Two single-column indexes vs one two-column index in MySQL?

Composite Index is Your Friend for Joint Queries

A two-column index is your go-to when dealing with queries that consistently include both columns in their condition. It shines the brightest when the two columns are used in conjunction in your WHERE, JOIN, or ORDER BY clause. It gives your database optimizer speed and efficiency for sorting and filtering.

Match the Index to the Pattern

Suppose you frequently use 'giver_id' and 'recipient_id' in your queries, a composite index can be very beneficial:

-- Looking for the ultimate giving and receiving couple
SELECT * FROM donations WHERE giver_id = 1 AND recipient_id = 2;
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But if most queries only involve 'giver_id' or 'recipient_id', single indexes suit just as well.

Indexing's Effect on CRUD Operations

Remember, every superpower comes with its kryptonite. Indexes speed up read operations, but they can slow down write operations like INSERT, UPDATE, and DELETE. Why? Because every time data changes, your index needs to update as well, resulting in overhead.

Check Your Storage Before You Index

An index, just like a new pair of shoes, requires storage. Take time to consider whether the performance gain of a two-column index outweighs the extra space it requires.

Cardinality for the Win

Before implementing a composite index, assess the cardinality of your columns. High-cardinality columns make a better candidate for indexing as they provide more specific, granular information for the query optimizer.

Dealing with Vintage MySQL

Say hello to the two-column covering index if you're dealing with older versions of MySQL. This type of index enhances query performance like a classic LP track.

Indexing: A Balancing Act

For every yin, there's a yang. Indexing provides query performance boosts, but it also increases maintenance costs. Balancing these factors is key to effective index usage.

The Lone Second Column in a Composite Index

A multi-column index may not serve well if your query pattern heavily involves only the second column. In such case, single indexes are the way to go.

Ensuring Uniqueness and Integrity with Index

A composite index can ensure data integrity by enforcing uniqueness over a two-columns combination. It's like a vigilant guard, ensuring no duplicates sneak in.

Tailoring Index to Your Data Size and Query Frequency

Remember to consider your data size and the frequency of your specific queries. The larger the data set and the more frequent complex queries are, the more you might lean toward composite indexes.

Let Performance Metrics Guide You

Performance metrics help in deciding between two-column and single-column indexes. Use tools like MySQL's EXPLAIN to unravel the mysteries of query execution and indexing strategies.

Automatic Indexing and Its Limitations

Keep in mind, automatic indexes on foreign keys might not meet performance requirements for complex queries. In this case, enter custom composite indexes.

Not All Columns Are Index-Worthy

Avoid indexing columns that:

• Contain many NULL values
• Have a high number of duplicate values
• Are rarely used in queries

You won't index just for the sake of it, right?

The Multi-Column Index as a Pseudo-Primary Key

In certain conditions, a multi-column index can act as a pseudo-primary key. This comes in handy in join tables where a combination of columns needs to be unique.