index prefetching #159
Description
--- As of 2026-02-09T22:44:11.000Z ---
Peter Geoghegan posted v10 of the index prefetching patch with two major improvements. First, the read stream callback (heapam_getnext_stream) now yields at key intervals to keep scans responsive, particularly beneficial for "ORDER BY ... LIMIT N" queries and merge joins. This involves complex trade-offs between responsiveness and maintaining adequate prefetch distance, with heuristics derived from adversarial benchmarking. Second, a new patch addresses the selfuncs.c problem where VISITED_PAGES_LIMIT is incompatible with the new table_index_getnext_slot interface. The solution adds IndexScanDescData.xs_read_extremal_only field for get_actual_variable_range, making nbtree stop after scanning one leaf page. This approach should solve existing VISITED_PAGES_LIMIT ineffectiveness issues, particularly when many index tuples have LP_DEAD bits set. Geoghegan plans to start a separate thread to discuss these issues further.
Peter Geoghegan发布了索引预取补丁的v10版本，包含两项主要改进。首先，读取流回调函数（heapam_getnext_stream）现在会在关键时刻让出控制权以保持扫描的响应性，这对"ORDER BY ... LIMIT N"查询和合并连接特别有益。这涉及响应性与维持足够预取距离之间的复杂权衡，使用了通过对抗性基准测试得出的启发式算法。其次，新补丁解决了selfuncs.c问题，即VISITED_PAGES_LIMIT与新的table_index_getnext_slot接口不兼容。解决方案为get_actual_variable_range添加了IndexScanDescData.xs_read_extremal_only字段，使nbtree在扫描一个叶页后停止。这种方法应该能解决现有的VISITED_PAGES_LIMIT无效性问题，特别是当许多索引元组设置了LP_DEAD位时。Geoghegan计划启动单独的讨论线程来进一步讨论这些问题。
participants - 参与者: