[Enhancement](agg-func) Make covar functions get greater precisions

[zclllyybb]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

covar function is more stable now:
before:

mysql> select col_decimal_37__12__undef_signed col_alias2425, col_largeint_undef_signed col_alias2426, covar(col_bigint_undef_signed_not_null, 1583928109) AS col_alias2427 from table_200_undef_partitions2_keys3_properties4_distributed_by54 where cos(col_tinyint_undef_signed) <= 811862185 and col_largeint_undef_signed = 245 GROUP BY col_alias2425,col_alias2426 ORDER BY col_alias2425,col_alias2426,col_alias2427 LIMIT 10;
+-----------------+---------------+---------------+
| col_alias2425   | col_alias2426 | col_alias2427 |
+-----------------+---------------+---------------+
|            NULL | 245           |            -8 |
......

or

mysql> select col_decimal_37__12__undef_signed col_alias2425, col_largeint_undef_signed col_alias2426, covar(col_bigint_undef_signed_not_null, 1583928109) AS col_alias2427 from table_200_undef_partitions2_keys3_properties4_distributed_by54 where cos(col_tinyint_undef_signed) <= 811862185 and col_largeint_undef_signed = 245 GROUP BY col_alias2425,col_alias2426 ORDER BY col_alias2425,col_alias2426,col_alias2427 LIMIT 10;
+-----------------+---------------+---------------+
| col_alias2425   | col_alias2426 | col_alias2427 |
+-----------------+---------------+---------------+
|            NULL | 245           |             0 |
......

now:

mysql> select col_decimal_37__12__undef_signed col_alias2425, col_largeint_undef_signed col_alias2426, covar(col_bigint_undef_signed_not_null, 1583928109) AS col_alias2427 from table_200_undef_partitions2_keys3_properties4_distributed_by54 where cos(col_tinyint_undef_signed) <= 811862185 and col_largeint_undef_signed = 245 GROUP BY col_alias2425,col_alias2426 ORDER BY col_alias2425,col_alias2426,col_alias2427 LIMIT 10;
+-----------------+---------------+---------------+
| col_alias2425   | col_alias2426 | col_alias2427 |
+-----------------+---------------+---------------+
|            NULL | 245           |          0.25 |
......

Release note

Some aggregate functions have greater precisions now

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason
      • this result may be unstable.

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

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[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[zclllyybb]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"