What is covering index?

A covering index in SQL is an index that includes all the columns required by a query, meaning the database system can fulfill the query entirely from the index itself without needing to access the actual table data. This significantly improves query performance by reducing disk I/O.

What is a Covering Index?

In relational databases, an index typically stores a subset of columns from a table and a pointer to the full row data in the base table. When a query needs columns that are not part of the index, the database must perform an additional 'bookmark lookup' or 'table fetch' operation to retrieve the remaining data from the table. A covering index, also known as a 'non-clustered index with included columns' (in SQL Server) or 'index-only scan' (in PostgreSQL), is designed to prevent this extra lookup.

By including all the columns a specific query might select, filter, or sort on directly within the index structure, the database engine can satisfy the query solely by scanning the index. This avoids the potentially costly operation of accessing the main table data, leading to faster query execution, especially for queries that are frequently run.

How it Works

When a query uses a covering index, the database optimizer determines that all necessary data for the query (columns in the SELECT, WHERE, ORDER BY, GROUP BY clauses) is available within the index. It then performs an 'index-only scan.' This means it reads only the index pages, which are typically smaller and more compact than table data pages, reducing the amount of data read from disk and processed in memory.

Example

Consider a table named 'Orders' with columns OrderID, CustomerID, OrderDate, TotalAmount, and OrderStatus. If you frequently query for the TotalAmount and OrderDate for a specific CustomerID, you can create a covering index like this:

CREATE INDEX IX_Orders_CustomerID_Covering ON Orders (CustomerID) INCLUDE (TotalAmount, OrderDate);

Now, for a query like the one below, the database can use IX_Orders_CustomerID_Covering to retrieve all the necessary data without touching the Orders table itself:

SELECT CustomerID, TotalAmount, OrderDate
FROM Orders
WHERE CustomerID = 12345
ORDER BY OrderDate DESC;

Benefits

• Improved Query Performance: Significantly reduces query execution time by eliminating table lookups.
• Reduced Disk I/O: Fewer disk reads are required since all data comes from the more compact index structure.
• Lower CPU Usage: Less data to process and fewer operations mean less CPU strain.
• Enhanced Concurrency: Reduced contention on table data pages, as queries can complete using only index pages.

Considerations and Trade-offs

• Increased Storage Space: Covering indexes are larger than standard indexes because they store more column data, consuming more disk space.
• Slower Write Operations: INSERT, UPDATE, and DELETE operations become slower because the database must maintain the larger index structure in addition to the base table and other indexes.
• Management Complexity: Deciding which columns to include requires careful analysis of query patterns. Over-indexing or creating too many covering indexes can lead to performance degradation rather than improvement.
• Not Always Applicable: Covering indexes are most effective for specific, frequently run queries that select a limited number of columns. They are generally not suitable for queries that select all columns (SELECT *) or a very large number of columns.