sql: fix pick up possible field (tidb#29399)

[lichunzhu]

cherry-pick pingcap/tidb#29386 to release-5.0

What problem does this PR solve?

close pingcap/tidb#29386
Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

Fix dumpling pick up possible field function by always pick the index whose 'seqInIndex' is "1".

Check List

Tests

• Unit test
• Integration test

Release note

fix the bug that dumpling gets very slow when dumping composite primary/unique key tables.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv

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[lichunzhu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7f237df

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #400.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #401.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #402.