From f94edb06ab60545a0e9e90aedc39e3b81b2b9e3f Mon Sep 17 00:00:00 2001
From: David Rowley <drowley@postgresql.org>
Date: Wed, 24 Nov 2021 14:57:07 +1300
Subject: Flush Memoize cache when non-key parameters change

It's possible that a subplan below a Memoize node contains a parameter
from above the Memoize node.  If this parameter changes then cache entries
may become out-dated due to the new parameter value.

Previously Memoize was mistakenly not aware of this.  We fix this here by
flushing the cache whenever a parameter that's not part of the cache
key changes.

Bug: #17213
Reported by: Elvis Pranskevichus
Author: David Rowley
Discussion: https://postgr.es/m/17213-988ed34b225a2862@postgresql.org
Backpatch-through: 14, where Memoize was added
---
 src/backend/executor/nodeMemoize.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

(limited to 'src/backend/executor/nodeMemoize.c')

diff --git a/src/backend/executor/nodeMemoize.c b/src/backend/executor/nodeMemoize.c
index cff7ee50469..fde8ebd5068 100644
--- a/src/backend/executor/nodeMemoize.c
+++ b/src/backend/executor/nodeMemoize.c
@@ -367,6 +367,37 @@ remove_cache_entry(MemoizeState *mstate, MemoizeEntry *entry)
 	pfree(key);
 }
 
+/*
+ * cache_purge_all
+ *		Remove all items from the cache
+ */
+static void
+cache_purge_all(MemoizeState *mstate)
+{
+	uint64		evictions = mstate->hashtable->members;
+	PlanState *pstate = (PlanState *) mstate;
+
+	/*
+	 * Likely the most efficient way to remove all items is to just reset the
+	 * memory context for the cache and then rebuild a fresh hash table.  This
+	 * saves having to remove each item one by one and pfree each cached tuple
+	 */
+	MemoryContextReset(mstate->tableContext);
+
+	/* Make the hash table the same size as the original size */
+	build_hash_table(mstate, ((Memoize *) pstate->plan)->est_entries);
+
+	/* reset the LRU list */
+	dlist_init(&mstate->lru_list);
+	mstate->last_tuple = NULL;
+	mstate->entry = NULL;
+
+	mstate->mem_used = 0;
+
+	/* XXX should we add something new to track these purges? */
+	mstate->stats.cache_evictions += evictions; /* Update Stats */
+}
+
 /*
  * cache_reduce_memory
  *		Evict older and less recently used items from the cache in order to
@@ -979,6 +1010,7 @@ ExecInitMemoize(Memoize *node, EState *estate, int eflags)
 	 * getting the first tuple.  This allows us to mark it as so.
 	 */
 	mstate->singlerow = node->singlerow;
+	mstate->keyparamids = node->keyparamids;
 
 	/*
 	 * Record if the cache keys should be compared bit by bit, or logically
@@ -1082,6 +1114,12 @@ ExecReScanMemoize(MemoizeState *node)
 	if (outerPlan->chgParam == NULL)
 		ExecReScan(outerPlan);
 
+	/*
+	 * Purge the entire cache if a parameter changed that is not part of the
+	 * cache key.
+	 */
+	if (bms_nonempty_difference(outerPlan->chgParam, node->keyparamids))
+		cache_purge_all(node);
 }
 
 /*
-- 
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