What is composite index in SQL Server?
An SQL composite index is an index with an index key of more than 1 column. It is good for covering searches and lookups like WHERE clause and joins. You can create composite indexes using CREATE INDEX or ALTER TABLE. An SQL GUI tool can also be used.
What is no lock in SQL?
The WITH (NOLOCK) table hint is used to override the default transaction isolation level of the table or the tables within the view in a specific query, by allowing the user to retrieve the data without being affected by the locks, on the requested data, due to another process that is changing it.
What is reindex in SQL Server?
Reindexing tables is an important part of good database housekeeping, because it reorganizes the indexes and restores speedy access. Microsoft’s SQL Server has a handy command that rebuilds all indexes in a table. Oracle users typically rebuild each index in a table individually.
What are table hints?
Table hints override the default behavior of the Query Optimizer for the duration of the data manipulation language (DML) statement by specifying a locking method, one or more indexes, a query-processing operation such as a table scan or index seek, or other options.
What is index scan and index seek in SQL Server?
Explanation. An index scan or table scan is when SQL Server has to scan the data or index pages to find the appropriate records. A scan is the opposite of a seek, where a seek uses the index to pinpoint the records that are needed to satisfy the query.
What is composite index example?
Composite index numbers allow us to measure, with a single number, the relative variations within a group of variables upon moving from one situation to another. The consumer price index, the wholesale price index, the employment index and the Dow-Jones index are all examples of composite index numbers.
What is Nolock and Rowlock in SQL Server?
The WITH (ROWLOCK) table hint can be used with either SELECT, INSERT, UPDATE, and DELETE statements, to instruct the server to only apply a Range-Lock(s) on the rows being modified or added, and to avoid escalating the lock to page or table level. The rest of rows are not locked and can be accessed by another query.
What is lock in SQL Server with example?
Locks are held on SQL Server resources, such as rows read or modified during a transaction, to prevent concurrent use of resources by different transactions. For example, if an exclusive (X) lock is held on a row within a table by a transaction, no other transaction can modify that row until the lock is released.
How often should I reindex SQL database?
There’s a general consensus that you should reorganize (“defragment”) your indices as soon as index fragmentation reaches more than 5 (sometimes 10%), and you should rebuild them completely when it goes beyond 30% (at least that’s the numbers I’ve heard advocated in a lot of places).
What is a dirty read in SQL?
Dirty Reads A dirty read occurs when a transaction reads data that has not yet been committed. For example, suppose transaction 1 updates a row. Transaction 2 reads the updated row before transaction 1 commits the update.
What is forceseek in SQL Server?
It forces the query optimizer to use an Index seek instead of Index scan. Forceseek applies to both clustered and nonclustered index seek operations. It can be specified for any table or view in the FROM clause of a SELECT statement and in the FROM clause of an UPDATE or DELETE statement.
What is forceforceseek hint in SQL Server?
Forceseek hint is a new addition to SQL Server 2008. It forces the query optimizer to use an Index seek instead of Index scan. Forceseek applies to both clustered and nonclustered index seek operations. It can be specified for any table or view in the FROM clause of a SELECT statement and in the FROM clause of an UPDATE or DELETE statement.
How to avoid using index hint with forceseek?
Do not unnecessarily use index hint with forceseek and avoid using index hint index (0) with forceseek hint as the former will cause a table scan. 3. Do not use the USE PLAN query hint with forceseek as the forceseek hint is ignored when you do so
What is forcescan hint in SQL Server?
FORCESCAN Applies to: SQL Server 2008 R2 SP1 and later. Specifies that the query optimizer use only an index scan operation as the access path to the referenced table or view. The FORCESCAN hint can be useful for queries in which the optimizer underestimates the number of affected rows and chooses a seek operation rather than a scan operation.