planner: generate index path as candidate when it is a single scan

[francis0407]

What problem does this PR solve?

fix #12021

When we only want to get primary key, we can consider index path if there is no condition on PK. For example:

create table t1 (a int primary key, b int, .... many columns, index idx_b(b));
create table t2 (a int, b int);

In this query, we can use IndexScan for t1.a:

select t2.a, t2.b
from t1, t2
where t1.a = t2.a;

We can also use a IndexScan for this one:

select count(1) from t1;

More general, we should consider the index path as a candidate when it is a single scan.

What is changed and how it works?

Change the condition that generates index path.

We can use index to generate physical plan if any of the following conditions is satisfied:

1. This path's access cond is not nil.
2. We have a non-empty prop to match.
3. This index is forced to choose.
4. The needed columns are all covered by index columns(and handleCol).

Check List

Tests

• Unit test
• Explain test

[francis0407]

Some unit tests about admin clean up index and admin check index failed. I'm gonna check these tests, maybe fix the issue in another PR.

[francis0407]

/rebuild

[francis0407]

/rebuild

[codecov]

Codecov Report

Merging #12022 into master will increase coverage by 0.1112%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##            master     #12022        +/-   ##
===============================================
+ Coverage   81.456%   81.5673%   +0.1112%     
===============================================
  Files          448        448                
  Lines        95945      96280       +335     
===============================================
+ Hits         78153      78533       +380     
+ Misses       12218      12178        -40     
+ Partials      5574       5569         -5

[francis0407]

All the tests have been fixed.
Some of the tests failed before because sql select count(*) from t chooses index path, but acutally, they want a table path. If we want to use a table scan, we must use index hint now.

[alivxxx]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

@francis0407 merge failed.

[francis0407]

/run-common-test tidb-test=pr/884

[francis0407]

/run-common-test tidb-test=pr/884

[francis0407]

/run-common-test tidb-test=pr/884

[francis0407]

/run-common-test tidb-test=pr/884

[francis0407]

/run-integration-common-test tidb-test=pr/884

[zz-jason]

@francis0407 please resolve conflicts

[francis0407]

/run-common-test

[francis0407]

/run-integration-common-test

[lzmhhh123]

/merge

[sre-bot]

/run-all-tests