alter column type from bytea to timestamp with data retention in a Postgres Database

Question

The field in the java class looked like this :

private LocalDateTime updateTime;

However, Hibernate stores it in the database like this:

(output of "select update_time from my_table;")

ac  ed  00  05  73  72  00  1b  6f  72  67  2e  6a  6f  64  61  2e  74  69  6d  65  2e  4c  6f  63  61  6c  44  61  74  65  54  69  6d  65  fc  45  53  6c  f5  a5  87  10  02  00  02  4a  00  0c  69  4c  6f  63  61  6c  4d  69  6c  6c  69  73  4c  00  0b  69  43  68  72  6f  6e  6f  6c  6f  67  79  74  00  1a  4c  6f  72  67  2f  6a  6f  64  61  2f  74  69  6d  65  2f  43  68  72  6f  6e  6f  6c  6f  67  79  3b  78  70  00  00  01  42  09  8e  35  21  73  72  00  27  6f  72  67  2e  6a  6f  64  61  2e  74  69  6d  65  2e  63  68  72  6f  6e  6f  2e  49  53  4f  43  68  72  6f  6e  6f  6c  6f  67  79  24  53  74  75  62  a9  c8  11  66  71  37  50  27  03  00  00  78  70  73  72  00  1f  6f  72  67  2e  6a  6f  64  61  2e  74  69  6d  65  2e  44  61  74  65  54  69  6d  65  5a  6f  6e  65  24  53  74  75  62  a6  2f  01  9a  7c  32  1a  e3  03  00  00  78  70  77  05  00  03  55  54  43  78  78

Now we decided to change the java code to look like:

@Type(type = "org.jadira.usertype.dateandtime.joda.PersistentLocalDateTime")
private LocalDateTime updateTime;

So that the value in the database looks like

(output of "select update_time from my_table;")
2013-12-18 00:00:00.0

However, the database table already has some values in this column. How can we now convert the datatype of update_time to timestamp so that the existing values are also correctly handled?

The database version and dependencies used are as below: database version:

psql --version
psql (EnterpriseDB) 9.1.6.10

mvn dependencies for hibernate:

              <groupId>org.hibernate</groupId>
                <artifactId>hibernate-core</artifactId>
                <version>4.0.1.Final</version>

                <groupId>org.hibernate</groupId>
                <artifactId>hibernate-validator</artifactId>
                <version>4.2.0.Final</version>

                <groupId>org.hibernate</groupId>
                <artifactId>hibernate-entitymanager</artifactId>
                <version>4.0.1.Final</version>

                <groupId>org.hibernate</groupId>
                <artifactId>hibernate-jpamodelgen</artifactId>
                <version>1.2.0.Final</version>

                <groupId>org.hibernate.javax.persistence</groupId>
                <artifactId>hibernate-jpa-2.0-api</artifactId>
                <version>1.0.1.Final</version>

Really stuck here. Any idea how can this be achieved? Any help here is truly appreciated.