Bitmap indexes in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a bitmap index?

A bitmap index is a type of database index that uses bit arrays (bitmaps) to quickly answer queries, especially useful for columns with a small number of distinct values.

beginner

How does a bitmap index represent data internally?

It uses a series of bits where each bit corresponds to a row in the table. A bit is set to 1 if the row has the value represented by that bitmap, otherwise 0.

intermediate

Why are bitmap indexes efficient for columns with low cardinality?

Because they use compact bit arrays, bitmap indexes require less space and can quickly combine bitmaps using bitwise operations for queries on columns with few distinct values.

intermediate

What types of queries benefit most from bitmap indexes?

Queries that filter on multiple low-cardinality columns, such as AND, OR, and NOT conditions, benefit because bitmaps can be combined quickly with bitwise operations.

intermediate

What is a limitation of bitmap indexes?

Bitmap indexes are not efficient for columns with high cardinality or for tables with frequent updates, because bitmaps can become large and costly to maintain.

What kind of data is best suited for bitmap indexes?

A Columns with few distinct values

B Columns with many unique values

C Large text columns

D Columns with continuous numeric values

Bitmap indexes work best on columns with low cardinality, meaning few distinct values.

How does a bitmap index speed up query processing?

A By compressing text data

B By sorting data physically

C By caching entire tables

D By using bitwise operations on bit arrays

Bitmap indexes speed up queries by combining bitmaps with fast bitwise operations like AND, OR, and NOT.

Which operation is NOT typically efficient with bitmap indexes?

A AND

B Frequent row insertions

C OR

D NOT

Frequent updates or insertions are inefficient for bitmap indexes because bitmaps need to be updated, which can be costly.

What does each bit in a bitmap index represent?

A A row in the table

B A column in the table

C A database user

D A query condition

Each bit corresponds to a specific row, indicating whether that row has the indexed value.

Why might bitmap indexes use less storage space than other indexes?

A Because they avoid indexing

B Because they store full row data

C Because bits are compact and can be compressed

D Because they use text compression

Bitmaps use bits which are very compact and can be further compressed, saving storage space.

Explain how bitmap indexes work and why they are useful for low-cardinality columns.

Think about how bits can quickly show which rows match a value.

Describe the main advantages and limitations of bitmap indexes in databases.

Consider when bitmap indexes shine and when they struggle.