commit f716536100e86bb410341daf3f84cf23f4e3adc2
Author: Lukas Fittl <lukas@fittl.com>
Date:   Sat Jan 9 23:42:42 2021 -0800

    Add AllocSetDeleteFreeList to clean up aset.c's freelist
    
    This frees up the memory allocated to memory contexts that are kept
    for future allocations. This behaves similar to changing aset.c's
    MAX_FREE_CONTEXTS to 0, but only does the cleanup when called, and
    allows the freelist approach to be used during Postgres operations.

diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index dede30dd86..16306c3216 100644
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -1722,3 +1722,25 @@ AllocSetCheck(MemoryContext context)
 }
 
 #endif							/* MEMORY_CONTEXT_CHECKING */
+
+void
+AllocSetDeleteFreeList(MemoryContext context)
+{
+	AllocSet set = (AllocSet) context;
+	if (set->freeListIndex >= 0)
+	{
+		AllocSetFreeList *freelist = &context_freelists[set->freeListIndex];
+
+		while (freelist->first_free != NULL)
+		{
+			AllocSetContext *oldset = freelist->first_free;
+
+			freelist->first_free = (AllocSetContext *) oldset->header.nextchild;
+			freelist->num_free--;
+
+			/* All that remains is to free the header/initial block */
+			free(oldset);
+		}
+		Assert(freelist->num_free == 0);
+	}
+}
diff --git a/src/include/utils/memutils.h b/src/include/utils/memutils.h
index cd9596ff21..fac1271673 100644
--- a/src/include/utils/memutils.h
+++ b/src/include/utils/memutils.h
@@ -114,6 +114,7 @@ extern MemoryContext AllocSetContextCreateInternal(MemoryContext parent,
 												   Size minContextSize,
 												   Size initBlockSize,
 												   Size maxBlockSize);
+extern void AllocSetDeleteFreeList(MemoryContext context);
 
 /*
  * This wrapper macro exists to check for non-constant strings used as context