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

diff --git a/src/backend/access/common/heaptuple.c b/src/backend/access/common/heaptuple.c
index a48a6cd757f4442ef291ac013db6a623488a79ea..ada089a3362e28691b9784e2e74d4ea6c6cd5b20 100644 (file)
--- a/src/backend/access/common/heaptuple.c
+++ b/src/backend/access/common/heaptuple.c
@@ -61,7 +61,11 @@
 #include "access/tupdesc_details.h"
 #include "access/tuptoaster.h"
 #include "executor/tuptable.h"
+#include "utils/datum.h"
 #include "utils/expandeddatum.h"
+#include "utils/hashutils.h"
+#include "utils/hsearch.h"
+#include "utils/memutils.h"
 
 
 /* Does att's datatype allow packing into the 1-byte-header varlena format? */
@@ -71,6 +75,58 @@
 #define VARLENA_ATT_IS_PACKABLE(att) \
        ((att)->attstorage != 'p')
 
+/*
+ * 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 DatumGetUInt32(
+               hash_any((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 +158,41 @@ 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;
                }
        }
 

diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index daebb7738706d63e13182597025ace591e85b041..0d08255ed7f9b59c18fb66a4befa1a57d54afae1 100644 (file)
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -3011,6 +3011,7 @@ mblen_converter
 mbverifier
 md5_ctxt
 metastring
+missing_cache_key
 mix_data_t
 mixedStruct
 mode_t