Similar to Read Committed Isolation level but you can still read the old version of the data. Before a row is locked in preparation for changes, that means an update statement that might be run to a given row, that row is then placed in a version store (TempDB). The big advantage that our SCI... » read more
Low isolation levels do allow for more users to be able to access the data so you can get greater concurrency, but they can also affect data integrity by creating the effect of lost updates and dirty reads.Dirty reads are a phenomenon where you read uncommitted data. That data could be wrong, because its transaction... » read more
Table of Differences char nchar varchar nvarchar Character Data Type ASCII Unicode ASCII Unicode Maximum Length up to 8,000 characters up to 4,000 characters up to 8,000 characters up to 4,000 characters Character Size takes up 1 byte per character takes up 2 bytes per Unicode/Non-Unicode character takes up 1 byte per character takes... » read more
Nvarchar stores UNICODE data. If you have requirements to store UNICODE or multilingual data, nvarchar is the choice. Varchar stores ASCII data and should be your data type of choice for normal use. Regarding memory usage, nvarchar uses 2 bytes per character, whereas varchar uses 1. VARCHAR is stored as regular 8-bit data. But NVARCHAR strings are stored in the database as UTF-16 — 16 bits... » read more
SQL Server gives us a limit of 900 bytes for clustering keys, and 1700 bytes for non-clustered indexes. Have to use full-text index for VARCHAR(MAX) or nVARCHAR(MAX) datatypes.
The I/O from an instance of SQL Server is divided into logical and physical I/O. 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. If... » read more
FillFactor is how much free space Microsoft leaves available in the index pages so that if an insert or an update happens, mainly an update, where the data grows, you don’t want those page splits to be within a row. So, FILLFACTOR can reduce your fragmentation. Typically on systems that see a lot of updates... » read more
Nonclustered indexes are secondary indexes used to help the performance of queries not served by the clustered index. You’re typically going to want to add a nonclustered index to the column that’s in your WHERE clause and your most commonly run queries. You want to use these involved on columns that are in joins and... » read more
In some scenarios, it might be beneficial to not have a clustered index on a table. E.T.L. staging tables is one scenario that you might not want to have a clustered index. We’re loading into a data warehouse a few times a day, and that could be hundreds of thousands of records at a time,... » read more
A heap is a table without a clustered index. One or more nonclustered indexes can be created on tables stored as a heap. Data is stored in the heap without specifying an order. Usually data is initially stored in the order in which is the rows are inserted into the table, but the Database Engine... » read more