Postgresql change column type from int to UUID

Question

I\'d like to change the column type from an int to a uuid. I am using the following statement

ALTER TABLE tableA ALTER COLUMN colA          


        

Answer 1

I had to convert from text to uuid type, and from a Django migration, so after solving this I wrote it up at http://baltaks.com/2015/08/how-to-change-text-fields-to-a-real-uuid-type-for-django-and-postgresql in case that helps anyone. The same techniques would work for an integer to uuid conversion.

Based on a comment, I've added the full solution here:

Django will most likely create a migration for you that looks something like:

class Migration(migrations.Migration):

    dependencies = [
        ('app', '0001_auto'),
    ]

    operations = [
        migrations.AlterField(
            model_name='modelname',
            name='uuid',
            field=models.UUIDField(db_index=True, unique=True),
        ),
    ]

First, put the auto created migration operations into a RunSQL operation as the state_operations parameter. This allows you to provide a custom migration, but keep Django informed about what's happened to the database schema.

class Migration(migrations.Migration):

    dependencies = [
        ('app', '0001_auto'),
    ]

    operations = [
    migrations.RunSQL(sql_commands, None, [
            migrations.AlterField(
                model_name='modelname',
                name='uuid',
                field=models.UUIDField(db_index=True, unique=True),
            ),
        ]),
    ]

Now you'll need to provide some SQL commands for that sql_commands variable. I opted to put the sql into a separate file and then load in with the following python code:

sql_path = os.path.join(os.path.dirname(os.path.abspath(__file__)), '0001.sql')
with open(sql_path, "r") as sqlfile:
    sql_commands = sqlfile.read()

Now for the real tricky part, where we actually perform the migration. The basic command you want looks like:

alter table tablename alter column uuid type uuid using uuid::uuid;

But the reason we are here is because of indexes. And as I discovered, Django likes to use your migrations to created randomly named indexes on your fields while running tests, so your tests will fail if you just delete and then recreate a fixed name index or two. So the following is sql that will delete one constraint and all indexes on the text field before converting to a uuid field. It also works for multiple tables in one go.

DO $$
DECLARE
    table_names text[];
    this_table_name text;
    the_constraint_name text;
    index_names record;

BEGIN

SELECT array['table1',
             'table2'
             ]
    INTO table_names;


FOREACH this_table_name IN array table_names
LOOP
    RAISE notice 'migrating table %', this_table_name;

    SELECT CONSTRAINT_NAME INTO the_constraint_name
    FROM information_schema.constraint_column_usage
    WHERE CONSTRAINT_SCHEMA = current_schema()
        AND COLUMN_NAME IN ('uuid')
        AND TABLE_NAME = this_table_name
    GROUP BY CONSTRAINT_NAME
    HAVING count(*) = 1;
    if the_constraint_name is not NULL then
        RAISE notice 'alter table % drop constraint %',
            this_table_name,
            the_constraint_name;
        execute 'alter table ' || this_table_name
            || ' drop constraint ' || the_constraint_name;
    end if;

    FOR index_names IN
    (SELECT i.relname AS index_name
     FROM pg_class t,
          pg_class i,
          pg_index ix,
          pg_attribute a
     WHERE t.oid = ix.indrelid
         AND i.oid = ix.indexrelid
         AND a.attrelid = t.oid
         AND a.attnum = any(ix.indkey)
         AND t.relkind = 'r'
         AND a.attname = 'uuid'
         AND t.relname = this_table_name
     ORDER BY t.relname,
              i.relname)
    LOOP
        RAISE notice 'drop index %', quote_ident(index_names.index_name);
        EXECUTE 'drop index ' || quote_ident(index_names.index_name);
    END LOOP; -- index_names

    RAISE notice 'alter table % alter column uuid type uuid using uuid::uuid;',
        this_table_name;
    execute 'alter table ' || quote_ident(this_table_name)
        || ' alter column uuid type uuid using uuid::uuid;';
    RAISE notice 'CREATE UNIQUE INDEX %_uuid ON % (uuid);',
        this_table_name, this_table_name;
    execute 'create unique index ' || this_table_name || '_uuid on '
        || this_table_name || '(uuid);';

END LOOP; -- table_names

END;
$$