8000 Cache by-reference missing values in a long lived context · postgres/postgres@2d13dab · GitHub
[go: up one dir, main page]
Skip to content

Commit 2d13dab

Browse files
adunstanadunstan
committed
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
1 parent c176d97 commit 2d13dab

File tree

2 files changed

+91
-1
lines changed

2 files changed

+91
-1
lines changed

‎src/backend/access/common/heaptuple.c

Lines changed: 90 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -61,7 +61,11 @@
6161
#include "access/tupdesc_details.h"
6262
#include "access/tuptoaster.h"
6363
#include "executor/tuptable.h"
64+
#include "utils/datum.h"
6465
#include "utils/expandeddatum.h"
66+
#include "utils/hashutils.h"
67+
#include "utils/hsearch.h"
68+
#include "utils/memutils.h"
6569

6670

6771
/* Does att's datatype allow packing into the 1-byte-header varlena format? */
@@ -71,6 +75,58 @@
7175
#define VARLENA_ATT_IS_PACKABLE(att) \
7276
((att)->attstorage != 'p')
7377

78+
/*
79+
* Setup for cacheing pass-by-ref missing attributes in a way that survives
80+
* tupleDesc destruction.
81+
*/
82+
83+
typedef struct
84+
{
85+
int len;
86+
Datum value;
87+
} missing_cache_key;
88+
89+
static HTAB *missing_cache = NULL;
90+
91+
static uint32
92+
missing_hash(const void *key, Size keysize)
93+
{
94+
const missing_cache_key *entry = (missing_cache_key *) key;
95+
96+
return DatumGetUInt32(
97+
hash_any((const unsigned char *) entry->value, entry->len));
98+
}
99+
100+
static int
101+
missing_match(const void *key1, const void *key2, Size keysize)
102+
{
103+
const missing_cache_key *entry1 = (missing_cache_key *) key1;
104+
const missing_cache_key *entry2 = (missing_cache_key *) key2;
105+
106+
if (entry1->len != entry2->len)
107+
return entry1->len > entry2->len ? 1 : -1;
108+
109+
return memcmp(DatumGetPointer(entry1->value),
110+
DatumGetPointer(entry2->value),
111+
entry1->len);
112+
}
113+
114+
static void
115+
init_missing_cache()
116+
{
117+
HASHCTL hash_ctl;
118+
119+
hash_ctl.keysize = sizeof(missing_cache_key);
120+
hash_ctl.entrysize = sizeof(missing_cache_key);
121+
hash_ctl.hcxt = TopMemoryContext;
122+
hash_ctl.hash = missing_hash;
123+
hash_ctl.match = missing_match;
124+
missing_cache =
125+
hash_create("Missing Values Cache",
126+
32,
127+
&hash_ctl,
128+
HASH_ELEM | HASH_CONTEXT | HASH_FUNCTION | HASH_COMPARE);
129+
}
74130

75131
/* ----------------------------------------------------------------
76132
* misc support routines
@@ -102,8 +158,41 @@ getmissingattr(TupleDesc tupleDesc,
102158

103159
if (attrmiss->am_present)
104160
{
161+
missing_cache_key key;
162+
missing_cache_key *entry;
163+
bool found;
164+
MemoryContext oldctx;
165+
105166
*isnull = false;
106-
return attrmiss->am_value;
167+
168+
/* no need to cache by-value attributes */
169+
if (att->attbyval)
170+
return attrmiss->am_value;
171+
172+
/* set up cache if required */
173+
if (missing_cache == NULL)
174+
init_missing_cache();
175+
176+
/* check if there's a cache entry */
177+
Assert(att->attlen > 0 || att->attlen == -1);
178+
if (att->attlen > 0)
179+
key.len = att->attlen;
180+
else
181+
key.len = VARSIZE_ANY(attrmiss->am_value);
182+
key.value = attrmiss->am_value;
183+
184+
entry = hash_search(missing_cache, &key, HASH_ENTER, &found);
185+
186+
if (!found)
187+
{
188+
/* cache miss, so we need a non-transient copy of the datum */
189+
oldctx = MemoryContextSwitchTo(TopMemoryContext);
190+
entry->value =
191+
datumCopy(attrmiss->am_value, false, att->attlen);
192+
MemoryContextSwitchTo(oldctx);
193+
}
194+
195+
return entry->value;
107196
}
108197
}
109198

‎src/tools/pgindent/typedefs.list

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2910,6 +2910,7 @@ mblen_converter
29102910
mbverifier
29112911
md5_ctxt
29122912
metastring
2913+
missing_cache_key
29132914
mix_data_t
29142915
mixedStruct
29152916
mode_t

0 commit comments

Comments
 (0)
0