composite index vs single column index

When deciding between using a composite index or single column indexes, it's essential to understand the differences and when one might be more beneficial than the other. Here's a comparison of the two:

Single Column Index

A single column index is an index on only one column of a table. It's most effective when queries filter or sort on that single column.

Pros:

• **Simplicity**: They are straightforward to implement and understand.

• **Versatility**: Single column indexes can be used by queries that filter or sort on the indexed column, regardless of other columns involved in the query.

• **Maintenance**: Less complex to maintain since each index is on a single column.

Cons:

• **Limited Scope**: If a query involves multiple columns for filtering or sorting, a single column index may not be as efficient as a composite index.

• **Index Overhead**: Having too many single column indexes can increase the overhead on write operations, as each index needs to be updated separately.

Composite Index

A composite index is an index on two or more columns of a table. It's beneficial when queries frequently filter or join on multiple columns simultaneously.

Pros:

• **Efficiency**: Can greatly improve query performance when filtering or joining on the indexed columns, as the database can quickly locate the desired data using the composite index.

• **Order-Sensitive**: Can efficiently support queries that specify conditions on the first column of the index, the first two columns, and so on, depending on the order of columns in the index.

Cons:

• **Complexity**: More complex to design and understand, as you need to consider the order of columns in the index based on your query patterns.

• **Less Versatile**: If a query does not use the leading column of the index (the first column specified in the index definition), the composite index may not be used.

• **Maintenance**: Can be more complex to maintain, as inserting, updating, or deleting rows requires updating the composite index, which can be more resource-intensive than updating a single column index.

Choosing Between Them

• **Query Patterns**: If your queries often involve filtering or sorting on multiple columns together, a composite index is likely more beneficial. If your queries typically involve only individual columns, single column indexes might be sufficient.

• **Column Selectivity**: High selectivity (columns with many unique values) might benefit more from single column indexes, while composite indexes are beneficial for columns with low selectivity when combined.

• **Index Column Order**: In composite indexes, the order of columns matters. The index will be most effective if the query conditions use the columns in the same order as they appear in the index.

In summary, the choice between composite and single column indexes depends on your specific query needs and data characteristics. Analyzing your most common and critical query patterns will guide you in choosing the appropriate indexing strategy.