backward compatibility

PostgreSQL 18 addresses common performance challenges for users, including managing query performance across composite indexes, diagnosing memory spills in materialized Common Table Expressions (CTEs), and upgrading major versions without plan regressions. Key enhancements include skip scan optimization for multicolumn indexes, improved EXPLAIN functionality, and optimizer statistics that persist through major version upgrades. Skip scan optimization allows PostgreSQL to efficiently utilize multicolumn B-tree indexes even when leading columns are not specified in the WHERE clause, significantly improving query performance. The EXPLAIN command has been enhanced to include buffer statistics by default, providing deeper insights into query execution and resource usage. PostgreSQL 18 also introduces visibility into the storage of materialized nodes in query plans, indicating whether intermediate results were stored in memory or spilled to disk. A new metric, Index Searches, has been added to EXPLAIN ANALYZE output, indicating how many times the database traversed the index tree during query execution. Additionally, Self-Join Elimination (SJE) automatically detects and removes unnecessary inner joins of a table to itself, optimizing query performance. The autovacuum mechanism has been improved with the introduction of autovacuum_vacuum_max_threshold, which caps the number of dead tuples that can accumulate before autovacuum triggers a VACUUM, addressing issues with large tables. The vacuum_truncate parameter provides a server-wide control point to disable VACUUM’s file truncation behavior, reducing locking issues on busy systems. PostgreSQL 18 also separates the allocation of autovacuum worker slots from their usage, allowing for dynamic adjustments to autovacuum_max_workers without requiring a server restart. Finally, new columns in pg_stat_all_tables track cumulative time spent on maintenance operations, providing better insights into maintenance overhead for each table.