ddl: skip delay for async commit during merging temp index

[tangenta]

What problem does this PR solve?

Issue Number: ref #35983

Problem Summary:

After @@tidb_ddl_enable_fast_reorg is enabled, there will be two reorganization steps: backfilll and merge. Because DDL has waited 2.5 seconds in the backfill step, it is guaranteed that none of the async commit transactions with old schema info(the newly adding index's schema state is none) are active. Thus, we don't have to wait another 2.5 seconds in the merge step.

What is changed and how it works?

Check List

Tests

• Unit test

• Integration test

• Manual test (add detailed scripts or steps below)
  Before this PR, adding index takes 6 seconds even the table is empty:

  mysql> set @@global.tidb_ddl_enable_fast_reorg = 1;
  Query OK, 0 rows affected (0.03 sec)
  
  mysql> alter table t add index idx(a);
  Query OK, 0 rows affected (6.17 sec)
  

  After this PR, it takes about 3 seconds:

  alter table t add index idx(a);
  Query OK, 0 rows affected (3.11 sec)
  

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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This pull request has been approved by:

• hawkingrei

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[tangenta]

/hold

[tangenta]

We need to keep it for the correctness, closed.