# PostgreSQL memtable extension implements cache invalidation for memory management

> **Key takeaway:** The memtable extension uses PostgreSQL relcache invalidation messages to prevent memory leaks caused by stale arena mappings after table drops or truncations.

**Published:** 2026-11-10T20:21:39+00:00 
**Source:** Planet PostgreSQL 
**Category:** databases-storage 
**Canonical URL:** https://fosswire.org/news/postgresql-memtable-extension-implements-cache-invalidation-.html 

## Summary
A technical deep dive by Mats Kindahl details the implementation of cache invalidation within the memtable extension for PostgreSQL. The extension utilizes a custom table access method that maps table arenas to backend memory, which can lead to significant memory retention if not properly managed.

Without invalidation, a backend that has read a table will continue to hold a mapping to that table's arena even after another session has dropped or truncated it. This implementation leverages PostgreSQL's internal relcache invalidation mechanism to ensure that stale mappings are detached, allowing the system to reclaim memory from dropped relations without waiting for backend disconnections.

## Technical implementation of relcache invalidation
- **Memory Leak Prevention:** The extension prevents DSM (Dynamic Shared Memory) segments from being pinned indefinitely by ensuring that mappings are detached when a relation changes.
- **Relcache Callback Integration:** The implementation uses `CacheRegisterRelcacheCallback` to register a callback function that triggers whenever `pg_class` changes via `DROP` or `TRUNCATE` operations.
- **Cache Inspection Tools:** A new set-returning function `memtable_cache_entries` and a corresponding view `memtable_cache` allow administrators to inspect current backend cache entries, including relation IDs and mapping status.
- **Invalidation Logic:** The `memtable_relcache_callback` handles both specific relation invalidations and global invalidations (where `relid` is `InvalidOid`) by iterating through the local state cache and removing entries via `memtable_forget_entry`.

## Why it matters
For database engineers and system administrators, this ensures that custom PostgreSQL access methods can manage large memory arenas safely. It prevents 'ghost' memory usage in connection pools where long-lived sessions might otherwise prevent the reclamation of memory from deleted tables.