The "Skip Scan" You Already Had Before v18

PostgreSQL 18 introduces native "Skip Scan" for multicolumn B-tree indexes, a major optimizer enhancement. However, a common misconception is that pre-v18 versions purely resort to sequential scans when the leading column isn't filtered. In reality, the cost-based planner has long been capable of leveraging such indexes when efficient. How it works under the hood: In pre-v18 versions, if an index…