Memoize

memory

{"description":"The Memoize cache evicts least-recently-used entries instead of spilling cached tuples to disk. If a result cannot fit, caching can enter bypass mode for that scan.","evidence":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeMemoize.c","path":"src/backend/executor/nodeMemoize.c","sha256":"1dec8adecc70763c97957ed94510c4734a399223fd2f81f5534a30351c06c5d6","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"mechanism":"eviction","source_notes":["The method of cache we use is a hash table. When the cache fills, we never spill tuples to disk, instead, we choose to evict the least recently used cache entry from the cache. We remember the least recently used entry by always pushing new entries and entries we look for onto the tail of a doubly linked list. This means that older items always bubble to the top of this LRU list.","It's possible when we're filling the cache for a given set of parameters that we're unable to free enough memory to store any more tuples. If this happens then we'll have already evicted all other cache entries. When caching another tuple would cause us to exceed our memory budget, we must free the entry that we're currently populating and move the state machine into MEMO_CACHE_BYPASS_MODE. This means that we'll not attempt to cache any further tuples for this particular scan. We don't have the memory for it. The state machine will be reset again on the next rescan. If the memory requirements to cache the next parameter's tuples are less demanding, then that may allow us to start putting useful entries back into the cache again.","cache_lookup Perform a lookup to see if we've already cached tuples based on the scan's current parameters. If we find an existing entry we move it to the end of the LRU list, set *found to true then return it. If we don't find an entry then we create a new one and add it to the end of the LRU list. We also update cache memory accounting and remove older entries if we go over the memory budget. If we managed to free enough memory we return the new entry, else we return NULL.","If we've gone over our memory budget, then we'll free up some space in the cache."]}

source inventory

{"executor":"src/backend/executor/execProcnode.c","explain":"src/backend/commands/explain.c","implementation":"src/backend/executor/nodeMemoize.c"}