Cache by-reference missing values in a long lived context

Attribute missing values might be needed past the lifetime of the tuple
descriptors from which they are extracted. To avoid possibly using
pointers for by-reference values which might thus be left dangling, we
cache a datumCopy'd version of the datum in the TopMemoryContext. Since
we first search for the value this only needs to be done once per
session for any such value.

Original complaint from Tom Lane, idea for mitigation by Andrew Dunstan,
tweaked by Tom Lane.

Backpatch to version 11 where missing values were introduced.

Discussion: https://postgr.es/m/1306569.1687978174@sss.pgh.pa.us

src/backend/access/common/heaptuple.c

@@ -60,8 +60,12 @@
 #include "access/heaptoast.h"
 #include "access/sysattr.h"
 #include "access/tupdesc_details.h"
+#include "common/hashfn.h"
 #include "executor/tuptable.h"
+#include "utils/datum.h"
 #include "utils/expandeddatum.h"
+#include "utils/hsearch.h"
+#include "utils/memutils.h"
 
 
 /* Does att's datatype allow packing into the 1-byte-header varlena format? */
@@ -71,6 +75,57 @@
 #define VARLENA_ATT_IS_PACKABLE(att) \
 	((att)->attstorage != TYPSTORAGE_PLAIN)
 
+/*
+ * Setup for cacheing pass-by-ref missing attributes in a way that survives
+ * tupleDesc destruction.
+ */
+
+typedef struct
+{
+	int			len;
+	Datum		value;
+} missing_cache_key;
+
+static HTAB *missing_cache = NULL;
+
+static uint32
+missing_hash(const void *key, Size keysize)
+{
+	const missing_cache_key *entry = (missing_cache_key *) key;
+
+	return hash_bytes((const unsigned char *) entry->value, entry->len);
+}
+
+static int
+missing_match(const void *key1, const void *key2, Size keysize)
+{
+	const missing_cache_key *entry1 = (missing_cache_key *) key1;
+	const missing_cache_key *entry2 = (missing_cache_key *) key2;
+
+	if (entry1->len != entry2->len)
+		return entry1->len > entry2->len ? 1 : -1;
+
+	return memcmp(DatumGetPointer(entry1->value),
+				  DatumGetPointer(entry2->value),
+				  entry1->len);
+}
+
+static void
+init_missing_cache()
+{
+	HASHCTL		hash_ctl;
+
+	hash_ctl.keysize = sizeof(missing_cache_key);
+	hash_ctl.entrysize = sizeof(missing_cache_key);
+	hash_ctl.hcxt = TopMemoryContext;
+	hash_ctl.hash = missing_hash;
+	hash_ctl.match = missing_match;
+	missing_cache =
+		hash_create("Missing Values Cache",
+					32,
+					&hash_ctl,
+					HASH_ELEM | HASH_CONTEXT | HASH_FUNCTION | HASH_COMPARE);
+}
 
 /* ----------------------------------------------------------------
  *						misc support routines
@@ -102,8 +157,41 @@ getmissingattr(TupleDesc tupleDesc,
 
 		if (attrmiss->am_present)
 		{
+			missing_cache_key key;
+			missing_cache_key *entry;
+			bool		found;
+			MemoryContext oldctx;
+
 			*isnull = false;
-			return attrmiss->am_value;
+
+			/* no  need to cache by-value attributes */
+			if (att->attbyval)
+				return attrmiss->am_value;
+
+			/* set up cache if required */
+			if (missing_cache == NULL)
+				init_missing_cache();
+
+			/* check if there's a cache entry */
+			Assert(att->attlen > 0 || att->attlen == -1);
+			if (att->attlen > 0)
+				key.len = att->attlen;
+			else
+				key.len = VARSIZE_ANY(attrmiss->am_value);
+			key.value = attrmiss->am_value;
+
+			entry = hash_search(missing_cache, &key, HASH_ENTER, &found);
+
+			if (!found)
+			{
+				/* cache miss, so we need a non-transient copy of the datum */
+				oldctx = MemoryContextSwitchTo(TopMemoryContext);
+				entry->value =
+					datumCopy(attrmiss->am_value, false, att->attlen);
+				MemoryContextSwitchTo(oldctx);
+			}
+
+			return entry->value;
 		}
 	}
 

src/tools/pgindent/typedefs.list

@@ -3479,6 +3479,7 @@ mbstr_verifier
 memoize_hash
 memoize_iterator
 metastring
+missing_cache_key
 mix_data_t
 mixedStruct
 mode_t