logical decoding: Fix handling of large old tuples with replica identity full.
authorAndres Freund <andres@anarazel.de>
Sun, 6 Mar 2016 02:02:20 +0000 (18:02 -0800)
committerAndres Freund <andres@anarazel.de>
Sun, 6 Mar 2016 02:02:20 +0000 (18:02 -0800)
When decoding the old version of an UPDATE or DELETE change, and if that
tuple was bigger than MaxHeapTupleSize, we either Assert'ed out, or
failed in more subtle ways in non-assert builds.  Normally individual
tuples aren't bigger than MaxHeapTupleSize, with big datums toasted.
But that's not the case for the old version of a tuple for logical
decoding; the replica identity is logged as one piece. With the default
replica identity btree limits that to small tuples, but that's not the
case for FULL.

Change the tuple buffer infrastructure to separate allocate over-large
tuples, instead of always going through the slab cache.

This unfortunately requires changing the ReorderBufferTupleBuf
definition, we need to store the allocated size someplace. To avoid
requiring output plugins to recompile, don't store HeapTupleHeaderData
directly after HeapTupleData, but point to it via t_data; that leaves
rooms for the allocated size.  As there's no reason for an output plugin
to look at ReorderBufferTupleBuf->t_data.header, remove the field. It
was just a minor convenience having it directly accessible.

Reported-By: Adam DratwiƄski
Discussion: CAKg6ypLd7773AOX4DiOGRwQk1TVOQKhNwjYiVjJnpq8Wo+i62Q@mail.gmail.com

contrib/test_decoding/expected/toast.out
contrib/test_decoding/sql/toast.sql
src/backend/replication/logical/decode.c
src/backend/replication/logical/reorderbuffer.c
src/include/replication/reorderbuffer.h

index 735b14c9786db5d211b985cafb2c240096c0983b..389748ff8c106a2bfd8468a2a834389fe414ec5d 100644 (file)
@@ -292,6 +292,64 @@ SELECT substr(data, 1, 200) FROM pg_logical_slot_get_changes('regression_slot',
  COMMIT
 (235 rows)
 
+-- test we can decode "old" tuples bigger than the max heap tuple size correctly
+DROP TABLE IF EXISTS toasted_several;
+NOTICE:  table "toasted_several" does not exist, skipping
+CREATE TABLE toasted_several (
+    id serial unique not null,
+    toasted_key text primary key,
+    toasted_col1 text,
+    toasted_col2 text
+);
+ALTER TABLE toasted_several REPLICA IDENTITY FULL;
+ALTER TABLE toasted_several ALTER COLUMN toasted_key SET STORAGE EXTERNAL;
+ALTER TABLE toasted_several ALTER COLUMN toasted_col1 SET STORAGE EXTERNAL;
+ALTER TABLE toasted_several ALTER COLUMN toasted_col2 SET STORAGE EXTERNAL;
+INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 2000));
+SELECT regexp_replace(data, '^(.{100}).*(.{100})$', '\1..\2') FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+                                                                                               regexp_replace                                                                                               
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ BEGIN
+ table public.toasted_several: INSERT: id[integer]:1 toasted_key[text]:'98765432109876543210987654321..098765432109876543210987654321098765432109876543210' toasted_col1[text]:null toasted_col2[text]:null
+ COMMIT
+(3 rows)
+
+-- test update of a toasted key without changing it
+UPDATE toasted_several SET toasted_col1 = toasted_key;
+UPDATE toasted_several SET toasted_col2 = toasted_col1;
+SELECT regexp_replace(data, '^(.{100}).*(.{100})$', '\1..\2') FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+                                                                                               regexp_replace                                                                                               
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ BEGIN
+ table public.toasted_several: INSERT: id[integer]:1 toasted_key[text]:'98765432109876543210987654321..098765432109876543210987654321098765432109876543210' toasted_col1[text]:null toasted_col2[text]:null
+ COMMIT
+ BEGIN
+ table public.toasted_several: UPDATE: old-key: id[integer]:1 toasted_key[text]:'98765432109876543210..432109876543210987654321098765432109876543210987654321098765432109876543210' toasted_col2[text]:null
+ COMMIT
+ BEGIN
+ table public.toasted_several: UPDATE: old-key: id[integer]:1 toasted_key[text]:'98765432109876543210..876543210987654321098765432109876543210987654321098765432109876543210987654321098765432109876543210'
+ COMMIT
+(9 rows)
+
+/*
+ * update with large tuplebuf, in a transaction large enough to force to spool to disk
+ */
+BEGIN;
+INSERT INTO toasted_several(toasted_key) SELECT * FROM generate_series(1, 10234);
+UPDATE toasted_several SET toasted_col1 = toasted_col2 WHERE id = 1;
+DELETE FROM toasted_several WHERE id = 1;
+COMMIT;
+DROP TABLE toasted_several;
+SELECT regexp_replace(data, '^(.{100}).*(.{100})$', '\1..\2') FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1')
+WHERE data NOT LIKE '%INSERT: %';
+                                                                                               regexp_replace                                                                                               
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ BEGIN
+ table public.toasted_several: UPDATE: old-key: id[integer]:1 toasted_key[text]:'98765432109876543210..7654321098765432109876543210987654321098765432109876543210' toasted_col2[text]:unchanged-toast-datum
+ table public.toasted_several: DELETE: id[integer]:1 toasted_key[text]:'98765432109876543210987654321..876543210987654321098765432109876543210987654321098765432109876543210987654321098765432109876543210'
+ COMMIT
+(4 rows)
+
 SELECT pg_drop_replication_slot('regression_slot');
  pg_drop_replication_slot 
 --------------------------
index 26d6b4fbdd6f330ec14f482093ada8d44c1637ce..dcb74e38c77dbf7c55e6e9c0abc73b7c5c37385e 100644 (file)
@@ -265,4 +265,41 @@ ALTER TABLE toasted_copy ALTER COLUMN data SET STORAGE EXTERNAL;
 203    untoasted200
 \.
 SELECT substr(data, 1, 200) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
+-- test we can decode "old" tuples bigger than the max heap tuple size correctly
+DROP TABLE IF EXISTS toasted_several;
+CREATE TABLE toasted_several (
+    id serial unique not null,
+    toasted_key text primary key,
+    toasted_col1 text,
+    toasted_col2 text
+);
+ALTER TABLE toasted_several REPLICA IDENTITY FULL;
+ALTER TABLE toasted_several ALTER COLUMN toasted_key SET STORAGE EXTERNAL;
+ALTER TABLE toasted_several ALTER COLUMN toasted_col1 SET STORAGE EXTERNAL;
+ALTER TABLE toasted_several ALTER COLUMN toasted_col2 SET STORAGE EXTERNAL;
+
+INSERT INTO toasted_several(toasted_key) VALUES(repeat('9876543210', 2000));
+
+SELECT regexp_replace(data, '^(.{100}).*(.{100})$', '\1..\2') FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
+-- test update of a toasted key without changing it
+UPDATE toasted_several SET toasted_col1 = toasted_key;
+UPDATE toasted_several SET toasted_col2 = toasted_col1;
+
+SELECT regexp_replace(data, '^(.{100}).*(.{100})$', '\1..\2') FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
+/*
+ * update with large tuplebuf, in a transaction large enough to force to spool to disk
+ */
+BEGIN;
+INSERT INTO toasted_several(toasted_key) SELECT * FROM generate_series(1, 10234);
+UPDATE toasted_several SET toasted_col1 = toasted_col2 WHERE id = 1;
+DELETE FROM toasted_several WHERE id = 1;
+COMMIT;
+
+DROP TABLE toasted_several;
+
+SELECT regexp_replace(data, '^(.{100}).*(.{100})$', '\1..\2') FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1')
+WHERE data NOT LIKE '%INSERT: %';
 SELECT pg_drop_replication_slot('regression_slot');
index c530fe9ba099eed1c9ce79843208245aa58e24df..3f8c5e50fba67a7a020ff19a2a92376c6dbd0d7f 100644 (file)
@@ -610,7 +610,8 @@ DecodeInsert(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
        Size        tuplelen;
        char       *tupledata = XLogRecGetBlockData(r, 0, &tuplelen);
 
-       change->data.tp.newtuple = ReorderBufferGetTupleBuf(ctx->reorder);
+       change->data.tp.newtuple =
+           ReorderBufferGetTupleBuf(ctx->reorder, tuplelen);
 
        DecodeXLogTuple(tupledata, tuplelen, change->data.tp.newtuple);
    }
@@ -656,7 +657,8 @@ DecodeUpdate(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
    {
        data = XLogRecGetBlockData(r, 0, &datalen);
 
-       change->data.tp.newtuple = ReorderBufferGetTupleBuf(ctx->reorder);
+       change->data.tp.newtuple =
+           ReorderBufferGetTupleBuf(ctx->reorder, datalen);
 
        DecodeXLogTuple(data, datalen, change->data.tp.newtuple);
    }
@@ -667,7 +669,8 @@ DecodeUpdate(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
        data = XLogRecGetData(r) + SizeOfHeapUpdate;
        datalen = XLogRecGetDataLen(r) - SizeOfHeapUpdate;
 
-       change->data.tp.oldtuple = ReorderBufferGetTupleBuf(ctx->reorder);
+       change->data.tp.oldtuple =
+           ReorderBufferGetTupleBuf(ctx->reorder, datalen);
 
        DecodeXLogTuple(data, datalen, change->data.tp.oldtuple);
    }
@@ -717,13 +720,15 @@ DecodeDelete(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
    /* old primary key stored */
    if (xlrec->flags & XLH_DELETE_CONTAINS_OLD)
    {
+       Size        len = XLogRecGetDataLen(r) - SizeOfHeapDelete;
+
        Assert(XLogRecGetDataLen(r) > (SizeOfHeapDelete + SizeOfHeapHeader));
 
-       change->data.tp.oldtuple = ReorderBufferGetTupleBuf(ctx->reorder);
+       change->data.tp.oldtuple =
+           ReorderBufferGetTupleBuf(ctx->reorder, len);
 
        DecodeXLogTuple((char *) xlrec + SizeOfHeapDelete,
-                       XLogRecGetDataLen(r) - SizeOfHeapDelete,
-                       change->data.tp.oldtuple);
+                       len, change->data.tp.oldtuple);
    }
 
    change->data.tp.clear_toast_afterwards = true;
@@ -783,35 +788,39 @@ DecodeMultiInsert(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
         */
        if (xlrec->flags & XLH_INSERT_CONTAINS_NEW_TUPLE)
        {
-           change->data.tp.newtuple = ReorderBufferGetTupleBuf(ctx->reorder);
+           HeapTupleHeader header;
+
+           xlhdr = (xl_multi_insert_tuple *) SHORTALIGN(data);
+           data = ((char *) xlhdr) + SizeOfMultiInsertTuple;
+           datalen = xlhdr->datalen;
+
+           change->data.tp.newtuple =
+               ReorderBufferGetTupleBuf(ctx->reorder, datalen);
 
            tuple = change->data.tp.newtuple;
+           header = tuple->tuple.t_data;
 
            /* not a disk based tuple */
            ItemPointerSetInvalid(&tuple->tuple.t_self);
 
-           xlhdr = (xl_multi_insert_tuple *) SHORTALIGN(data);
-           data = ((char *) xlhdr) + SizeOfMultiInsertTuple;
-           datalen = xlhdr->datalen;
-
            /*
             * We can only figure this out after reassembling the
             * transactions.
             */
            tuple->tuple.t_tableOid = InvalidOid;
-           tuple->tuple.t_data = &tuple->t_data.header;
+
            tuple->tuple.t_len = datalen + SizeofHeapTupleHeader;
 
-           memset(&tuple->t_data.header, 0, SizeofHeapTupleHeader);
+           memset(header, 0, SizeofHeapTupleHeader);
 
-           memcpy((char *) &tuple->t_data.header + SizeofHeapTupleHeader,
+           memcpy((char *) tuple->tuple.t_data + SizeofHeapTupleHeader,
                   (char *) data,
                   datalen);
            data += datalen;
 
-           tuple->t_data.header.t_infomask = xlhdr->t_infomask;
-           tuple->t_data.header.t_infomask2 = xlhdr->t_infomask2;
-           tuple->t_data.header.t_hoff = xlhdr->t_hoff;
+           header->t_infomask = xlhdr->t_infomask;
+           header->t_infomask2 = xlhdr->t_infomask2;
+           header->t_hoff = xlhdr->t_hoff;
        }
 
        /*
@@ -877,31 +886,31 @@ DecodeXLogTuple(char *data, Size len, ReorderBufferTupleBuf *tuple)
 {
    xl_heap_header xlhdr;
    int         datalen = len - SizeOfHeapHeader;
+   HeapTupleHeader header;
 
    Assert(datalen >= 0);
-   Assert(datalen <= MaxHeapTupleSize);
 
    tuple->tuple.t_len = datalen + SizeofHeapTupleHeader;
+   header = tuple->tuple.t_data;
 
    /* not a disk based tuple */
    ItemPointerSetInvalid(&tuple->tuple.t_self);
 
    /* we can only figure this out after reassembling the transactions */
    tuple->tuple.t_tableOid = InvalidOid;
-   tuple->tuple.t_data = &tuple->t_data.header;
 
    /* data is not stored aligned, copy to aligned storage */
    memcpy((char *) &xlhdr,
           data,
           SizeOfHeapHeader);
 
-   memset(&tuple->t_data.header, 0, SizeofHeapTupleHeader);
+   memset(header, 0, SizeofHeapTupleHeader);
 
-   memcpy((char *) &tuple->t_data.header + SizeofHeapTupleHeader,
+   memcpy(((char *) tuple->tuple.t_data) + SizeofHeapTupleHeader,
           data + SizeOfHeapHeader,
           datalen);
 
-   tuple->t_data.header.t_infomask = xlhdr.t_infomask;
-   tuple->t_data.header.t_infomask2 = xlhdr.t_infomask2;
-   tuple->t_data.header.t_hoff = xlhdr.t_hoff;
+   header->t_infomask = xlhdr.t_infomask;
+   header->t_infomask2 = xlhdr.t_infomask2;
+   header->t_hoff = xlhdr.t_hoff;
 }
index 2f4489d1b3ec44164a270293b167e90d89c56315..a04d8ec6e633d4ba0287374f5a61d8483125e4b4 100644 (file)
@@ -444,27 +444,48 @@ ReorderBufferReturnChange(ReorderBuffer *rb, ReorderBufferChange *change)
 
 
 /*
- * Get an unused, possibly preallocated, ReorderBufferTupleBuf
+ * Get an unused, possibly preallocated, ReorderBufferTupleBuf fitting at
+ * least a tuple of size tuple_len (excluding header overhead).
  */
 ReorderBufferTupleBuf *
-ReorderBufferGetTupleBuf(ReorderBuffer *rb)
+ReorderBufferGetTupleBuf(ReorderBuffer *rb, Size tuple_len)
 {
    ReorderBufferTupleBuf *tuple;
+   Size        alloc_len;
 
-   /* check the slab cache */
-   if (rb->nr_cached_tuplebufs)
+   alloc_len = tuple_len + SizeofHeapTupleHeader;
+
+   /*
+    * Most tuples are below MaxHeapTupleSize, so we use a slab allocator for
+    * those. Thus always allocate at least MaxHeapTupleSize. Note that tuples
+    * tuples generated for oldtuples can be bigger, as they don't have
+    * out-of-line toast columns.
+    */
+   if (alloc_len < MaxHeapTupleSize)
+       alloc_len = MaxHeapTupleSize;
+
+
+   /* if small enough, check the slab cache */
+   if (alloc_len <= MaxHeapTupleSize && rb->nr_cached_tuplebufs)
    {
        rb->nr_cached_tuplebufs--;
        tuple = slist_container(ReorderBufferTupleBuf, node,
                                slist_pop_head_node(&rb->cached_tuplebufs));
 #ifdef USE_ASSERT_CHECKING
-       memset(tuple, 0xa9, sizeof(ReorderBufferTupleBuf));
+       memset(&tuple->tuple, 0xa9, sizeof(HeapTupleData));
+#endif
+       tuple->tuple.t_data = ReorderBufferTupleBufData(tuple);
+#ifdef USE_ASSERT_CHECKING
+       memset(tuple->tuple.t_data, 0xa8, tuple->alloc_tuple_size);
 #endif
    }
    else
    {
        tuple = (ReorderBufferTupleBuf *)
-           MemoryContextAlloc(rb->context, sizeof(ReorderBufferTupleBuf));
+           MemoryContextAlloc(rb->context,
+                              sizeof(ReorderBufferTupleBuf) + alloc_len);
+       tuple->alloc_tuple_size = alloc_len;
+       tuple->tuple.t_data = ReorderBufferTupleBufData(tuple);
    }
 
    return tuple;
@@ -479,13 +500,16 @@ ReorderBufferGetTupleBuf(ReorderBuffer *rb)
 void
 ReorderBufferReturnTupleBuf(ReorderBuffer *rb, ReorderBufferTupleBuf *tuple)
 {
-   /* check whether to put into the slab cache */
-   if (rb->nr_cached_tuplebufs < max_cached_tuplebufs)
+   /* check whether to put into the slab cache, oversized tuples never are */
+   if (tuple->alloc_tuple_size == MaxHeapTupleSize &&
+       rb->nr_cached_tuplebufs < max_cached_tuplebufs)
    {
        rb->nr_cached_tuplebufs++;
        slist_push_head(&rb->cached_tuplebufs, &tuple->node);
+       VALGRIND_MAKE_MEM_UNDEFINED(tuple->tuple.t_data, tuple->alloc_tuple_size);
        VALGRIND_MAKE_MEM_UNDEFINED(tuple, sizeof(ReorderBufferTupleBuf));
        VALGRIND_MAKE_MEM_DEFINED(&tuple->node, sizeof(tuple->node));
+       VALGRIND_MAKE_MEM_DEFINED(&tuple->alloc_tuple_size, sizeof(tuple->alloc_tuple_size));
    }
    else
    {
@@ -2092,15 +2116,18 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
                newtup = change->data.tp.newtuple;
 
                if (oldtup)
-                   oldlen = offsetof(ReorderBufferTupleBuf, t_data) +
-                       oldtup->tuple.t_len;
+               {
+                   sz += sizeof(HeapTupleData);
+                   oldlen = oldtup->tuple.t_len;
+                   sz += oldlen;
+               }
 
                if (newtup)
-                   newlen = offsetof(ReorderBufferTupleBuf, t_data) +
-                       newtup->tuple.t_len;
-
-               sz += oldlen;
-               sz += newlen;
+               {
+                   sz += sizeof(HeapTupleData);
+                   newlen = newtup->tuple.t_len;
+                   sz += newlen;
+               }
 
                /* make sure we have enough space */
                ReorderBufferSerializeReserve(rb, sz);
@@ -2111,14 +2138,20 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
                if (oldlen)
                {
-                   memcpy(data, oldtup, oldlen);
+                   memcpy(data, &oldtup->tuple, sizeof(HeapTupleData));
+                   data += sizeof(HeapTupleData);
+
+                   memcpy(data, oldtup->tuple.t_data, oldlen);
                    data += oldlen;
                }
 
                if (newlen)
                {
-                   memcpy(data, newtup, newlen);
-                   data += newlen;
+                   memcpy(data, &newtup->tuple, sizeof(HeapTupleData));
+                   data += sizeof(HeapTupleData);
+
+                   memcpy(data, newtup->tuple.t_data, newlen);
+                   data += oldlen;
                }
                break;
            }
@@ -2337,27 +2370,46 @@ ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:
            if (change->data.tp.oldtuple)
            {
-               Size        len = offsetof(ReorderBufferTupleBuf, t_data) +
-               ((ReorderBufferTupleBuf *) data)->tuple.t_len;
+               Size        tuplelen = ((HeapTuple) data)->t_len;
+
+               change->data.tp.oldtuple =
+                   ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);
 
-               change->data.tp.oldtuple = ReorderBufferGetTupleBuf(rb);
-               memcpy(change->data.tp.oldtuple, data, len);
+               /* restore ->tuple */
+               memcpy(&change->data.tp.oldtuple->tuple, data,
+                      sizeof(HeapTupleData));
+               data += sizeof(HeapTupleData);
+
+               /* reset t_data pointer into the new tuplebuf */
                change->data.tp.oldtuple->tuple.t_data =
-                   &change->data.tp.oldtuple->t_data.header;
-               data += len;
+                   ReorderBufferTupleBufData(change->data.tp.oldtuple);
+
+               /* restore tuple data itself */
+               memcpy(change->data.tp.oldtuple->tuple.t_data, data, tuplelen);
+               data += tuplelen;
            }
 
            if (change->data.tp.newtuple)
            {
-               Size        len = offsetof(ReorderBufferTupleBuf, t_data) +
-               ((ReorderBufferTupleBuf *) data)->tuple.t_len;
+               Size        tuplelen = ((HeapTuple) data)->t_len;
+
+               change->data.tp.newtuple =
+                   ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);
 
-               change->data.tp.newtuple = ReorderBufferGetTupleBuf(rb);
-               memcpy(change->data.tp.newtuple, data, len);
+               /* restore ->tuple */
+               memcpy(&change->data.tp.newtuple->tuple, data,
+                      sizeof(HeapTupleData));
+               data += sizeof(HeapTupleData);
+
+               /* reset t_data pointer into the new tuplebuf */
                change->data.tp.newtuple->tuple.t_data =
-                   &change->data.tp.newtuple->t_data.header;
-               data += len;
+                   ReorderBufferTupleBufData(change->data.tp.newtuple);
+
+               /* restore tuple data itself */
+               memcpy(change->data.tp.newtuple->tuple.t_data, data, tuplelen);
+               data += tuplelen;
            }
+
            break;
        case REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT:
            {
@@ -2734,7 +2786,7 @@ ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn,
     */
    tmphtup = heap_form_tuple(desc, attrs, isnull);
    Assert(newtup->tuple.t_len <= MaxHeapTupleSize);
-   Assert(&newtup->t_data.header == newtup->tuple.t_data);
+   Assert(ReorderBufferTupleBufData(newtup) == newtup->tuple.t_data);
 
    memcpy(newtup->tuple.t_data, tmphtup->t_data, tmphtup->t_len);
    newtup->tuple.t_len = tmphtup->t_len;
index cf47a117200a5c77aa983d31dda95f98a0dab3fe..fe35edf311255c3924080de1f043d4556400e924 100644 (file)
@@ -26,16 +26,19 @@ typedef struct ReorderBufferTupleBuf
    /* position in preallocated list */
    slist_node  node;
 
-   /* tuple, stored sequentially */
+   /* tuple header, the interesting bit for users of logical decoding */
    HeapTupleData tuple;
-   union
-   {
-       HeapTupleHeaderData header;
-       char        data[MaxHeapTupleSize];
-       double      align_it;   /* ensure t_data is MAXALIGN'd */
-   }           t_data;
+
+   /* pre-allocated size of tuple buffer, different from tuple size */
+   Size    alloc_tuple_size;
+
+   /* actual tuple data follows */
 } ReorderBufferTupleBuf;
 
+/* pointer to the data stored in a TupleBuf */
+#define ReorderBufferTupleBufData(p) \
+   ((HeapTupleHeader) MAXALIGN(((char *) p) + sizeof(ReorderBufferTupleBuf)))
+
 /*
  * Types of the change passed to a 'change' callback.
  *
@@ -344,7 +347,7 @@ struct ReorderBuffer
 ReorderBuffer *ReorderBufferAllocate(void);
 void       ReorderBufferFree(ReorderBuffer *);
 
-ReorderBufferTupleBuf *ReorderBufferGetTupleBuf(ReorderBuffer *);
+ReorderBufferTupleBuf *ReorderBufferGetTupleBuf(ReorderBuffer *, Size tuple_len);
 void       ReorderBufferReturnTupleBuf(ReorderBuffer *, ReorderBufferTupleBuf *tuple);
 ReorderBufferChange *ReorderBufferGetChange(ReorderBuffer *);
 void       ReorderBufferReturnChange(ReorderBuffer *, ReorderBufferChange *);