With hbm2ddl.auto=update on Postgresql, columns for properties persisted as timestamps (like Instant, OffsetDateTime, LocalDateTime, …) are updated on every startup, even if the alter statement does not perform any actual change.

hbm2ddl.auto=validate does not detect any problem.

This did not happen on 6.1.7 (because it did not have the capability to update existing columns anyway).

Sample

My entity:

@Entity
@Getter
@Setter
public class Test {

    @Id
    private String id;
    private Instant instant;
    private OffsetDateTime offsetDateTime;
    private ZonedDateTime zonedDateTime;
    private LocalDateTime localDateTime;
    private Date date;
    
}

Startup logs (even when columns already exist with the right type):

[Hibernate] alter table if exists auth0.test alter column date set data type timestamp(6)
[Hibernate] alter table if exists auth0.test alter column instant set data type timestamp(6) with time zone
[Hibernate] alter table if exists auth0.test alter column local_date_time set data type timestamp(6)
[Hibernate] alter table if exists auth0.test alter column offset_date_time set data type timestamp(6) with time zone
[Hibernate] alter table if exists auth0.test alter column zoned_date_time set data type timestamp(6) with time zone

Quick analysis (using 6.2.0.CR4)

With hbm2ddl.auto=update: StandardTableMigrator#sqlAlterStrings checks both type and length

//StandardTableMigrator line 88
if ( !ColumnDefinitions.hasMatchingType( column, columnInformation, metadata, dialect )
						|| !ColumnDefinitions.hasMatchingLength( column, columnInformation, metadata, dialect ) )

With hbm2ddl.auto=validate: AbstractSchemaValidator#validateColumnType only checks the type (which matches)

//AbstractSchemaValidator line 163
if ( !ColumnDefinitions.hasMatchingType( column, columnInformation, metadata, dialect ) ) {

So the problem is in ColumnDefinitions.hasMatchingLength:

final int actualSize = columnInformation.getColumnSize();
if ( actualSize == 0 ) {
	return true;
}
else {
	final Size size = column.getColumnSize( dialect, metadata );
	final Long requiredLength = size.getLength();
	final Integer requiredPrecision = size.getPrecision();
	return requiredLength != null && requiredLength == actualSize
		|| requiredPrecision != null && requiredPrecision == actualSize
		|| requiredPrecision == null && requiredLength == null;
}

For a timestamp with zone column:

• actualSize = 35 (no idea why)
• requiredLength is null
• requiredPrecision = 6
• ==> requiredPrecision != null && requiredPrecision == actualSize is false