What is the optimal strategy if there is an existing index in db that has more fields than what I need for my query? Should I use it or create a new index?

If there already exists an index in the database that includes more fields than you’re planning to use in your query, you have a few considerations to weigh:

1. Query Performance: Evaluate whether the existing index adequately supports the performance requirements of your query. If the existing index includes all the fields necessary for your query and performs well, you may not need to create another index.
2. Index Usage: Consider whether the existing index is being used efficiently for other queries in the system. Adding unnecessary indexes can lead to increased storage and maintenance overhead.
3. Index Size: Keep in mind that adding more fields to an index increases its size, which can impact insert, update, and delete operations on the table. If the existing index is already large, creating another index with fewer fields might be preferable to avoid unnecessary overhead.
4. Index Maintenance: Consider the maintenance overhead associated with additional indexes. Each index requires storage space and maintenance during database operations such as inserts, updates, and deletes.
5. Query Optimization: If the existing index is not optimal for your query or if you frequently run queries that could benefit from a more specialized index, creating a new index tailored to your specific query might improve performance.

In summary, the best approach depends on various factors such as query performance, index usage, size, and maintenance overhead. Evaluate these factors carefully to determine whether to use the existing index or create a new one for your query.

Will it be more efficient to query with an existing index that has extra fields than I need, rather than querying without any index?

Using an existing index that includes more fields than you are planning to use in your query can still be beneficial compared to not using any index at all, but the effectiveness depends on several factors:

1. Index Structure: If the existing index covers the fields used in your query as well as additional fields, the database engine might still utilize it to speed up your query. However, if the extra fields significantly increase the size of the index, there might be some overhead.
2. Query Optimization: The database engine’s query optimizer will evaluate the available indexes and choose the one it deems most efficient for executing the query. If the existing index can satisfy the query’s requirements and is deemed more efficient than a table scan, it will likely be utilized.
3. Index Usage Patterns: If the existing index is heavily utilized by other queries in the system, adding your query to utilize it might lead to contention for the index resources. In such cases, it might be better to create a separate index tailored specifically to your query.
4. Data Distribution: Consider the distribution of data in the indexed fields. If the additional fields in the existing index result in high cardinality and selective filtering, it might still provide good performance for your query.

In most cases, using an existing index, even if it includes more fields than needed, will likely be faster than not using any index at all. However, for optimal performance, you may want to periodically review and optimize indexes based on the actual query workload and data access patterns.