IndexMerge didn't check if pushdown is enabled or not

[guo-shaoge]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table t1;
create table t1(c1 varchar(100), c2 varchar(100), key(c1), key(c2), c3 varchar(100));
insert into t1 values('ab', '10', '10');
explain select /*+ use_index_merge(t1) */ * from t1 where c1 = 'ab' or c2 = '10' and substring(c3, 10) = 10;
select /*+ use_index_merge(t1) */ * from t1 where c1 = 'ab' or c2 = '10' and substring(c3, 10) = 10;
<!-- a step by step guide for reproducing the bug. -->

2. What did you expect to see? (Required)

mysql> select /*+ use_index_merge(t1) */ * from t1 where c1 = 'ab' or c2 = '10' and substring(c3, 10) = 10;
+------+------+------+
| c1   | c2   | c3   |
+------+------+------+
| ab   | 10   | 10   |
+------+------+------+

3. What did you see instead (Required)

[components/tidb_query_expr/src/lib.rs:705]: ScalarFunction Substring2ArgsUtf8 is not supported in batch mode

4. What is your TiDB version? (Required)

master

[aytrack]

duplicated with #30200