Contents
How many logical reads are there in SQL Server?
Each page needed to be read only once and all values were retrieved. You can verify this by examining the number of pages used by both tables – the Customer table is ~110 pages large and the SalesOrderHeader table is ~700 pages. This accounts for the ~800 reads we saw in the trace.
When does a logical read occur in a database?
A logical read occurs every time the database engine requests a page from the buffer cache. If the page is not currently in the buffer cache, a physical read is then performed to read the page into the buffer cache.
Is it bad to have a lot of logical reads?
Large numbers of logical reads may not necessarily be bad — or, rather, not necessarily preventable. What’s bad is an inordinate number of logical reads.
Why is memory usage so high in SQL Server?
This is the precise reason why memory usage in SQL Server is high even if no one is using SQL Server. Once a page is modified in memory due to data modification (Insert/update/delete), it is called “dirty” page. On the other hand, if a page which is not modified is called “clean” page.
When does a physical read occur in SQL Server?
The I/O from an instance of the SQL Server Database Engine includes logical and physical reads. A logical read occurs every time the Database Engine requests a page from the buffer cache. If the page is not currently in the buffer cache, a physical read first copies the page from disk into the cache.
How to reduce logical reads in SQL Server?
Reduce logical reads (and the according large intermediate result) by applying both criteria in one step: (This may also effect physical reads, but doesn’t necessarily have to. For instance the first query may access 100 records and then reduce that to 10, whereas the second only reads those 10.
Note that the total_logical_reads now shows 2358 (3 x 786), but the total_physical_reads remains at 780. This enables us to calculate how often the pages are retrieved from cache and how often from disk when a query is run repeatedly over time.
When does a logical read result in a physical read?
When a block is requested by a query, Oracle looks for it in the Buffer Cache and if found, it results in a Logical read and if it does not find the Block in there it results in a physical read (disk I/O). I understand that physical I/O is the most expensive component and finding the block in the buffer will reduce most of the work.
What is the last logical reads in Java?
We can see that the statement has been executed twice and that the last_logical_reads value is 786, but the last_physical_reads value is 0. This confirms that the pages for this query are being retrieved from Buffer cache and the query is not waiting for physical disk operations.
What happens when there is insufficient memory in SQL Server?
If there is insufficient memory available for the SQL Server Buffer Cache, client sessions will have to wait while buffer pages are freed and new pages are read from disk. Ideally we would like to see that most of our reads and writes are being satisfied from cache and not incurring physical IO waits.