Now let’s say you wanted (for some reason) to replace your WHERE statement with a variable that can be used to call a single row each time but 1051 times you can use a cursor to do so: Results: This cursor will fetch exactly the same set of data but it does it on a row by row basis, and it takes heck of a lot longer to do so, as a matter of fact 48 seconds as opposed to 87 milliseconds, that’s 55172% slower!This is because the set-based logic for which RDBMS systems like SQL Server are optimized is completely broken and the entire query process has to be repeated for each row.
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Having worked on highly transactional production systems and advanced corporate business intelligence, Evan is now using this experience make a difference in the e Health world.
Take this simple SELECT statement as an example: When you write a SELECT statement like this (that returns 1051 rows) to fetch a bunch of data from that database the system receives the statement and creates or uses an existing query plan, then it uses indexes to locate the data on the disk, fetches the data in one foul swoop and returns the data as a set.
If your indexes are correctly placed the query can be sped up.
I have been told these types of operations are what SQL Server is designed to process and it should be quicker than serial processing. I assume Microsoft created them for a reason so they must have a place where they can be used in an efficient manner.
I know cursors exist but I am not sure how to use them. In some circles cursors are never used, in others they are a last resort and in other groups they are used regularly.
To illustrate how a while loop works I have massaged the above query example to return the same data again but this time, instead of row-by-row or a full set, it does something in between and returns 100 rows at a time.
Even this clumsy WHILE loop is blisteringly fast in comparison to the SQL Server cursor approach.
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In my T-SQL code I always use set based operations.
However, other than for backwards compatibility they can still serve us well in the right situations.