Re: Alter domain type / avoiding table rewrite

On Wed, 17 Apr 2019 at 15:23, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

>
>
> The table definition and the size of the data set would help with
> interpreting the below.

The below example shows the issue on a table with just a single field. I
can demonstrate the problem wether there are 100 records or a million
records.

In every case:
Altering the type from a domain of varchar(9) to a raw varchar(9) results
in a full table rewrite (as identified by relfilenode).
Altering the type from a raw varchar(9) to a domain of varchar(9) occurs
for free, with no change to relfilenode.

The timing of each ALTER operation appears to back this up.

postgres(at)[local]=# create domain old_type as varchar(9);
CREATE DOMAIN

postgres(at)[local]=# create table test (values old_type);
CREATE TABLE

postgres(at)[local]=# with data as (select generate_series(1,1000000),
md5(random()::text))
postgres(at)[local]-# insert into test select substring(md5, 1, 9) from data;
INSERT 0 1000000
Time: 4097.162 ms

postgres(at)[local]=# \d test
Table "alpha_core.test"
Column | Type | Modifiers
--------+----------+-----------
values | old_type |

postgres(at)[local]=# \dD old_type
List of domains
Schema | Name | Type | Modifier | Check
------------+----------+----------------------+----------+-------
alpha_core | old_type | character varying(9) | |
(1 row)

postgres(at)[local]=# select count(*) from test;
count
---------
1000000
(1 row)

postgres(at)[local]=# select relfilenode from pg_class where relname='test';
relfilenode
-------------
20689856
(1 row)

postgres(at)[local]=# alter table test alter COLUMN values set data type
varchar(9);
ALTER TABLE
Time: 993.271 ms

postgres(at)[local]=# select relfilenode from pg_class where relname='test';
relfilenode
-------------
20691283
(1 row)

postgres(at)[local]=# alter table test alter COLUMN values set data type
old_type;
ALTER TABLE
Time: 21.569 ms

postgres(at)[local]=# select relfilenode from pg_class where relname='test';
relfilenode
-------------
20691283
(1 row)

postgres(at)[local]=# drop table test;
DROP TABLE