chose the manipulation method for the matched rows. Finally, we added the semicolon ; sign because the MERGE statements must end with the semicolon signs.
The major characteristic of the subquery is, they can only be executed with the external query. The noticeable difference in this method is, it might be a convenient way to update one column for the tables that have a small number of the rows.
Now we will execute the following query and then will analyze it. You're right, I was typing the example by hand. I've added a third and fourth clause to the where statement to deal with that. Col1, T1. Col1, T2. Col2 is more concise. Depends on if you want to replace nulls in the destination with nulls from the source. Frequently, I don't. But if you do, Martin's construction of the where clause is the best thing to use. Show 1 more comment. Col2, T2. Stewart 3, 3 3 gold badges 25 25 silver badges 36 36 bronze badges.
Martin Smith Martin Smith k 81 81 gold badges silver badges bronze badges. Just an arbitrary alias in this case. Adrian Macneil Adrian Macneil Shiva Patrick Frenette Patrick Frenette 6 6 silver badges 2 2 bronze badges. Ryan Ryan 2, 6 6 gold badges 33 33 silver badges 45 45 bronze badges. I feel this should be the accepted answer, because it keeps things simple and to the point.
Simon Hughes 3, 3 3 gold badges 23 23 silver badges 44 44 bronze badges. Hentold Hentold 7 7 silver badges 11 11 bronze badges. What if we want to update Table2. BSMP 4, 8 8 gold badges 33 33 silver badges 43 43 bronze badges. Jason Clark Jason Clark 6 6 gold badges 17 17 silver badges 47 47 bronze badges. And if you wanted to join the table with itself which won't happen too often : update t1 -- just reference table alias here set t1. Aleksandr Fedorenko Aleksandr Fedorenko Richard Richard 1, 10 10 silver badges 13 13 bronze badges.
Consolidating all the different approaches here. Abdul Azeez Abdul Azeez 8 8 silver badges 18 18 bronze badges. Yaman Yaman 16 16 silver badges 31 31 bronze badges. Bartosz X Bartosz X 2, 23 23 silver badges 32 32 bronze badges.
Or even use table variable like tbl, "on PermTable. Have you tried? I am using a phone to reply this, no computer to try. Johannes Wentu Johannes Wentu 1 1 gold badge 12 12 silver badges 27 27 bronze badges. Cornezuelo del Centeno Cornezuelo del Centeno 7 7 silver badges 19 19 bronze badges. Mateen Mateen 1, 1 1 gold badge 19 19 silver badges 26 26 bronze badges.
Govind Tupkar Govind Tupkar 2 2 silver badges 5 5 bronze badges. This answer turned up in the low quality review queue, presumably because you don't provide any explanation of the code. If this code answers the question, consider adding adding some text explaining the code in your answer.
This way, you are far more likely to get more upvotes — and help the questioner learn something new. The MERGE statement is useful for manipulating data in the target table based on the source table data for both matched and unmatched rows.
It is a straightforward method to update the existing table data from other tables. The merge join uses an inner join for matching data rows between the source and target data. It also has the maximum relative cost for sort operator. The subquery is the fastest method to update column data. It uses the clustered index update and clustered index scan as highlighted. The subquery works efficiently, but it has its own limitations, as highlighted earlier.
The overall performance of your database depends on the table data, the number of updates, table relationships, indexes , and statistics. SQL Server execution plans are a gateway for query optimizer and query executions. Learn how to read, interpret and analyze execution plan data. Here are three key areas to focus your SQL Server performance tuning efforts to keep your database in peak condition. You may withdraw your consent at any time.
0コメント