An index makes the query fast.
The data structure that is used for indexes is a B-tree (balanced tree). That's because the tree depth is equal for all nodes (the distance between root and leaf nodes).
Why is a query not using an index?
The execution plan of a query can show that the database does not use the index, and uses sequential scan. Possible situations:
- majority of rows are getting fetched as part of the SQL query
- although the database reads more data, it might need to execute fewer read operations
- there is no index available for specified columns
NOTE: Using an index does not mean a query is executed in the best way!
It is possible to define an index on two or more columns - that kind of an index is called composite (or concatenated, combined) index.
Order matters - the most important thing is how to choose the column order so the index can be used as often as possible! We as developers should have a feeling for the data (business domain) and properly choose columns for an index.
CREATE INDEX idx_employee_department_1 ON employees(employee_id, department_id); -- different approach CREATE INDEX idx_employee_department_2 ON employees(department_id, employee_id);
What index would you create, depends on data usage from the domain perspective. If queries will mostly use the
department_id column in the
WHERE part of a query, then it's recommended to use the
idx_employee_department_2 index. Otherwise, if
employee_id will be used repeatedly, it would probably be more performant to use the
When both of the columns would be used on a similar frequency, use the column with more scattered data, on the most left position of an index (in our example it would be
When you have an index whose definition contains a function or an expression, you have a function-based index. Instead of copying the column data directly into the index, the database applies a function on the column and stores the result into the index.
SELECT * FROM users WHERE LOWER(email) = 'email@example.com' -- we store lower-cased email as an index value CREATE INDEX idx_users_email ON users(LOWER(email))
In some situations, where we query only a part of the table, it makes sense to index only a specific partition of the table.
For example, we don't want to index posts that are deleted (
deleted_at IS NOT NULL). So, we will create a partial index:
-- index not deleted posts by published_at CREATE INDEX idx_posts_undeleted ON posts(published_at) WHERE deleted_at IS NULL
The main benefit of partial indexes is the smaller size of the index.
The index size is reduced vertically and horizontally:
- reduced number of rows - only posts that are not deleted (
deleted_at IS NULL)
- reduced number of columns - you can skip
deleted_atcolumn from the index definition