Move heap-specific detoasting logic into a separate function.

The new function, heap_fetch_toast_slice, is shared between
toast_fetch_datum_slice and toast_fetch_datum, and does all the
work of scanning the TOAST table, fetching chunks, and storing
them into the space allocated for the result varlena.

As an incidental side effect, this allows toast_fetch_datum_slice
to perform the scan with only a single scankey if all chunks are
being fetched, which might have some tiny performance benefit.

Discussion: http://postgr.es/m/CA+TgmobBzxwFojJ0zV0Own3dr09y43hp+OzU2VW+nos4PMXWEg@mail.gmail.com

Author: robertmhaas

src/backend/access/common/detoast.c

@@ -27,6 +27,9 @@ static struct varlena *toast_fetch_datum(struct varlena *attr);
 static struct varlena *toast_fetch_datum_slice(struct varlena *attr,
 											   int32 sliceoffset,
 											   int32 slicelength);
+static void heap_fetch_toast_slice(Relation toastrel, Oid valueid,
+								   int32 attrsize, int32 sliceoffset,
+								   int32 slicelength, struct varlena *result);
 static struct varlena *toast_decompress_datum(struct varlena *attr);
 static struct varlena *toast_decompress_datum_slice(struct varlena *attr, int32 slicelength);
 
@@ -325,19 +328,9 @@ static struct varlena *
 toast_fetch_datum(struct varlena *attr)
 {
 	Relation	toastrel;
-	Relation   *toastidxs;
-	ScanKeyData toastkey;
-	SysScanDesc toastscan;
-	HeapTuple	ttup;
-	TupleDesc	toasttupDesc;
 	struct varlena *result;
 	struct varatt_external toast_pointer;
 	int32		attrsize;
-	int32		expectedchunk;
-	int32		totalchunks;
-	int			num_indexes;
-	int			validIndex;
-	SnapshotData SnapshotToast;
 
 	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
 		elog(ERROR, "toast_fetch_datum shouldn't be called for non-ondisk datums");
@@ -346,7 +339,6 @@ toast_fetch_datum(struct varlena *attr)
 	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
 
 	attrsize = toast_pointer.va_extsize;
-	totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
 
 	result = (struct varlena *) palloc(attrsize + VARHDRSZ);
 
@@ -355,130 +347,19 @@ toast_fetch_datum(struct varlena *attr)
 	else
 		SET_VARSIZE(result, attrsize + VARHDRSZ);
 
+	if (attrsize == 0)
+		return result;		/* Probably shouldn't happen, but just in case. */
+
 	/*
 	 * Open the toast relation and its indexes
 	 */
 	toastrel = table_open(toast_pointer.va_toastrelid, AccessShareLock);
-	toasttupDesc = toastrel->rd_att;
-
-	/* Look for the valid index of the toast relation */
-	validIndex = toast_open_indexes(toastrel,
-									AccessShareLock,
-									&toastidxs,
-									&num_indexes);
 
-	/*
-	 * Setup a scan key to fetch from the index by va_valueid
-	 */
-	ScanKeyInit(&toastkey,
-				(AttrNumber) 1,
-				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(toast_pointer.va_valueid));
+	/* Fetch all chunks */
+	heap_fetch_toast_slice(toastrel, toast_pointer.va_valueid, attrsize, 0,
+						   attrsize, result);
 
-	/*
-	 * Read the chunks by index
-	 *
-	 * Note that because the index is actually on (valueid, chunkidx) we will
-	 * see the chunks in chunkidx order, even though we didn't explicitly ask
-	 * for it.
-	 */
-	expectedchunk = 0;
-
-	init_toast_snapshot(&SnapshotToast);
-	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										   &SnapshotToast, 1, &toastkey);
-	while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
-	{
-		int32		curchunk;
-		Pointer		chunk;
-		bool		isnull;
-		char	   *chunkdata;
-		int32		chunksize;
-		int32		expected_size;
-
-		/*
-		 * Have a chunk, extract the sequence number and the data
-		 */
-		curchunk = DatumGetInt32(fastgetattr(ttup, 2, toasttupDesc, &isnull));
-		Assert(!isnull);
-		chunk = DatumGetPointer(fastgetattr(ttup, 3, toasttupDesc, &isnull));
-		Assert(!isnull);
-		if (!VARATT_IS_EXTENDED(chunk))
-		{
-			chunksize = VARSIZE(chunk) - VARHDRSZ;
-			chunkdata = VARDATA(chunk);
-		}
-		else if (VARATT_IS_SHORT(chunk))
-		{
-			/* could happen due to heap_form_tuple doing its thing */
-			chunksize = VARSIZE_SHORT(chunk) - VARHDRSZ_SHORT;
-			chunkdata = VARDATA_SHORT(chunk);
-		}
-		else
-		{
-			/* should never happen */
-			elog(ERROR, "found toasted toast chunk for toast value %u in %s",
-				 toast_pointer.va_valueid,
-				 RelationGetRelationName(toastrel));
-			chunksize = 0;		/* keep compiler quiet */
-			chunkdata = NULL;
-		}
-
-		/*
-		 * Some checks on the data we've found
-		 */
-		if (curchunk != expectedchunk)
-			ereport(ERROR,
-					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg_internal("unexpected chunk number %d (expected %d) for toast value %u in %s",
-									 curchunk, expectedchunk,
-									 toast_pointer.va_valueid,
-									 RelationGetRelationName(toastrel))));
-		if (curchunk > totalchunks - 1)
-			ereport(ERROR,
-					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg_internal("unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
-									 curchunk,
-									 0, totalchunks - 1,
-									 toast_pointer.va_valueid,
-									 RelationGetRelationName(toastrel))));
-		expected_size = curchunk < totalchunks - 1 ? TOAST_MAX_CHUNK_SIZE
-			: attrsize - ((totalchunks - 1) * TOAST_MAX_CHUNK_SIZE);
-		if (chunksize != expected_size)
-			ereport(ERROR,
-					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg_internal("unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s",
-									 chunksize, expected_size,
-									 curchunk, totalchunks,
-									 toast_pointer.va_valueid,
-									 RelationGetRelationName(toastrel))));
-
-		/*
-		 * Copy the data into proper place in our result
-		 */
-		memcpy(VARDATA(result) + curchunk * TOAST_MAX_CHUNK_SIZE,
-			   chunkdata,
-			   chunksize);
-
-		expectedchunk++;
-	}
-
-	/*
-	 * Final checks that we successfully fetched the datum
-	 */
-	if (expectedchunk != totalchunks)
-		ereport(ERROR,
-				(errcode(ERRCODE_DATA_CORRUPTED),
-				 errmsg_internal("missing chunk number %d for toast value %u in %s",
-								 expectedchunk,
-								 toast_pointer.va_valueid,
-								 RelationGetRelationName(toastrel))));
-
-	/*
-	 * End scan and close relations
-	 */
-	systable_endscan_ordered(toastscan);
-	toast_close_indexes(toastidxs, num_indexes, AccessShareLock);
+	/* Close toast table */
 	table_close(toastrel, AccessShareLock);
 
 	return result;
@@ -500,22 +381,9 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 						int32 slicelength)
 {
 	Relation	toastrel;
-	Relation   *toastidxs;
-	ScanKeyData toastkey[3];
-	int			nscankeys;
-	SysScanDesc toastscan;
-	HeapTuple	ttup;
-	TupleDesc	toasttupDesc;
 	struct varlena *result;
 	struct varatt_external toast_pointer;
 	int32		attrsize;
-	int32		expectedchunk;
-	int			startchunk;
-	int			endchunk;
-	int			totalchunks;
-	int			num_indexes;
-	int			validIndex;
-	SnapshotData SnapshotToast;
 
 	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
 		elog(ERROR, "toast_fetch_datum_slice shouldn't be called for non-ondisk datums");
@@ -531,7 +399,6 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 	Assert(!VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) || 0 == sliceoffset);
 
 	attrsize = toast_pointer.va_extsize;
-	totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
 
 	if (sliceoffset >= attrsize)
 	{
@@ -560,35 +427,74 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 	if (slicelength == 0)
 		return result;			/* Can save a lot of work at this point! */
 
-	startchunk = sliceoffset / TOAST_MAX_CHUNK_SIZE;
-	endchunk = (sliceoffset + slicelength - 1) / TOAST_MAX_CHUNK_SIZE;
-	Assert(endchunk < totalchunks);
-
-	/*
-	 * Open the toast relation and its indexes
-	 */
+	/* Open the toast relation */
 	toastrel = table_open(toast_pointer.va_toastrelid, AccessShareLock);
-	toasttupDesc = toastrel->rd_att;
+
+	/* Fetch all chunks */
+	heap_fetch_toast_slice(toastrel, toast_pointer.va_valueid, attrsize,
+						   sliceoffset, slicelength, result);
+
+	/* Close toast table */
+	table_close(toastrel, AccessShareLock);
+
+	return result;
+}
+
+/*
+ * Fetch a TOAST slice from a heap table.
+ *
+ * toastrel is the relation from which chunks are to be fetched.
+ * valueid identifies the TOAST value from which chunks are being fetched.
+ * attrsize is the total size of the TOAST value.
+ * sliceoffset is the byte offset within the TOAST value from which to fetch.
+ * slicelength is the number of bytes to be fetched from the TOAST value.
+ * result is the varlena into which the results should be written.
+ */
+static void
+heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
+					   int32 sliceoffset, int32 slicelength,
+					   struct varlena *result)
+{
+	Relation   *toastidxs;
+	ScanKeyData toastkey[3];
+	TupleDesc	toasttupDesc = toastrel->rd_att;
+	int			nscankeys;
+	SysScanDesc toastscan;
+	HeapTuple	ttup;
+	int32		expectedchunk;
+	int32		totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
+	int			startchunk;
+	int			endchunk;
+	int			num_indexes;
+	int			validIndex;
+	SnapshotData SnapshotToast;
 
 	/* Look for the valid index of toast relation */
 	validIndex = toast_open_indexes(toastrel,
 									AccessShareLock,
 									&toastidxs,
 									&num_indexes);
 
+	startchunk = sliceoffset / TOAST_MAX_CHUNK_SIZE;
+	endchunk = (sliceoffset + slicelength - 1) / TOAST_MAX_CHUNK_SIZE;
+	Assert(endchunk <= totalchunks);
+
 	/*
 	 * Setup a scan key to fetch from the index. This is either two keys or
 	 * three depending on the number of chunks.
 	 */
 	ScanKeyInit(&toastkey[0],
 				(AttrNumber) 1,
 				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(toast_pointer.va_valueid));
+				ObjectIdGetDatum(valueid));
 
 	/*
-	 * Use equality condition for one chunk, a range condition otherwise:
+	 * No additional condition if fetching all chunks. Otherwise, use an
+	 * equality condition for one chunk, and a range condition otherwise.
 	 */
-	if (startchunk == endchunk)
+	if (startchunk == 0 && endchunk == totalchunks - 1)
+		nscankeys = 1;
+	else if (startchunk == endchunk)
 	{
 		ScanKeyInit(&toastkey[1],
 					(AttrNumber) 2,
@@ -609,15 +515,17 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 		nscankeys = 3;
 	}
 
+	/* Prepare for scan */
+	init_toast_snapshot(&SnapshotToast);
+	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
+										   &SnapshotToast, nscankeys, toastkey);
+
 	/*
 	 * Read the chunks by index
 	 *
 	 * The index is on (valueid, chunkidx) so they will come in order
 	 */
-	init_toast_snapshot(&SnapshotToast);
 	expectedchunk = startchunk;
-	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										   &SnapshotToast, nscankeys, toastkey);
 	while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
 	{
 		int32		curchunk;
@@ -651,8 +559,7 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 		{
 			/* should never happen */
 			elog(ERROR, "found toasted toast chunk for toast value %u in %s",
-				 toast_pointer.va_valueid,
-				 RelationGetRelationName(toastrel));
+				 valueid, RelationGetRelationName(toastrel));
 			chunksize = 0;		/* keep compiler quiet */
 			chunkdata = NULL;
 		}
@@ -664,16 +571,14 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg_internal("unexpected chunk number %d (expected %d) for toast value %u in %s",
-									 curchunk, expectedchunk,
-									 toast_pointer.va_valueid,
+									 curchunk, expectedchunk, valueid,
 									 RelationGetRelationName(toastrel))));
 		if (curchunk > endchunk)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg_internal("unexpected chunk number %d (out of range %d..%d) for toast value %u in %s",
 									 curchunk,
-									 startchunk, endchunk,
-									 toast_pointer.va_valueid,
+									 startchunk, endchunk, valueid,
 									 RelationGetRelationName(toastrel))));
 		expected_size = curchunk < totalchunks - 1 ? TOAST_MAX_CHUNK_SIZE
 			: attrsize - ((totalchunks - 1) * TOAST_MAX_CHUNK_SIZE);
@@ -682,8 +587,7 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg_internal("unexpected chunk size %d (expected %d) in chunk %d of %d for toast value %u in %s",
 									 chunksize, expected_size,
-									 curchunk, totalchunks,
-									 toast_pointer.va_valueid,
+									 curchunk, totalchunks, valueid,
 									 RelationGetRelationName(toastrel))));
 
 		/*
@@ -711,18 +615,12 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 		ereport(ERROR,
 				(errcode(ERRCODE_DATA_CORRUPTED),
 				 errmsg_internal("missing chunk number %d for toast value %u in %s",
-								 expectedchunk,
-								 toast_pointer.va_valueid,
+								 expectedchunk, valueid,
 								 RelationGetRelationName(toastrel))));
 
-	/*
-	 * End scan and close relations
-	 */
+	/* End scan and close indexes. */
 	systable_endscan_ordered(toastscan);
 	toast_close_indexes(toastidxs, num_indexes, AccessShareLock);
-	table_close(toastrel, AccessShareLock);
-
-	return result;
 }
 
 /* ----------