The document discusses local secondary indexes in Apache Phoenix. Local indexes are stored in the same region as the base table data, providing faster index building and reads compared to global indexes. The write process involves preparing index updates along with data updates and writing them atomically to memstores and the write ahead log. Reads scan the local index and retrieve any missing columns from the base table. Local indexes improve write performance over global indexes due to reduced network utilization. The document provides performance results and tips on using local indexes.