Improve unparser MySQL compatibility

[sgrebnov]

Please let me know if you prefer me to split this into separate PRs. Combined as the changes are related/follow the same logic and to avoid resolving conflicts after PRs are merged.

Which issue does this PR close?

PR addresses unparser issues producing invalid sql for MySQL and implements support for IntervalStyle::MySQL

1. Invalid CAST(col AS Timestamp) SQL for MySQL.

MySQL cast function does not support Timestamp for CAST and requires DATETIME

DATETIME[(M)]
Produces a DATETIME value. If the optional M value is given, it specifies the fractional seconds precision.

Timezone information is correctly handled by MySQL when casting with DATETIME.

2. Unsupported date_part function to extract Datetime subfield

MySQL does not support date_part function so PR introduces configurable option to specify Datetime subfield extraction style for unparsing. Different DBMSs follow different standards; popular ones are:

date_part('YEAR', date '2001-02-16')
EXTRACT(YEAR from date '2001-02-16')

Some DBMSs, like Postgres, support both, whereas others like MySQL, Amazon Athena require EXTRACT.

3. invalid CAST(col AS BigInt) SQL for MySQL.

MySQL cast function does not support BigInt and requires SIGNED for CAST

SIGNED [INTEGER]
Produces a signed BIGINT value.

Note: sqlparser crate does not have Signed type so we use ast::DataType::Custom as a workaround

Rationale for this change

PRs implements IntervalStyle::MySQL and adds support for alternate formats for Timestamp, Int64, Datetime part unparsing via Dialect customization.

What changes are included in this PR?

Please see above

Are these changes tested?

Manual testing + unit test for each modification.

Are there any user-facing changes?

CustomDialectBuilder now supports more options for Dialect customization (Timestamp, Int64 unparsing and Datetime part extraction).

[alamb]

Thank you @sgrebnov -- this makes sense to me

[alamb]

❤️

[alamb]

Thanks again @sgrebnov

[sgrebnov]

Thank you Andrew @alamb!