Under what conditions should we use indexes?

Under what conditions should we use indexes?

When should indexes be created –

• A column contains a wide range of values.
• A column does not contain a large number of null values.
• One or more columns are frequently used together in a where clause or a join condition.

How do you create an index on a joined table?

A star-join index is one in which a single table at the center of the star is joined to multiple tables in a one-to-many relationship. To define a star-join index, you must define single-column key and primary keys, and then use the key join syntax in the CREATE JOIN INDEX statement.

Do indexes help with joins?

Indexes can help improve the performance of a nested-loop join in several ways. The biggest benefit often comes when you have a clustered index on the joining column in one of the tables. The presence of a clustered index on a join column frequently determines which table SQL Server chooses as the inner table.

Which fields should you index in a table?

Primary key columns are typically great for indexing because they are unique and are often used to lookup rows.

When should you create an index on a table?

Index the Correct Tables and Columns

1. Create an index if you frequently want to retrieve less than about 15% of the rows in a large table.
2. Index columns that are used for joins to improve join performance.

How are indexes used in the join condition?

Without useful indexes, the optimizer’s main join strategy is hash join, where the join keys from the smaller table are used to build a hash table, then join keys from the larger table are used to probe into that hash table for matches. Hash join requires memory to hold the complete hash table.

Is it beneficial to index table table a?

In above join, is it beneficial to index table table_a (col1,col2) or table table_b (col1,col2) and does order of columns in index impact performance? If suppose there is an index on table_b (col1,col2). Does it mean we need to change the query as follows?

When to apply an index on a column?

For equi-join, index key order is not significant. Seeking on (col1, col2) is possible whether the index keys are (col1, col2) or (col2, col1). For your example, an index on (col1, col2) or (col2, col1) for either table would enable an indexed nested loops join strategy.

Is it possible to update an indexed column in Oracle?

No. Updating an indexed column will have an impact. The Oracle 11g performance manual states that: UPDATE statements that modify indexed columns and INSERT and DELETE statements that modify indexed tables take longer than if there were no index. Such SQL statements must modify data in indexes and data in tables.