Database Interview: What are statistics, under what circumstances they go out of date, how do you update them?
Statistics determine the selectivity of the indexes. If an indexed
column has unique values then the selectivity of that index is more,
as opposed to an index with non-unique values. Query optimizer uses
these indexes in determining whether to choose an index or not while
executing a query.
Some situations under which you should update statistics:
1) If there is significant change in the key values in the index
2) If a large amount of data in an indexed column has been added,
changed, or removed (that is, if the distribution of key values has
changed), or the table has been truncated using the TRUNCATE TABLE
statement and then repopulated
3) Database is upgraded from a previous version
Interview Questions: Indexes in SQL, What is Index? It’s purpose?
What is Index? It's purpose?
Indexes in databases are similar to indexes in books. In a database,
an index allows the database program to find data in a table without
scanning
the entire table. An index in a database is a list of values
in a table with the storage locations of rows in the table that
contain each value. Indexes can be created on either a single column
or a combination of columns in a table and are implemented in the form
of B-trees. An index contains an entry with one or more columns (the
search key) from each row in a table. A B-tree is sorted on the search
key, and can be searched efficiently on any leading subset of the
search key. For example, an index on columns A, B, C can be searched
efficiently on A, on A, B, and A, B, C.
