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 f89769f37937448858ae44d5982536b29c5fb38b..09b9edda45609f4e74a08d3159a4ad24dea8e1dc 100644 (file)
--- a/src/backend/access/common/heaptuple.c
+++ b/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;
        }
    }
 

diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 50fdb17fa3f2313004d9c990a15383a270a402b6..5a0ff42ece45c4eb78719f38c43ee35480281b42 100644 (file)
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -3104,6 +3104,7 @@ mblen_converter
 mbverifier
 md5_ctxt
 metastring
+missing_cache_key
 mix_data_t
 mixedStruct
 mode_t