Revert r799, which was supposed to prevent similar cases as Bug #21638.

In reality, the patch breaks the handling of prefix indexes of
variable-length columns in ROW_FORMAT=COMPACT.  Reverting the patch
is only a partial fix of Bug #28138.

dict/dict0dict.c

@@ -1587,9 +1587,6 @@ dict_index_copy_types(
 		ifield = dict_index_get_nth_field(index, i);
 		dfield_type = dfield_get_type(dtuple_get_nth_field(tuple, i));
 		dict_col_copy_type(dict_field_get_col(ifield), dfield_type);
-		if (UNIV_UNLIKELY(ifield->prefix_len)) {
-			dfield_type->len = ifield->prefix_len;
-		}
 	}
 }
 

row/row0row.c

@@ -142,21 +142,16 @@ row_build_index_entry(
 		dfield_copy(dfield, dfield2);
 
 		/* If a column prefix index, take only the prefix */
-		if (ind_field->prefix_len) {
-			if (dfield_get_len(dfield2) != UNIV_SQL_NULL) {
+		if (ind_field->prefix_len > 0
+		    && dfield_get_len(dfield2) != UNIV_SQL_NULL) {
 
 			storage_len = dtype_get_at_most_n_mbchars(
-					col->prtype,
-					col->mbminlen, col->mbmaxlen,
+				col->prtype, col->mbminlen, col->mbmaxlen,
 				ind_field->prefix_len,
-					dfield_get_len(dfield2),
-					dfield2->data);
+				dfield_get_len(dfield2), dfield2->data);
 
 			dfield_set_len(dfield, storage_len);
 		}
-
-			dfield_get_type(dfield)->len = ind_field->prefix_len;
-		}
 	}
 
 	ut_ad(dtuple_check_typed(entry));