fix substring wrong result bug (#9603) (#9618)

close #9604

Signed-off-by: guo-shaoge <shaoge1994@163.com>

Co-authored-by: guo-shaoge <shaoge1994@163.com>
Co-authored-by: ti-chi-bot[bot] <108142056+ti-chi-bot[bot]@users.noreply.github.com>

Author: 3 people

dbms/src/Functions/FunctionsString.cpp

@@ -847,6 +847,7 @@ struct SubstringUTF8Impl
                 res_data[res_offset] = 0;
                 ++res_offset;
                 res_offsets[column_index] = res_offset;
+                prev_offset = offsets[column_index];
                 return;
             }
             start = start_offsets.size() - original_start_abs + 1;

tests/fullstack-test/expr/substring_utf8.test

@@ -44,3 +44,27 @@ a
 1
 
 mysql> drop table if exists test.t
+
+mysql> drop table if exists test.t1
+mysql> create table test.t1 (c1 int, c2 int)
+mysql> insert into test.t1 (c1, c2) values (-82 ,  -4), (  1 ,   4)
+mysql> alter table test.t1 set tiflash replica 1
+func> wait_table test t1
+
+mysql> set tidb_enforce_mpp=1; set tidb_isolation_read_engines='tiflash'; select c2, substring('abc', c2) as b FROM test.t1 order by 1, 2
++------+------+
+| c2   | b    |
++------+------+
+|   -4 |      |
+|    4 |      |
++------+------+
+
+mysql> set session tidb_isolation_read_engines='tiflash'; set tidb_allow_mpp=0; set tidb_allow_tiflash_cop = ON; select c2, substring('abc', c2) as b FROM test.t1 order by 1, 2
++------+------+
+| c2   | b    |
++------+------+
+|   -4 |      |
+|    4 |      |
++------+------+
+
+mysql> drop table if exists test.t1