Add date and timestamp format to COPY TO (CSV, JSON)

[vbmithr]

What type of enhancement is this?

User experience

What does the enhancement do?

It should be possible to choose which format one wants for date and timestamp. RFC3339 (current default) and various unix formats is a minimum, but it would be even better to additionally support a format string (like here https://docs.greptime.com/reference/sql/functions/overview/#date_format)

Implementation challenges

No response

[KR-bluejay]

Spec Proposal: Date/Time/Timestamp format options for COPY TO

Proposal:

• DATE_FORMAT → applies only to DATE columns
• TIME_FORMAT → applies only to TIME columns (if supported)
• TIMESTAMP_FORMAT → applies to both TIMESTAMP and DATETIME columns

Scope:

• Output only (for COPY TO)
• Applies to CSV/JSON export (string serialization)
• Ignored or error for Parquet (to be decided)
• If not specified, keep the current default output format
• 1:1 mapping between column type and format option (no ambiguity)

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References:

• GreptimeDB SQL Data Types
• GreptimeDB COPY command

[linyihai]

Could you assign this to me? I almost finished it based on #6860