[6X][WIP]Enable direct dispatch in deep-slice queries

[QingMa-gp]

About ten years ago, direct dispatch in deep-slice queries has been disabled due to the following case that found DTM issue with targeted dispatch and deepslice queries.

set test_print_direct_dispatch_info = on;
create table t1(x int);
insert into t1 select x from generate_series(1,300000)x;
analyze t1;
select * from t1 a, t1 b where a.x=b.x and a.x=23;
select count(*) from t1 a, t1 b where a.x=b.x union all select count(*) from t1 a, t1 b where a.x=b.x and a.x=23;
select count(*) from t1 a, t1 b where a.x=b.x and b.x=12 union all select count(*) from t1 a, t1 b where a.x=b.x and a.x =23;

Re-enable it since the DTM issue can't reproduce.

Here are some reminders before you submit the pull request

• [ ] Add tests for the change
• [ ] Document changes
• [ ] Communicate in the mailing list if needed
• [ ] Pass make installcheck
• [ ] Review a PR in return to support the community

[ashwinstar]

Would be helpful for reviewers to clarify what exactly was the problem earlier and what changed in DTM that avoids problem now? Yes will need to dig through the history. Having full clarity is important.

[ashwinstar]

Closing this PR for now. Once we have full clarity feel free to reopen