Ignorance in effectively implementing and managing MySQL indexing can have a significant and equally detrimental impact on both performance and scalability. Here's how improper indexing can affect these aspects:

Impact on Performance

1. Slow Query Execution: Without proper indexes, MySQL has to perform full table scans, which are inefficient and slow, especially with large tables. Indexes are crucial for speeding up data retrieval.
2. Increased I/O Operations: Lack of indexes results in increased disk I/O as more data needs to be read from disk. This can be particularly problematic for read-heavy applications.
3. Locking and Concurrency Issues: Full table scans due to missing indexes can lead to longer locking times on rows and tables, reducing concurrency and increasing the chance of lock contention.
4. Inefficient Joins: Without proper indexing, join operations can become highly inefficient, leading to performance issues in queries involving multiple tables.
5. Overhead in Sorting and Grouping: Without indexes, operations like sorting (ORDER BY) and grouping (GROUP BY) can become resource-intensive, as they may require creating temporary tables and performing full table scans.

Impact on Scalability

1. Poor Read Scalability: As the data volume grows, the lack of indexes exponentially increases the time taken for full table scans, making the system less scalable.
2. Write Scalability Issues: While indexes improve read operations, they can introduce overhead for write operations (INSERT, UPDATE, DELETE). Over-indexing, or having too many unnecessary indexes, can thus adversely affect write scalability.
3. Replication Delays: In a replicated setup, lack of efficient indexing can lead to delays in replication, as each query might take longer to execute on the replicas, impacting overall scalability.
4. Resource Utilization: Without indexes, database servers may require more CPU and I/O resources to perform the same tasks. This increased resource demand can limit the ability to scale, as adding more data or queries disproportionately increases resource needs.
5. Inability to Optimize for Specific Workloads: Different applications have different access patterns. Ignorance in indexing means missing out on tailoring the database performance to the specific needs of an application, thereby impacting its scalability.

Best Practices

• Index Important Columns: Columns used in WHERE, JOIN, ORDER BY, and GROUP BY clauses should typically be indexed.
• Avoid Over-indexing: While indexes are beneficial, too many indexes can degrade write performance. Balance is key.
• Regular Index Review and Optimization: Periodically review the query performance and index usage. Remove unused or redundant indexes.
• Use EXPLAIN: Regularly use the EXPLAIN command to analyze query execution plans and understand how indexes are being used.
• Monitor and Adjust: Continuously monitor database performance and adjust indexing strategies based on evolving data access patterns.

Conclusion

A well-thought-out indexing strategy is crucial for both performance and scalability in MySQL. Ignorance in this area can lead to severe inefficiencies, particularly as data volume grows. Balancing indexing for optimizing both read and write operations is essential for maintaining a high-performing and scalable database system.