lightning: pick the first file to check schema

[3pointer]

What problem does this PR solve?

Issue Number: close #27605
Problem Summary:
check all files costs too much time.

What is changed and how it works?

What's Changed:
pick one random file to check schema is valid.

Check List

Tests

• Unit test

Release note

Fix the issue that pre-check cost too much time when import too many files for tables.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[3pointer]

/component lightning

[3pointer]

/sig migrate

[kennytm]

? move them back

[3pointer]

Fixed

[3pointer]

/label needs-cherry-pick-5.2

[kennytm]

/merge

[ti-chi-bot]

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

Commit hash: 772016b

[ti-srebot]

cherry pick to release-5.2 in PR #27623

[zhangjinpeng87]

Is it safe to check schema for one random file? cc @lonng

[glorv]

Is it safe to check schema for one random file? cc @lonng

In the common case that the source files are exported by mydumper/dumpling or other similar tools, source files' schema should be the same, so only check one scheme should be enough.

In other case, e.g. user manually created source files with different columns, this check may not find potential error and lightning will still raise an error after parse the incompatible file. Since this kind of situation is rare, so I think this compromise between performance and correctness is ok.

cc @IANTHEREAL