Learn MongoDB Indexing: Boost Your Query Speed by 10x

MongoDB indexes are used to improve query performance, solving the inefficiency of "full table scan" (with a time complexity of O(n)) when no index is present. With an index, the complexity is reduced to O(log n), similar to using a library catalog to locate books. An index is a special data structure (based on B-tree/B+ tree) that stores mappings between field values and document positions. Basic types include single-field indexes (the most common, e.g., `db.users.createIndex({age:1})`); compound indexes (multi-field, e.g., `{age:1, gender:1}`, which must follow the "leftmost prefix principle"). There are also advanced types such as multikey, geospatial, and text indexes. Indexes are created using `createIndex()`, and verification is done using `explain()` to view the execution plan. It is recommended to create indexes on frequently queried, sorted, or compound query fields. Indexes are not suitable for small datasets, extremely frequent writes, low-cardinality, or highly repeated fields. Avoid over-indexing and duplicate indexes. Use `explain` to verify effectiveness and prevent index failure due to field type mismatches.