Indexing in Database Management

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Questions and Answers

What is a minor disadvantage of B+-trees?

A complete reorganization of the file is needed for updates.

They have extra insertion and deletion overhead. (correct)

They require significant memory space for operation.

They are slower than regular binary trees.

Which statement accurately describes the use of indices in databases?

Indices are only helpful for primary key attributes.

They can significantly speed up lookups but may slow down updates. (correct)

They automatically degrade the performance of updates.

Creating indices always requires manual tuning and cannot be automated.

Which command would you use to delete an existing index in SQL?

remove index <index_name>

delete index <index_name>

erase index <index_name>

drop index <index_name> (correct)

Why are indices on primary keys created automatically by databases?

To enhance the uniqueness constraint of the primary key. Signup and view all the answers

What advantage do B+-trees offer over other types of trees?

They maintain sorted order efficiently and facilitate search operations. Signup and view all the answers

What is the purpose of the 'create unique index' command in SQL?

To enforce that the search key must be a candidate key. Signup and view all the answers

What role do index tuning assistants play in database management?

They help choose indices based on query and update workloads. Signup and view all the answers

What is an example of a SQL command to create an index?

create index student_index on student(name); Signup and view all the answers

What is the main purpose of indexing in a database system?

To speed up access to desired data Signup and view all the answers

In which type of index are search keys stored in sorted order?

Ordered index Signup and view all the answers

What distinguishes a clustering index from a secondary index?

A clustering index matches the sequential order of the file Signup and view all the answers

Which of the following statements is true regarding dense index files?

They contain pointers to every search-key value in the file Signup and view all the answers

What mechanism does a hash index use to distribute search keys?

It employs a hash function to place keys in buckets Signup and view all the answers

Which type of index provides a record for only some search-key values?

Sparse index Signup and view all the answers

What is typically true about the size of index files compared to the original file?

Index files are smaller than the original file Signup and view all the answers

Which index is also known as a nonclustering index?

Secondary index Signup and view all the answers

What is the primary disadvantage of indexed-sequential files as mentioned?

Performance degrades as the file size increases. Signup and view all the answers

Which indexing technique is generally slower when locating records?

Sparse index Signup and view all the answers

What is a characteristic of secondary indices?

They must be dense. Signup and view all the answers

What is a benefit of using a B+-tree index file?

It automatically reorganizes itself with small changes. Signup and view all the answers

What is the purpose of using a sparse index in the context of clustering?

To index only the least search-key value in each block. Signup and view all the answers

What happens when a record is updated in the context of maintaining indices?

All indices on the updated attribute must be updated. Signup and view all the answers

What does a multi-level index allow when the basic index is too large for main memory?

To create an outer sparse index on the basic index. Signup and view all the answers

What is a common overhead associated with maintaining indices on a database?

Updating indices upon record modifications. Signup and view all the answers

Study Notes

Basic Concepts

Indexing is used to speed up access to data by creating an index file that contains records (index entries) of the form: search-key - pointer
The index file is typically much smaller than the original file
Two basic kinds of indices are ordered indices and hash indices

Ordered Indices

Ordered indices store index entries sorted on the search key value
A clustering index specifies the sequential order of a file
A clustering index is also called a primary index
A secondary index specifies a different order than the sequential order of the file
A secondary index is also called a non-clustering index
An index-sequential file is a sequential file ordered on a search key, with a clustering index on the search key

Dense Index Files

A dense index has an index record for every search-key value in the file
Dense indices are efficient for locating records
Dense indices are also more space consuming and require more maintenance overhead for insertions and deletions

Sparse Index Files

A sparse index only contains index records for some search-key values
Sparse indices are suitable for sequentially ordered records
To locate a record with a search-key value, a sparse index will find the index record with the largest search-key value less than the target search-key value and then search sequentially from the record pointed to by that index record
Sparse indices are less space consuming and have less maintenance overhead compared to dense indices, but they are generally slower for locating records
A good tradeoff for a clustered index is a sparse index with an index entry for every block in the file, corresponding to the least search-key value in the block
For an unclustered index, a sparse index on top of a dense index can be used for multilevel indexing

Secondary Indices Example

A secondary index on the salary field of the instructor relation can point to a bucket that contains pointers to records with that particular salary value
Secondary indices must be dense

Clustering vs Non-Clustering Indices

Indices offer substantial benefits when searching for records, but they impose overhead on database modifications
For updates, all indices on a relation must be updated when a record is inserted or deleted
For updates, any index on an updated attribute must be updated if a record is updated
Sequential scanning using a clustering index is efficient, but it is expensive on magnetic disk for a non-clustering index because each record access may need a new block from disk, which takes about 5-10 milliseconds

Multilevel Index

A multilevel index is used to efficiently manage large indexes that don't fit in memory
It involves constructing a sparse index (outer index) on top of the basic index (inner index)
If the outer index is too large, another level of index can be created, forming a hierarchical structure
All levels of indices must be updated on insertion or deletion from the file

B+-Tree Index Files

B+-tree index files offer an advantage over indexed-sequential files by automatically reorganizing themselves with small, local changes during insertions and deletions, thus eliminating the need for periodic reorganization of the entire file
This reorganization capability makes them efficient even when the file grows
B+-trees are used extensively due to their advantages
B+-trees impose minor insertion and deletion overhead and some space overhead

Creation of Indices

The command create index <index-name> on <relation> (<attribute-list>) is used to create indices
The command drop index <index-name> is used to drop indices
Indices on primary keys are created automatically by all database systems
Some database systems also create indices on foreign key attributes, which can speed up joins
Indices can significantly speed up lookups but impose costs on updates
Index tuning assistants/wizards are available in several databases to help in choosing indices based on query and update workload

Index Definition in SQL

An index can be created using the command CREATE INDEX <index-name> ON <table-name> (<column-list>)
The command CREATE UNIQUE INDEX can be used to indirectly specify and enforce the condition that the search key is a candidate key
An index can be dropped using the command DROP INDEX <index-name>
Most database systems allow the specification of index type and clustering

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Description

This quiz covers the fundamental concepts of indexing in database management, including ordered and dense indices. Learn the difference between clustering and secondary indices, and explore how indexing helps improve data retrieval efficiency.