What happens if equality predicates are not at the start of a compound index?

When equality predicates are not positioned at the start of a compound index, it can lead to less efficient query performance and potentially slower execution times for those queries. Compound indexes in MongoDB work best when the most selective fields are placed first. This is because MongoDB can utilize the index to quickly narrow down the result set based on the specified equality conditions.

When equality predicates are at the beginning of the index, MongoDB can efficiently filter the documents that match those specific criteria before applying any additional sort or filter operations based on the other fields in the index. If equality predicates are located further along in the index, MongoDB may not effectively leverage the index for quick lookups, as it will first need to scan through a larger number of entries before reaching the equality conditions that specifically narrow down the result set.

In essence, the placement of equality predicates in compound indexes is crucial for optimization, as the order directly affects query performance. Having them at the start allows MongoDB to use the index efficiently, making queries faster; otherwise, it can result in less optimal query plans and longer runtimes.