We just launched W3Schools videos. Get certified by completing a course today! If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail:. Exercise: Delete all the records from the Customers table where the Country value is 'Norway'. Report Error. Your message has been sent to W3Schools. Instead of deleting , rows in one large transaction, you can delete or 1, or some arbitrary number of rows at a time, in several smaller transactions, in a loop.
In addition to reducing the impact on the log, you could provide relief to long-running blocking. So, I thought it might be time for a refresh to give a better picture of how this pans out in SQL Server Or copy the keeper rows out, truncate the table, and then copy them back in.
Again, if it turns out you have to delete rows, you will want to minimize the impact on the transaction log and how the operations affect the rest of the workload.
To create a table with 10 million rows, I made a copy of Sales. SalesOrderDetail, with its own identity column, and added a filler column just to give each row a little more meat and reduce page density:. Then, to generate the 10,, rows, I inserted , rows at a time, and ran the insert times:.
I did not create any indexes on the table; depending on the storage approach, I will create a new clustered index columnstore half of the time after the database is restored as a part of each test. Once the 10 million row table existed, I set a few options, backed up the database, backed up the log twice, and then backed up the database again so that the log would have the least possible used space when restored :.
Next, I created a Control database, where I would store the stored procedures that would run the tests, and the tables that would hold the test results just the start and end time of each test and performance metrics captured throughout all of the tests.
Capturing the permutations of all the tests I wanted to perform took a few tries, but I ended up with this:. Next, I created a stored procedure to capture the set of metrics described earlier. I am also monitoring the instance with SentryOne SQL Sentry, so there will certainly be some other interesting information available there, but I also wanted to capture the important details without the use of any third party tools.
Here is the procedure, which goes out of its way to produce all of the metrics for any given timestamp in a single row:. I put that stored procedure in a job step and started it. You may want to use a delay other than three seconds — there is a trade-off between the cost of the collection and the completeness of the data that may lean more one way than the other for you.
Finally, I created the procedure that would contain all the logic to determine exactly what to do with the combination of parameters for each individual test. This again took several iterations, but the final product is as follows:. I created a stored procedure and put this into a job also:. Part of that was because I had originally included a 0. So I removed those from the table a few days in, and can easily say: if you are removing 1,, rows, deleting 1, rows at a time is highly unlikely to be an optimal choice, regardless of any other variables:.
There are some exceptions to this behavior for InnoDB tables, as discussed in Section See Section 3. Errors encountered during the parsing stage are processed in the usual manner. If you are deleting many rows from a large table, you may exceed the lock table size for an InnoDB table. To avoid this problem, or simply to minimize the time that the table remains locked, the following strategy which does not use DELETE at all might be helpful:.
Select the rows not to be deleted into an empty table that has the same structure as the original table:. In this case, the holes left by deleted values are reused. Here is an example of such a scenario:. Insert many rows into the table. Each insert results in an index value that is added to the high end of the index.
In this scenario, the index blocks associated with the deleted index values become underfilled but are not merged with other index blocks due to the use of QUICK. They remain underfilled when new inserts occur, because new rows do not have index values in the deleted range. This rebuilds the index rather than performing many index block merge operations. For the first multiple-table syntax, only matching rows from the tables listed before the FROM clause are deleted.
The effect is that you can delete rows from many tables at the same time and have additional tables that are used only for searching:. These statements use all three tables when searching for rows to delete, but delete matching rows only from tables t1 and t2. The syntax permits. In this case, the statement fails and rolls back. If you declare an alias for a table, you must use the alias when referring to the table:. Elsewhere, alias references are permitted but not alias declarations.
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