# What does cardinality mean in index?

## What does cardinality mean in index?

It is an estimate of the number of unique values in the index. For a table with a single primary key column, the cardinality should normally be equal to the number of rows in the table.

## What is curse cardinality?

One of the most serious ones, which we call the curse of cardinality, is that both index sizes and query response time increase as the number of distinct values in an attribute increases.

What do you mean by bitmap index?

A bitmap index is a special kind of database index that uses bitmaps. Bitmap indexes use bit arrays (commonly called bitmaps) and answer queries by performing bitwise logical operations on these bitmaps. Bitmap indexes have a significant space and performance advantage over other structures for query of such data.

What is a bitmap index used for?

Bitmap indexes are widely used in data warehousing applications, which have large amounts of data and ad hoc queries but a low level of concurrent transactions. For such applications, bitmap indexing provides: Reduced response time for large classes of ad hoc queries.

### What is cardinality example?

The cardinality of a set is a measure of a set’s size, meaning the number of elements in the set. For instance, the set A = { 1 , 2 , 4 } A = \{1,2,4\} A={1,2,4} has a cardinality of 3 for the three elements that are in it.

### What is the symbol of cardinality?

Table of set theory symbols

Symbol Symbol Name Meaning / definition
|A| cardinality the number of elements of set A
#A cardinality the number of elements of set A
| vertical bar such that
ℵ0 aleph-null infinite cardinality of natural numbers set

When would you use a bitmap index?

You can use bitmap indexes even when retrieving large percentages (20–80 percent) of a table. You can also use bitmaps to retrieve conditions based on NULLs (because NULLs are also indexed), and can be used for not-equal conditions for the same reason. The best way to find out is to test!

What is difference between B-tree and bitmap index?

The basic differences between b-tree and bitmap indexes include: 2: Cardinality differences: The bitmap index is generally for columns with lots of duplicate values (low cardinality), while b-tree indexes are best for high cardinality columns.

#### What is ∈ called?

The relation “is an element of”, also called set membership, is denoted by the symbol “∈”. Writing. means that “x is an element of A”. Equivalent expressions are “x is a member of A”, “x belongs to A”, “x is in A” and “x lies in A”.

#### What is difference between b-tree and bitmap index?

How do I create a bitmap index?

To create a bitmap index, use the BITMAP clause of the CREATE INDEX command, as shown in the following listing. You should indicate its nature as a bitmap index within the index name so it will be easy to detect during tuning operations.

When to use bitmap indexing in a table?

A bitmap indexing can also be built on columns with higher cardinality, consider a table with million rows having 50000 rows with distinct values. B-tree indexing can be used for data with high cardinality.

## What are the drawbacks of bit map indexing?

One bitmap is “male = 11010” and the other bitmap is “female = 00101”. But there is a drawback with bitmap indices, that is bit map index is suffering from the column cardinality. If the column cardinality is very high (if there are many number of distinct attribute values ), then the bitmap index file will be large in terms of size.

## Which is the smallest bit-slice Index in bitmap?

The smallest bit-slice index is a single bitmap index. Bit-slice index has solved the problem of high cardinality in bitmap indices. It is possible to create a bit-slice index to a numeric column (eg:- to salary column in Employee table above) and we normally create bit-slice indices on numeric columns.

Where does an oracle bitmap index store pointers?

In a bitmap index, Oracle stores a bitmap for each index key. Each index key stores pointers to multiple rows. For example, if you create a bitmap index on the gender column of the members table.