Does ORDER BY affect GROUP BY?
No, the order doesn’t matter for the GROUP BY clause. MySQL and SQLite are the only databases I’m aware of that allow you to select columns which are omitted from the group by (non-standard, not portable) but the order doesn’t matter there either.
Which comes first GROUP BY or ORDER BY?
Important Points:
- GROUP BY clause is used with the SELECT statement.
- In the query, GROUP BY clause is placed after the WHERE clause.
- In the query, GROUP BY clause is placed before ORDER BY clause if used any.
Does GROUP BY improve performance?
GROUP BY performs better if you keep the number of grouping columns small. When you’re grouping joined tables, the GROUP BY column should be from the same table as the column(s) on which you’re applying a set function. You can improve performance on some grouped joins by replacing the join with a subquery.
Does GROUP BY maintain order?
1 Answer. Found answer on MSDN: Yes. The IGrouping objects are yielded in an order based on the order of the elements in source that produced the first key of each IGrouping . Elements in a grouping are yielded in the order they appear in source.
Why is an ORDER BY clause bad performance?
I found Bad performance of SQL query due to ORDER BY clause. Is there any other circumstances where an order by clause may negatively effect performance? An ORDER BY clause will always add a sort operation if there is no usable index to retrieve the data in order.
When to test performance against a production database?
If there is a question about performance, I test the query against a production database before testing alternatives. It depends on the query and size of result set. If sorting can be done in memory its still “fast”. But if its too large for memory Oracle will write the result to TEMP tablespace and this could be worse.
Can a large table be run ORDER BY clause?
ORDER BY on large tables can be real performance disaster , try to apply the ORDER BY clause on smallest possible dataset , in theory the optimizer does good job to choose the best way to run the ORDER BY so performance is not affected significantly but in real heavy environments i saw ORDER BY clause which affect the performance significantly.
Do you need to sort data before presenting results?
For larger sets of data the data being sorted may be written to disk adding additional processing. For smaller result sets where indexes are being used, there may be no requirement for a sort step. If sorting is required, the database will have to retrieve all the sort key data and sort it before presenting results.