branches/zip: trx_undo_rec_get_partial_row(): Set up the row_ext cache

only for those externally stored columns that occur in the ordering columns
of indexes.  Prefetch the prefixes of those columns, because the clustered
index record and the BLOBs may have been deleted by the time when the
purge thread needs to read the BLOB prefixes.

row_ext_create(): Add the debug assertion ut_ad(ut_is_2pow(zip_size)).

include/row0ext.ic

@@ -42,6 +42,8 @@ row_ext_create(
 	row_ext_t*	ret = mem_heap_alloc(heap, (sizeof *ret)
 					     + (n_ext - 1) * sizeof ret->len);
 
+	ut_ad(ut_is_2pow(zip_size));
+
 	ret->n_ext = n_ext;
 	ret->ext = ext;
 	ret->zip_size = zip_size;

trx/trx0rec.c

@@ -894,7 +894,11 @@ trx_undo_rec_get_partial_row(
 	ulint		n_ext_cols;
 	ulint*		ext_cols;
 
-	ut_ad(index && ptr && row && ext && heap);
+	ut_ad(index);
+	ut_ad(ptr);
+	ut_ad(row);
+	ut_ad(heap);
+	ut_ad(dict_index_is_clust(index));
 
 	row_len = dict_table_get_n_cols(index->table);
 	n_ext_cols = 0;
@@ -908,15 +912,17 @@ trx_undo_rec_get_partial_row(
 	ptr += 2;
 
 	while (ptr != end_ptr) {
-		dfield_t*	dfield;
-		byte*		field;
-		ulint		field_no;
-		ulint		col_no;
-		ulint		len;
+		dfield_t*		dfield;
+		byte*			field;
+		ulint			field_no;
+		const dict_col_t*	col;
+		ulint			col_no;
+		ulint			len;
 
 		ptr = trx_undo_update_rec_get_field_no(ptr, &field_no);
 
-		col_no = dict_index_get_nth_col_no(index, field_no);
+		col = dict_index_get_nth_col(index, field_no);
+		col_no = dict_col_get_no(col);
 
 		ptr = trx_undo_rec_get_col_val(ptr, &field, &len);
 
@@ -929,14 +935,40 @@ trx_undo_rec_get_partial_row(
 			dfield_set_len(dfield,
 				       len - UNIV_EXTERN_STORAGE_FIELD);
 			dfield_set_ext(dfield);
-			ext_cols[n_ext_cols++] = col_no;
+			if (col->ord_part) {
+				/* We will have to fetch prefixes of
+				externally stored columns that are
+				referenced by column prefixes. */
+				ext_cols[n_ext_cols++] = col_no;
+			}
 		}
 	}
 
 	if (n_ext_cols) {
+		ulint	i;
+
 		*ext = row_ext_create(n_ext_cols, ext_cols,
 				      dict_table_zip_size(index->table),
 				      heap);
+
+		/* Fetch the BLOB prefixes, because the clustered
+		index record (and the BLOBs) may have been deleted by
+		the time row_ext_lookup() is called later in the purge
+		thread. */
+		for (i = 0; i < n_ext_cols; i++) {
+			const dfield_t*	dfield;
+			ulint		len;
+			byte*		b;
+
+			dfield = dtuple_get_nth_field(*row, ext_cols[i]);
+
+			b = row_ext_lookup_ith(*ext, i,
+					       dfield_get_data(dfield),
+					       dfield_get_len(dfield),
+					       &len);
+			ut_a(b);
+			ut_a(b != field_ref_zero);
+		}
 	} else {
 		*ext = NULL;
 	}