Dense vs sparse indexes in DBMS Theory - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference between dense and sparse indexes

Which statement correctly describes the difference between dense and sparse indexes in database systems?

A A sparse index has an index entry for every search key value in the data file, while a dense index has entries only for some of the search key values.

B A dense index has an index entry for every search key value in the data file, while a sparse index has entries only for some of the search key values.

C Both dense and sparse indexes have entries for every search key value, but sparse indexes store less data per entry.

D Dense indexes are used only for unordered data, while sparse indexes are used only for ordered data.

💡 Hint

Think about how many entries each index type contains relative to the data file.

Dense indexes contain an entry for every record in the data file, making lookups fast but requiring more space. Sparse indexes contain entries for only some records, typically one per data block, saving space but requiring extra steps to find records.

🚀 Application

intermediate

Choosing index type for sorted data

You have a large sorted data file stored on disk. Which type of index is generally more space-efficient and suitable for this data?

A Dense index, because it stores only the first record of each block.

B Dense index, because it stores an entry for every record making searches faster.

C Sparse index, because it stores fewer entries by indexing only some records, reducing space usage.

D Sparse index, because it stores entries for every record to speed up access.

💡 Hint

Consider how indexing fewer records affects space and search efficiency for sorted files.

Sparse indexes are more space-efficient for sorted files because they store entries only for some records, usually one per block. This reduces index size while still allowing efficient block-level searching.

🔍 Analysis

advanced

Impact of index type on search performance

Consider a database with a dense index and another with a sparse index on the same sorted data file. Which statement best explains the difference in search performance?

A The dense index allows direct access to every record, so searches are faster than with a sparse index which requires an extra step to locate the block.

B The sparse index allows direct access to every record, so searches are faster than with a dense index which requires scanning blocks.

C Both indexes provide the same search speed because they index the same data.

D Sparse indexes are always slower because they store more entries than dense indexes.

💡 Hint

Think about how many steps are needed to find a record using each index type.

Dense indexes have an entry for every record, allowing direct access without extra block scanning. Sparse indexes have fewer entries, so after locating the block, a search within the block is needed, making it slightly slower.

❓ Comparison

advanced

When to prefer sparse index over dense index

In which scenario is a sparse index generally preferred over a dense index?

A When the data file is sorted and space efficiency is important.

B When the data file is unsorted and random access is required.

C When every record must be accessed quickly without scanning.

D When the data file is very small and fits entirely in memory.

💡 Hint

Consider the trade-off between space and access speed for sorted data.

Sparse indexes are preferred for sorted data files where saving space is important. They index fewer records, reducing index size, while still allowing efficient block-level searching.

❓ Reasoning

expert

Effect of block size on sparse index entries

How does increasing the block size of a sorted data file affect the number of entries in a sparse index built on it?

A The number of sparse index entries increases because more records fit in each block.

B The number of sparse index entries becomes equal to the number of records.

C The number of sparse index entries stays the same regardless of block size.

D The number of sparse index entries decreases because fewer blocks exist, so fewer entries are needed.

💡 Hint

Think about how block size affects the total number of blocks and index entries.

Increasing block size means each block holds more records, so the total number of blocks decreases. Since sparse indexes have one entry per block, the number of index entries decreases.