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Fix failure to detoast fields in composite elements of structured types.
If we have an array of records stored on disk, the individual record fields cannot contain out-of-line TOAST pointers: the tuptoaster.c mechanisms are only prepared to deal with TOAST pointers appearing in top-level fields of a stored row. The same applies for ranges over composite types, nested composites, etc. However, the existing code only took care of expanding sub-field TOAST pointers for the case of nested composites, not for other structured types containing composites. For example, given a command such as UPDATE tab SET arraycol = ARRAY[(ROW(x,42)::mycompositetype] ... where x is a direct reference to a field of an on-disk tuple, if that field is long enough to be toasted out-of-line then the TOAST pointer would be inserted as-is into the array column. If the source record for x is later deleted, the array field value would become a dangling pointer, leading to errors along the line of "missing chunk number 0 for toast value ..." when the value is referenced. A reproducible test case for this was provided by Jan Pecek, but it seems likely that some of the "missing chunk number" reports we've heard in the past were caused by similar issues. Code-wise, the problem is that PG_DETOAST_DATUM() is not adequate to produce a self-contained Datum value if the Datum is of composite type. Seen in this light, the problem is not just confined to arrays and ranges, but could also affect some other places where detoasting is done in that way, for example form_index_tuple(). I tried teaching the array code to apply toast_flatten_tuple_attribute() along with PG_DETOAST_DATUM() when the array element type is composite, but this was messy and imposed extra cache lookup costs whether or not any TOAST pointers were present, indeed sometimes when the array element type isn't even composite (since sometimes it takes a typcache lookup to find that out). The idea of extending that approach to all the places that currently use PG_DETOAST_DATUM() wasn't attractive at all. This patch instead solves the problem by decreeing that composite Datum values must not contain any out-of-line TOAST pointers in the first place; that is, we expand out-of-line fields at the point of constructing a composite Datum, not at the point where we're about to insert it into a larger tuple. This rule is applied only to true composite Datums, not to tuples that are being passed around the system as tuples, so it's not as invasive as it might sound at first. With this approach, the amount of code that has to be touched for a full solution is greatly reduced, and added cache lookup costs are avoided except when there actually is a TOAST pointer that needs to be inlined. The main drawback of this approach is that we might sometimes dereference a TOAST pointer that will never actually be used by the query, imposing a rather large cost that wasn't there before. On the other side of the coin, if the field value is used multiple times then we'll come out ahead by avoiding repeat detoastings. Experimentation suggests that common SQL coding patterns are unaffected either way, though. Applications that are very negatively affected could be advised to modify their code to not fetch columns they won't be using. In future, we might consider reverting this solution in favor of detoasting only at the point where data is about to be stored to disk, using some method that can drill down into multiple levels of nested structured types. That will require defining new APIs for structured types, though, so it doesn't seem feasible as a back-patchable fix. Note that this patch changes HeapTupleGetDatum() from a macro to a function call; this means that any third-party code using that macro will not get protection against creating TOAST-pointer-containing Datums until it's recompiled. The same applies to any uses of PG_RETURN_HEAPTUPLEHEADER(). It seems likely that this is not a big problem in practice: most of the tuple-returning functions in core and contrib produce outputs that could not possibly be toasted anyway, and the same probably holds for third-party extensions. This bug has existed since TOAST was invented, so back-patch to all supported branches.
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‎src/backend/access/common/heaptuple.c

Lines changed: 43 additions & 37 deletions
Original file line numberDiff line numberDiff line change
@@ -617,6 +617,41 @@ heap_copytuple_with_tuple(HeapTuple src, HeapTuple dest)
617617
memcpy((char *) dest->t_data, (char *) src->t_data, src->t_len);
618618
}
619619

620+
/* ----------------
621+
* heap_copy_tuple_as_datum
622+
*
623+
* copy a tuple as a composite-type Datum
624+
* ----------------
625+
*/
626+
Datum
627+
heap_copy_tuple_as_datum(HeapTuple tuple, TupleDesc tupleDesc)
628+
{
629+
HeapTupleHeader td;
630+
631+
/*
632+
* If the tuple contains any external TOAST pointers, we have to inline
633+
* those fields to meet the conventions for composite-type Datums.
634+
*/
635+
if (HeapTupleHasExternal(tuple))
636+
return toast_flatten_tuple_to_datum(tuple->t_data,
637+
tuple->t_len,
638+
tupleDesc);
639+
640+
/*
641+
* Fast path for easy case: just make a palloc'd copy and insert the
642+
* correct composite-Datum header fields (since those may not be set if
643+
* the given tuple came from disk, rather than from heap_form_tuple).
644+
*/
645+
td = (HeapTupleHeader) palloc(tuple->t_len);
646+
memcpy((char *) td, (char *) tuple->t_data, tuple->t_len);
647+
648+
HeapTupleHeaderSetDatumLength(td, tuple->t_len);
649+
HeapTupleHeaderSetTypeId(td, tupleDesc->tdtypeid);
650+
HeapTupleHeaderSetTypMod(td, tupleDesc->tdtypmod);
651+
652+
return PointerGetDatum(td);
653+
}
654+
620655
/*
621656
* heap_form_tuple
622657
* construct a tuple from the given values[] and isnull[] arrays,
@@ -635,7 +670,6 @@ heap_form_tuple(TupleDesc tupleDescriptor,
635670
data_len;
636671
int hoff;
637672
bool hasnull = false;
638-
Form_pg_attribute *att = tupleDescriptor->attrs;
639673
int numberOfAttributes = tupleDescriptor->natts;
640674
int i;
641675

@@ -646,28 +680,14 @@ heap_form_tuple(TupleDesc tupleDescriptor,
646680
numberOfAttributes, MaxTupleAttributeNumber)));
647681

648682
/*
649-
* Check for nulls and embedded tuples; expand any toasted attributes in
650-
* embedded tuples. This preserves the invariant that toasting can only
651-
* go one level deep.
652-
*
653-
* We can skip calling toast_flatten_tuple_attribute() if the attribute
654-
* couldn't possibly be of composite type. All composite datums are
655-
* varlena and have alignment 'd'; furthermore they aren't arrays. Also,
656-
* if an attribute is already toasted, it must have been sent to disk
657-
* already and so cannot contain toasted attributes.
683+
* Check for nulls
658684
*/
659685
for (i = 0; i < numberOfAttributes; i++)
660686
{
661687
if (isnull[i])
662-
hasnull = true;
663-
else if (att[i]->attlen == -1 &&
664-
att[i]->attalign == 'd' &&
665-
att[i]->attndims == 0 &&
666-
!VARATT_IS_EXTENDED(DatumGetPointer(values[i])))
667688
{
668-
values[i] = toast_flatten_tuple_attribute(values[i],
669-
att[i]->atttypid,
670-
att[i]->atttypmod);
689+
hasnull = true;
690+
break;
671691
}
672692
}
673693

@@ -697,7 +717,8 @@ heap_form_tuple(TupleDesc tupleDescriptor,
697717

698718
/*
699719
* And fill in the information. Note we fill the Datum fields even though
700-
* this tuple may never become a Datum.
720+
* this tuple may never become a Datum. This lets HeapTupleHeaderGetDatum
721+
* identify the tuple type if needed.
701722
*/
702723
tuple->t_len = len;
703724
ItemPointerSetInvalid(&(tuple->t_self));
@@ -1389,7 +1410,6 @@ heap_form_minimal_tuple(TupleDesc tupleDescriptor,
13891410
data_len;
13901411
int hoff;
13911412
bool hasnull = false;
1392-
Form_pg_attribute *att = tupleDescriptor->attrs;
13931413
int numberOfAttributes = tupleDescriptor->natts;
13941414
int i;
13951415

@@ -1400,28 +1420,14 @@ heap_form_minimal_tuple(TupleDesc tupleDescriptor,
14001420
numberOfAttributes, MaxTupleAttributeNumber)));
14011421

14021422
/*
1403-
* Check for nulls and embedded tuples; expand any toasted attributes in
1404-
* embedded tuples. This preserves the invariant that toasting can only
1405-
* go one level deep.
1406-
*
1407-
* We can skip calling toast_flatten_tuple_attribute() if the attribute
1408-
* couldn't possibly be of composite type. All composite datums are
1409-
* varlena and have alignment 'd'; furthermore they aren't arrays. Also,
1410-
* if an attribute is already toasted, it must have been sent to disk
1411-
* already and so cannot contain toasted attributes.
1423+
* Check for nulls
14121424
*/
14131425
for (i = 0; i < numberOfAttributes; i++)
14141426
{
14151427
if (isnull[i])
1416-
hasnull = true;
1417-
else if (att[i]->attlen == -1 &&
1418-
att[i]->attalign == 'd' &&
1419-
att[i]->attndims == 0 &&
1420-
!VARATT_IS_EXTENDED(values[i]))
14211428
{
1422-
values[i] = toast_flatten_tuple_attribute(values[i],
1423-
att[i]->atttypid,
1424-
att[i]->atttypmod);
1429+
hasnull = true;
1430+
break;
14251431
}
14261432
}
14271433

‎src/backend/access/common/indextuple.c

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -158,6 +158,11 @@ index_form_tuple(TupleDesc tupleDescriptor,
158158
if (tupmask & HEAP_HASVARWIDTH)
159159
infomask |= INDEX_VAR_MASK;
160160

161+
/* Also assert we got rid of external attributes */
162+
#ifdef TOAST_INDEX_HACK
163+
Assert((tupmask & HEAP_HASEXTERNAL) == 0);
164+
#endif
165+
161166
/*
162167
* Here we make sure that the size will fit in the field reserved for it
163168
* in t_info.

‎src/backend/access/heap/tuptoaster.c

Lines changed: 44 additions & 48 deletions
Original file line numberDiff line numberDiff line change
@@ -946,6 +946,9 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
946946
*
947947
* "Flatten" a tuple to contain no out-of-line toasted fields.
948948
* (This does not eliminate compressed or short-header datums.)
949+
*
950+
* Note: we expect the caller already checked HeapTupleHasExternal(tup),
951+
* so there is no need for a short-circuit path.
949952
* ----------
950953
*/
951954
HeapTuple
@@ -1023,59 +1026,61 @@ toast_flatten_tuple(HeapTuple tup, TupleDesc tupleDesc)
10231026

10241027

10251028
/* ----------
1026-
* toast_flatten_tuple_attribute -
1029+
* toast_flatten_tuple_to_datum -
1030+
*
1031+
* "Flatten" a tuple containing out-of-line toasted fields into a Datum.
1032+
* The result is always palloc'd in the current memory context.
1033+
*
1034+
* We have a general rule that Datums of container types (rows, arrays,
1035+
* ranges, etc) must not contain any external TOAST pointers. Without
1036+
* this rule, we'd have to look inside each Datum when preparing a tuple
1037+
* for storage, which would be expensive and would fail to extend cleanly
1038+
* to new sorts of container types.
1039+
*
1040+
* However, we don't want to say that tuples represented as HeapTuples
1041+
* can't contain toasted fields, so instead this routine should be called
1042+
* when such a HeapTuple is being converted into a Datum.
10271043
*
1028-
* If a Datum is of composite type, "flatten" it to contain no toasted fields.
1029-
* This must be invoked on any potentially-composite field that is to be
1030-
* inserted into a tuple. Doing this preserves the invariant that toasting
1031-
* goes only one level deep in a tuple.
1044+
* While we're at it, we decompress any compressed fields too. This is not
1045+
* necessary for correctness, but reflects an expectation that compression
1046+
* will be more effective if applied to the whole tuple not individual
1047+
* fields. We are not so concerned about that that we want to deconstruct
1048+
* and reconstruct tuples just to get rid of compressed fields, however.
1049+
* So callers typically won't call this unless they see that the tuple has
1050+
* at least one external field.
10321051
*
1033-
* Note that flattening does not mean expansion of short-header varlenas,
1034-
* so in one sense toasting is allowed within composite datums.
1052+
* On the other hand, in-line short-header varlena fields are left alone.
1053+
* If we "untoasted" them here, they'd just get changed back to short-header
1054+
* format anyway within heap_fill_tuple.
10351055
* ----------
10361056
*/
10371057
Datum
1038-
toast_flatten_tuple_attribute(Datum value,
1039-
Oid typeId, int32 typeMod)
1058+
toast_flatten_tuple_to_datum(HeapTupleHeader tup,
1059+
uint32 tup_len,
1060+
TupleDesc tupleDesc)
10401061
{
1041-
TupleDesc tupleDesc;
1042-
HeapTupleHeader olddata;
10431062
HeapTupleHeader new_data;
10441063
int32 new_header_len;
10451064
int32 new_data_len;
10461065
int32 new_tuple_len;
10471066
HeapTupleData tmptup;
1048-
Form_pg_attribute *att;
1049-
int numAttrs;
1067+
Form_pg_attribute *att = tupleDesc->attrs;
1068+
int numAttrs = tupleDesc->natts;
10501069
int i;
1051-
bool need_change = false;
10521070
bool has_nulls = false;
10531071
Datum toast_values[MaxTupleAttributeNumber];
10541072
bool toast_isnull[MaxTupleAttributeNumber];
10551073
bool toast_free[MaxTupleAttributeNumber];
10561074

1057-
/*
1058-
* See if it's a composite type, and get the tupdesc if so.
1059-
*/
1060-
tupleDesc = lookup_rowtype_tupdesc_noerror(typeId, typeMod, true);
1061-
if (tupleDesc == NULL)
1062-
return value; /* not a composite type */
1063-
1064-
att = tupleDesc->attrs;
1065-
numAttrs = tupleDesc->natts;
1066-
1067-
/*
1068-
* Break down the tuple into fields.
1069-
*/
1070-
olddata = DatumGetHeapTupleHeader(value);
1071-
Assert(typeId == HeapTupleHeaderGetTypeId(olddata));
1072-
Assert(typeMod == HeapTupleHeaderGetTypMod(olddata));
10731075
/* Build a temporary HeapTuple control structure */
1074-
tmptup.t_len = HeapTupleHeaderGetDatumLength(olddata);
1076+
tmptup.t_len = tup_len;
10751077
ItemPointerSetInvalid(&(tmptup.t_self));
10761078
tmptup.t_tableOid = InvalidOid;
1077-
tmptup.t_data = olddata;
1079+
tmptup.t_data = tup;
10781080

1081+
/*
1082+
* Break down the tuple into fields.
1083+
*/
10791084
Assert(numAttrs <= MaxTupleAttributeNumber);
10801085
heap_deform_tuple(&tmptup, tupleDesc, toast_values, toast_isnull);
10811086

@@ -1099,20 +1104,10 @@ toast_flatten_tuple_attribute(Datum value,
10991104
new_value = heap_tuple_untoast_attr(new_value);
11001105
toast_values[i] = PointerGetDatum(new_value);
11011106
toast_free[i] = true;
1102-
need_change = true;
11031107
}
11041108
}
11051109
}
11061110

1107-
/*
1108-
* If nothing to untoast, just return the original tuple.
1109-
*/
1110-
if (!need_change)
1111-
{
1112-
ReleaseTupleDesc(tupleDesc);
1113-
return value;
1114-
}
1115-
11161111
/*
11171112
* Calculate the new size of the tuple.
11181113
*
@@ -1121,7 +1116,7 @@ toast_flatten_tuple_attribute(Datum value,
11211116
new_header_len = offsetof(HeapTupleHeaderData, t_bits);
11221117
if (has_nulls)
11231118
new_header_len += BITMAPLEN(numAttrs);
1124-
if (olddata->t_infomask & HEAP_HASOID)
1119+
if (tup->t_infomask & HEAP_HASOID)
11251120
new_header_len += sizeof(Oid);
11261121
new_header_len = MAXALIGN(new_header_len);
11271122
new_data_len = heap_compute_data_size(tupleDesc,
@@ -1133,14 +1128,16 @@ toast_flatten_tuple_attribute(Datum value,
11331128
/*
11341129
* Copy the existing tuple header, but adjust natts and t_hoff.
11351130
*/
1136-
memcpy(new_data, olddata, offsetof(HeapTupleHeaderData, t_bits));
1131+
memcpy(new_data, tup, offsetof(HeapTupleHeaderData, t_bits));
11371132
HeapTupleHeaderSetNatts(new_data, numAttrs);
11381133
new_data->t_hoff = new_header_len;
1139-
if (olddata->t_infomask & HEAP_HASOID)
1140-
HeapTupleHeaderSetOid(new_data, HeapTupleHeaderGetOid(olddata));
1134+
if (tup->t_infomask & HEAP_HASOID)
1135+
HeapTupleHeaderSetOid(new_data, HeapTupleHeaderGetOid(tup));
11411136

1142-
/* Reset the datum length field, too */
1137+
/* Set the composite-Datum header fields correctly */
11431138
HeapTupleHeaderSetDatumLength(new_data, new_tuple_len);
1139+
HeapTupleHeaderSetTypeId(new_data, tupleDesc->tdtypeid);
1140+
HeapTupleHeaderSetTypMod(new_data, tupleDesc->tdtypmod);
11441141

11451142
/* Copy over the data, and fill the null bitmap if needed */
11461143
heap_fill_tuple(tupleDesc,
@@ -1157,7 +1154,6 @@ toast_flatten_tuple_attribute(Datum value,
11571154
for (i = 0; i < numAttrs; i++)
11581155
if (toast_free[i])
11591156
pfree(DatumGetPointer(toast_values[i]));
1160-
ReleaseTupleDesc(tupleDesc);
11611157

11621158
return PointerGetDatum(new_data);
11631159
}

‎src/backend/executor/execQual.c

Lines changed: 9 additions & 23 deletions
Original file line numberDiff line numberDiff line change
@@ -896,8 +896,6 @@ ExecEvalWholeRowFast(WholeRowVarExprState *wrvstate, ExprContext *econtext,
896896
{
897897
Var *variable = (Var *) wrvstate->xprstate.expr;
898898
TupleTableSlot *slot;
899-
HeapTuple tuple;
900-
TupleDesc tupleDesc;
901899
HeapTupleHeader dtuple;
902900

903901
if (isDone)
@@ -927,32 +925,20 @@ ExecEvalWholeRowFast(WholeRowVarExprState *wrvstate, ExprContext *econtext,
927925
if (wrvstate->wrv_junkFilter != NULL)
928926
slot = ExecFilterJunk(wrvstate->wrv_junkFilter, slot);
929927

930-
tuple = ExecFetchSlotTuple(slot);
931-
tupleDesc = slot->tts_tupleDescriptor;
932-
933928
/*
934-
* We have to make a copy of the tuple so we can safely insert the Datum
935-
* overhead fields, which are not set in on-disk tuples.
929+
* Copy the slot tuple and make sure any toasted fields get detoasted.
936930
*/
937-
dtuple = (HeapTupleHeader) palloc(tuple->t_len);
938-
memcpy((char *) dtuple, (char *) tuple->t_data, tuple->t_len);
939-
940-
HeapTupleHeaderSetDatumLength(dtuple, tuple->t_len);
931+
dtuple = DatumGetHeapTupleHeader(ExecFetchSlotTupleDatum(slot));
941932

942933
/*
943-
* If the Var identifies a named composite type, label the tuple with that
944-
* type; otherwise use what is in the tupleDesc.
934+
* If the Var identifies a named composite type, label the datum with that
935+
* type; otherwise we'll use the slot's info.
945936
*/
946937
if (variable->vartype != RECORDOID)
947938
{
948939
HeapTupleHeaderSetTypeId(dtuple, variable->vartype);
949940
HeapTupleHeaderSetTypMod(dtuple, variable->vartypmod);
950941
}
951-
else
952-
{
953-
HeapTupleHeaderSetTypeId(dtuple, tupleDesc->tdtypeid);
954-
HeapTupleHeaderSetTypMod(dtuple, tupleDesc->tdtypmod);
955-
}
956942

957943
return PointerGetDatum(dtuple);
958944
}
@@ -1029,13 +1015,13 @@ ExecEvalWholeRowSlow(WholeRowVarExprState *wrvstate, ExprContext *econtext,
10291015
}
10301016

10311017
/*
1032-
* We have to make a copy of the tuple so we can safely insert the Datum
1033-
* overhead fields, which are not set in on-disk tuples.
1018+
* Copy the slot tuple and make sure any toasted fields get detoasted.
10341019
*/
1035-
dtuple = (HeapTupleHeader) palloc(tuple->t_len);
1036-
memcpy((char *) dtuple, (char *) tuple->t_data, tuple->t_len);
1020+
dtuple = DatumGetHeapTupleHeader(ExecFetchSlotTupleDatum(slot));
10371021

1038-
HeapTupleHeaderSetDatumLength(dtuple, tuple->t_len);
1022+
/*
1023+
* Reset datum's type ID fields to match the Var.
1024+
*/
10391025
HeapTupleHeaderSetTypeId(dtuple, variable->vartype);
10401026
HeapTupleHeaderSetTypMod(dtuple, variable->vartypmod);
10411027

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