Question

Why can we have at most one primary or clustering index on a file, but several secondary indexes?

Short answer

We can only have one primary or clustering index on a file because it uniquely orders the records based on one field. Multiple primary indices would mean several unique ordering, which is not possible on a single file. However, secondary indexes don't uniquely order the file, but provide a fast search capability on various non-ordering fields, hence we can have several secondary indexes.

Step-by-step solution

Understanding Primary or Clustering index

A primary or clustering index is used to order the records in a file based on one field, which is the indexing field. This field has to be unique; thus we can only have one primary or clustering index on a file, because only one field can play this role. This type of index is also best used for range queries, because all the tuples with values within a certain range will be stored together.

Understanding Secondary Indexes

On the other hand, a secondary index, as the name indicates, is also an index, but it can be used on fields other than the one used for the primary index. It can be created for any field that we want to be searchable in a quick way, whether it is unique or not. Consequently, we can have more than one secondary index on a file, as there can be multiple fields in each record that we want to have fast access to. Secondary indexes provide pointers to the blocks, and within blocks, to the tuples.

Key concepts

Primary Index

A primary index is like the main marker that helps organize and speed up access to records in a database. Think of it as a librarian who sorts books based on their ISBN numbers – each book has a unique number, just like each record in a database file has a distinct field used as the primary index.

• It orders the data records based on a unique key.
• Primary indexes play a pivotal role in the efficient retrieval of data.
• They are especially useful for range queries.

For instance, if you wanted to find all records between certain dates, a primary index can save the day. It ensures that all data in those date ranges are nicely grouped together, making it quicker to fetch them. This efficiency becomes possible because there's only one primary index directing the order of records, maintaining a neat structure.

Clustering Index

A clustering index stands out because it groups data rows together based on certain non-unique fields. You can imagine it as a shelf where books are grouped by their genre – not each genre is unique, but clustering helps you find related books more effortlessly.

• Clustering indexes improve access speed by grouping similar records.
• They are effective for columns where values are not unique yet logically associated.
• Once set, reorganizing data with clustering can be complex.

Because records with similar values are physically close in the database thanks to the clustering index, searchers benefit when they look for related data chunks, akin to finding all mystery novels on the same shelf. However, like primary indexes, only one field can act as a clustering index in a file, so choose wisely! The constraints of having one clustering index arise from its organizing function.

Secondary Index

Secondary indexes provide additional paths to reach data fields not covered by primary or clustering indexes. Imagine you need another way to locate a book besides its ISBN or genre, such as by the author's name. Secondary indexes offer such alternative routes.

• They can be created on multiple non-unique fields.
• More than one secondary index can exist simultaneously.
• Secondary indexes enable rapid searches on supplemental fields.

Unlike primary and clustering indexes, secondary indexes are flexible. They don't rearrange or order the actual records but rather maintain pointers that direct to data locations. This flexibility allows several secondary indexes to coexist, enhancing the database's searchability. They serve as a map with different paths leading to the same destination, aiding in quick access to frequently queried fields without altering the primary data structure.