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What is a low cardinality column?
Low-cardinality refers to columns with few unique values. Low-cardinality column values are typically status flags, Boolean values, or major classifications such as gender. An example of a data table column with low-cardinality would be a CUSTOMER table with a column named NEW_CUSTOMER.
How do you select a column index?
Choose the Order of Columns in Composite Indexes In general, you should put the column expected to be used most often first in the index. You can create a composite index (using several columns), and the same index can be used for queries that reference all of these columns, or just some of them.
What is cardinality MySQL index?
Index cardinality refers to the uniqueness of values stored in a specified column within an index. MySQL generates the index cardinality based on statistics stored as integers, therefore, the value may not be necessarily exact.
What is cardinality of a column?
Cardinality refers to the uniqueness of data values contained in a particular column (attribute) of a database table. The lower the cardinality, the more duplicated elements in a column. There are 3 types of cardinality: high-cardinality, normal-cardinality, and low-cardinality.
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.
When to use an index that will have a low cardinality?
When the index covers all fields used in the query: MySQL will use INDEX FOR GROUP-BY, and if you have few colors, this query will be instant even with millions of records. This is an example of a scenario when the index on a low cardinality field is more efficient than that on a high cardinality field.
Why does an index on a column have high selectivity?
Thus, an index on that column ends up having high selectivity as the number of “deleted” records grows since each record will have a unique (not strictly speaking) value. The query would have “… AND dateInactivated is NULL …”
When does it make sense to put an index on a column?
When half of the records’ values will be 1 and the other half 0, no point of putting an index on that column. The query optimizer is likely not to make use of it. Typically, however, you have a small set of “active” records and an increasingly larger set of “inactive”.