Navigating PostgreSQL - Index, Index Only and Bitmap Index Scans Explained

PostgreSQL uses various access methods to retrieve data, including index scans, index only scans, and bitmap scans. An index scan traverses the index structure to find matching entries and then retrieves the corresponding row data from the table. Index scans are efficient when the query is highly selective, the table is large, and the index covers the columns used in the WHERE clause. An index only scan is a more specialized version of the index scan that retrieves all required data directly from the index, eliminating the need to visit the table. Index only scans are particularly efficient when the query only needs columns that are included in the index and the table is large. A bitmap index scan is a sophisticated access method that combines the selectivity of indexes with the efficiency of sequential disk access patterns. Bitmap scans are used when the query is expected to return a moderate number of rows, the rows are scattered throughout the table, and multiple indexes can be combined. The Explain Analyze command provides a detailed breakdown of how PostgreSQL executes a query, including actual execution time and resource usage. Query plans in PostgreSQL follow a hierarchical structure, with each indented arrow representing a node in the execution tree. Understanding query plans is essential to optimizing database performance and identifying bottlenecks in query execution.