Thread: [GENERAL] How to drop column from interrelated views

Guyren Howe wrote:
> I’ve a set of interrelated views. I want to drop a column from a table and from all the views that
> cascade from it.
>
> I’ve gone to the leaf dependencies and removed the field from them. But I can’t remove the field
> from the intermediate views because Postgres doesn’t appear to be clever enough to see that the
> leafs no longer depend on the column. Or did I just miss one?
>
> In general, this seems like a major weakness expressing a model in Postgres (I get that any such
> weakness derives from SQL; that doesn’t stop me wanting a solution).
>
> Thoughts? Comments?

This usually involves a pg_dump in the custom format, editing the list file, creating a script with
pg_restore.

I described a way I have had success with it at one point at


https://www.postgresql.org/message-id/55C3F0B4.5010600%40computer.org



-- B

Guyren Howe wrote:
> On Jul 8, 2017, at 16:11 , Berend Tober <btober@computer.org <mailto:btober@computer.org>>
> wrote:
>>
>> Guyren Howe wrote:
>>> I’ve a set of interrelated views. I want to drop a column from a table and from all the views
>>> that cascade from it.
>>>
>>> I’ve gone to the leaf dependencies and removed the field from them. But I can’t remove the
>>> field from the intermediate views because Postgres doesn’t appear to be clever enough to see
>>> that the leafs no longer depend on the column. Or did I just miss one?
>>>
>>> In general, this seems like a major weakness expressing a model in Postgres (I get that any
>>> such weakness derives from SQL; that doesn’t stop me wanting a solution).
>>>
>>> Thoughts? Comments?
>>
>> This usually involves a pg_dump in the custom format, editing the list file, creating a script
>> with pg_restore.
>>
>> I described a way I have had success with it at one point at
>>
>>
>> https://www.postgresql.org/message-id/55C3F0B4.5010600%40computer.org
>
> I was wondering if I changed up all the things that interrelate in a transaction, whether that
> would bundle them up so they’re all correct afterward. I was hoping so.


Well, nothing beats empirical evidence ... set up a test case and try it!

You definitely want to do it in a transaction anyway, so that if you get it wrong the first few
times and have to iterate, the data base rolls back to where you started.

Note the method suggested in the earlier link appears to have a error. Step 4 should be


   pg_restore -c -1  -L mydatabase.list mydatabase.dump > sql


The lower case "c" flag will include DROP statements for the views. The "1" will wrap in a
transaction, like you want.


BTW, please generally use the "reply-all" functionality of your email client when interacting with
this list ... the server puts the list alias in the CC line, so you have to reply all to keep the
conversation publicly available for others to learn from.

-- B

I've got some functions on my own :)
If you prefer to do it from SQL level I've got two functions prepared which
first secure then restore all missing views definitions.


-- secure all views
DO $$
BEGIN
--drop schema migration cascade
  CREATE SCHEMA migration;

  CREATE TABLE migration.views AS
    SELECT
      table_schema,
      table_name,
      view_definition
    FROM INFORMATION_SCHEMA.views
    WHERE table_schema NOT IN ('pg_catalog', 'information_schema');

  CREATE TABLE migration.view_count AS
    SELECT
      count(*),
      'before' :: TEXT AS desc
    FROM INFORMATION_SCHEMA.views
    WHERE table_schema NOT IN ('pg_catalog', 'information_schema');

END;
$$;

/*
 HERE DO YOUR EVIL DROP CASCADE
 YOUR VIEWS DEFINITIONS ARE SAFE IN MIGRATION SCHEMA
 REMEMBER YOU BACKED UP ONLY VIEWS DEFINITIONS - NO TABLES, RULES OR DATA
 */

-- restore all dropped views / only not existing views
DO $$

DECLARE
  l_string TEXT;
BEGIN

  FOR l_string IN SELECT 'CREATE VIEW ' || table_schema || '.' || table_name
|| ' AS '
                         || view_definition
                  FROM migration.views
  LOOP
    BEGIN
      EXECUTE l_string;
      EXCEPTION WHEN OTHERS THEN
      -- do nothing
    END;
  END LOOP;

  IF ((SELECT count
       FROM migration.view_count) = (SELECT count(*)
                                     FROM INFORMATION_SCHEMA.views
                                     WHERE table_schema NOT IN
('pg_catalog', 'information_schema')))
  THEN
    RAISE NOTICE 'Migration successful';
  ELSE
    RAISE NOTICE 'Something went wrong';
  END IF;

END;
$$;



If migration was successful you can drop schema migration.





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On Sat, Jul 08, 2017 at 03:18:39PM -0700, Guyren Howe wrote:

> I’ve a set of interrelated views. I want to drop a column from a table and from all the views that cascade from it.
>
> I’ve gone to the leaf dependencies and removed the field from them. But I can’t remove the field from the
intermediateviews because Postgres doesn’t appear to be clever enough to see that the leafs no longer depend on the
column.Or did I just miss one? 
>
> In general, this seems like a major weakness expressing a model in Postgres (I get that any such weakness derives
fromSQL; that doesn’t stop me wanting a solution). 

Not that it helps much with your immediate problem but this
is typically the point where one realizes that database
definitions should live under version control.

That doesn't enable easy dropping of a column from
interrelated views but does allow for more convenient ways of
writing the required DDL change script.

Regards,
Karsten
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