What is a Clustered Index?

Clustered Index stores a key value in the same data or row page. It is attached with data rows.

Clustered indexes sort and store the data rows in the table or view based on their key values.

For example, each and every book has attached one page number on each page so this called clustered index of that book.

How many clustered indexes there can be in one table?

You can create only one clustered index.

What is a Non-clustered Index?

Nonclustered indexes have a structure separate from the data rows. A nonclustered index contains the nonclustered index key values and each key value entry has a pointer to the data row that contains the key value.

The pointer from an index row in a nonclustered index to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a heap or a clustered table. For a heap, a row locator is a pointer to the row. For a clustered table, the row locator is the clustered index key.

You can add nonkey columns to the leaf level of the nonclustered index to by-pass existing index key limits, 900 bytes and 16 key columns, and execute fully covered, indexed, queries.

Can you make non-clustered indexes as unique index?

Yes, you can create unique non-clustered index in which no two rows can have the same value for the index key.

What is a Hash Index?

With a hash index, data is accessed through an in-memory hash table. Hash indexes consume a fixed amount of memory, which is a function of the bucket count.

hash indexes are used is because hash tables are extremely efficient when it comes to just looking up values. So, queries that compare for equality to a string can retrieve values very fast if they use a hash index.

What is the main disadvantage of the Hash Index?

Hash tables are not sorted data structures, and there are many types of queries which hash indexes can not even help with.

For instance, suppose you want to find out all of the employees who are less than 40 years old. How could you do that with a hash table index? Well, it’s not possible because a hash table is only good for looking up key value pairs – which means queries that check for equality condition.

What is a Unique Index?

A unique index ensures that the index key contains no duplicate values and therefore every row in the table or view is in some way unique.

Uniqueness can be a property of both clustered and nonclustered indexes.

Why CLUSTERED INDEX is a Special Index in the SQL Server?

The clustered index in any table has special uses and significance. This index is the data itself, and it will be used in every nonclustered index in the table. If you are defining a new clustered index or changing a clustered index, SQL Server will need to do IO on every nonclustered index on the table as well.

T-SQL Script to create a CLUSTERED INDEX.

CREATE CLUSTERED INDEX Index_Name ON Schema.TableName(Column);

T-SQL Script to create a NONCLUSTERED INDEX.

CREATE NONCLUSTERED INDEX Index_Name ON Schema.TableName(Column);

T-SQL Script to create a NONCLUSTERED INDEX with covered nonkey columns.

CREATE NONCLUSTERED INDEX Index_Name ON Schema.TableName(Column) INCLUDE (ColumnA, ColumnB);

What is a Filtered Index?

An optimized nonclustered index, especially suited to cover queries that select from a well-defined subset of data. It uses a filter predicate to index a portion of rows in the table. A well-designed filtered index can improve query performance, reduce index maintenance costs, and reduce index storage costs compared with full-table indexes.

What is a Full-text Index?

A special type of token-based functional index that is built and maintained by the Microsoft Full-Text Engine for SQL Server. It provides efficient support for sophisticated word searches in character string data.

What is a Spatial Index?

A spatial index provides the ability to perform certain operations more efficiently on spatial objects (spatial data) in a column of the geometry data type. The spatial index reduces the number of objects on which relatively costly spatial operations need to be applied.

What are Covering Indexes and Covered Queries in SQL Server?

If all the columns requested in the select list of query, are available in the index, then the query engine doesn’t have to lookup the table again which can significantly increase the performance of the query. Since all the requested columns are available with in the index, the index is covering the query. So, the query is called a covering query and the index is a covering index.

A clustered index can always cover a query, if the columns in the select list are from the same table.

How Indexes are used by the Query Optimizer?

Well-designed indexes can reduce disk I/O operations and consume fewer system resources therefore improving query performance. Indexes can be helpful for a variety of queries that contain SELECT, UPDATE, DELETE, or MERGE statements.

When the query optimizer uses an index, it searches the index key columns, finds the storage location of the rows needed by the query and extracts the matching rows from that location. Generally, searching the index is much faster than searching the table because unlike a table, an index frequently contains very few columns per row and the rows are in sorted order.

What is an Active Log?

The section of the log file from the MinLSN to the last-written log record is called the active portion of the log, or the active log. This is the section of the log required to do a full recovery of the database. No part of the active log can ever be truncated. All log records must be truncated from the parts of the log before the MinLSN.

What is “Write Ahead Transaction Log”?

SQL Server uses a write-ahead log (WAL), which guarantees that no data modifications are written to disk before the associated log record is written to disk.

Data modifications are not made directly to disk, but are made to the copy of the page in the buffer cache. The modification is not written to disk until a checkpoint occurs in the database.

What is a Clustered table?

Clustered tables are tables that have a clustered index. The data rows are stored in order based on the clustered index key.

What is a Heap Table?

Heaps are tables that have no clustered index. The data rows are not stored in any particular order, and there is no particular order to the sequence of the data pages.

What is a Worktables?

The relational engine may need to build a worktable to perform a logical operation specified in an SQL statement. Worktables are internal tables that are used to hold intermediate results. Worktables are generated for certain GROUP BY, ORDER BY, or UNION queries.

What is a COMPATIBILITY_LEVEL option of the Database?

This lets SQL Server know with which version of SQL Server to make the database compatible.

What is AUTO_SHRINK option of the Database?

This option either turns on or off the feature that will automatically shrink your database files when free space is available. In almost all cases, this should be set to OFF.

What is AUTO_UPDATE_STATISTICS option of the Database?

When turned ON, as it should be in most cases, the optimizer will automatically keep statistics updated, in response to data modications.

What is READ_WRITE option of the Database?

This is the default option and the one to use if you want users to be able to update the database. We could also set the database to READ_ONLY to prevent any users making updates to the database.

What is Lazy writer?

Lazy writer can adjust the number of buffers in the buffer pool if dynamic memory management is enabled. The lazy writer is a system process that keeps free buffers available by removing infrequently used pages from the buffer cache.

Ensure that a specified number of buffers are free in the Buffer Pool so they can be allocated for use by the server.

How you explicitly recompile your execution plan?

• You can use sp_recompile to explicitly recompile your execution plan.
• You can also use WITH RECOMPILE to recompile the stored procedure.

What is buffer pool?

SQL Server builds a buffer pool in memory to hold pages read from the database. You can minimize physical I/O to the database files by maximizing the size of the buffer pool.

What is a query wait option?

Use the query wait option to specify the time in seconds (from 0 through 2147483647) that a query waits for resources before timing out. If the default value of -1 is used, or if –1 is specified, then the time-out is calculated as 25 times of the estimated query cost.

You can change query wait value using sp_configure.

How SQL Server executes a single SELECT statement?

• The parser scans the SELECT statement and breaks it into logical units such as keywords, expressions, operators, and identifiers.
• A query tree, sometimes referred to as a sequence tree, is built describing the logical steps needed to transform the source data into the format required by the result set.
• The query optimizer analyzes different ways the source tables can be accessed and create an optimized version of the query tree is called the execution plan.
• The relational engine starts executing the execution plan.
• The relational engine processes the data and returns the result set to the client.

What is Query Plan?

The bulk of the execution plan is a re-entrant, read-only data structure used by any number of users. This is referred to as the query plan. No user context is stored in the query plan. There are never more than one or two copies of the query plan in memory.

What is Execution Context?

Each user that is currently executing the query has a data structure that holds the data specific to their execution, such as parameter values. This data structure is referred to as the execution context.

How SQL Server uses procedure cache?

When any SQL statement is executed, it first looks through the procedure cache to verify that an existing execution plan for the same SQL statement exists.

It reuses any existing plan it finds, saving the overhead of recompiling the SQL statement

What is a BACKUP WITH FORMAT option?

BACKUP WITH FORMAT option tells SQL Server whether or not to overwrite the media header information. The FORMAT option will erase any information in a backup set that already exists when the backup is initialized.

What is a BACKUP WITH SKIP option?

Using the SKIP parameter will cause SQL Server to skip the expiration check that it normally does on the backup set. It doesn’t care if any backups existing in the backup set have been marked for availability to be overwritten.

What is BACKUP WITH CHECKSUM option?

When generating a backup checksum, BACKUP verifies that the data read from the database is consistent with any checksum or torn-page indication that is present in the database.

When creating a backup checksum, a backup operation does not add any checksums to pages. Pages are backed up as they exist in the database, and the pages are unmodified by backup.

How many copies are allowed when taking a backup using MIRROR Backup option?

Three copies are allowed in a Mirror backup apart from the original copy.

What is MAXTRANSFERSIZE option in Backup database command?

MAXTRANSFERSIZE : specifies the largest unit of transfer in bytes to be used between SQL Server and the backup media. The possible values are multiples of 64 KB ranging up to 4194304 bytes (4 MB).  The default is 1 MB.

What is BUFFERCOUNT option in Backup database command?

BUFFERCOUNT specifies the total number of I/O buffers to be used for the backup operation.   The total space that will be used by the buffers is determined by: buffercount * maxtransfersize.

What is Windows Azure Blob storage service Database backups?

SQL Server 2012 SP1 CU2, enables SQL Server backup and restore directly to the Windows Azure Blob service. Backup to cloud offers benefits such as availability, limitless geo-replicated off-site storage, and ease of migration of data to and from the cloud.

What are the Benefits with Windows Azure Blob storage service?

Flexible, reliable, and limitless off-site storage: Storing your backups on Windows Azure Blob service can be a convenient, flexible, and easy to access off-site option.

No overhead of hardware management

Cost Benefits: Pay only for the service that is used. Can be cost-effective as an off-site and backup archive option.

What is the Difference between WITH RECOVERY and WITH NORECOVERY parameter?

The RECOVERY command requests the current RESTORE to roll back all uncommitted transactions.  When all uncommitted transactions are rolled back the database is placed into a ready state. The database is ready to use.

NORECOVERY allows the Database Administrator to restore additional backup files such as Differential or Transactional backups. While the database is in this state then users are not able to connect or access this database.

What is a Disaster Recovery?

Disaster recovery is a process that you can use to help recover information systems and data, if a disaster occurs.

Some examples of disasters include a natural or a man-made disaster such as a fire, or a technical disaster such as a two-disk failure in a Redundant Array of Independent Disks (RAID) 5 array.

What are your strategies to recover data when the disaster occur?

• Failover Clustering
• AlwaysOn Avaliability
• Database Mirroring
• Transaction Replication
• Log Shipping
• Strong backup strategies
• Virtual Machine Replication

Before designing a backup strategy, what should be your questions?

• How many hours a day do applications have to access the database?
• How frequently are changes and updates likely to occur?
• Are changes likely to occur in only a small part of the database or in a large part of the database?
• How much disk space will a full database backup require?

Can you encrypt data while creating a backup?

Starting in SQL Server 2014, SQL Server has the ability to encrypt the data while creating a backup. By specifying the encryption algorithm and the encryptor (a Certificate or Asymmetric Key) when creating a backup, you can create an encrypted backup file.

What is Partial Backups?

Partial backups are designed for use under the simple recovery model to improve flexibility for backing up very large databases that contain one or more read-only filegroups.

Partial backups are useful whenever you want to exclude read-only filegroups. A partial backup resembles a full database backup, but a partial backup does not contain all the filegroups.

Why can’t I just backup SQL Server’s data files with my Windows backup tool?

SQL Server is not like a word processing application. It manages its own files internally in order to guarantee the ACID (Atomic, Consistent, Isolated, Durable) properties of its databases.

If you simply copy the data file, ignoring the locks and ignoring the transactions that may be currently in progress, it means that when you attempt to attach that database later you will have a database file that is in an inconsistent state. It will generate errors.

It is only a small database. Why can’t I just write every table to disk to back it up?

Yes, you can use a tool like SQLCMD to write the tables to flat file but then, instead of a straightforward, single statement to restore the database, you will need a whole series of commands.

Can someone change the contents of a backup?

There is no direct way to modify the contents of a backup file. Since the backup is a page-by-page copy of the database.

When SQL Server reads each page, during the restore, it will calculate a checksum, based on its contents, and compare it to the value when it read the page during the backup. If someone did manage to change the data within the backup, these values won’t match and SQL Server will flag the page as corrupt.

What is Piecemeal Restores?

Piecemeal restore allows databases that contain multiple filegroups to be restored and recovered in stages. Piecemeal restore involves a series of restore sequences, starting with the primary filegroup and, in some cases, one or more secondary filegroups. Piecemeal restore maintains checks to ensure that the database will be consistent in the end.