[Feature] Add expression-based column default values

[nooneuse]

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Description

Previously, column DEFAULT values in Doris were primarily treated as literal constants (or a small set of special built-ins such as CURRENT_TIMESTAMP), which limited usability for common patterns like “derive default from current date/time” or “compose a formatted default string”.

Use case

Example (date/datetime defaults):
d DATEV2 NOT NULL DEFAULT to_date(now())
dt DATETIMEV2(3) NOT NULL DEFAULT now(3)
Example (composed string default):
s STRING NOT NULL DEFAULT concat('a-', DATE_FORMAT(now(), '%M %e, %Y'))

Related issues

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