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

/messages/by-id/55C3F0B4.5010600@computer.org

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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

/messages/by-id/55C3F0B4.5010600@computer.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.

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On Sun, Jul 9, 2017 at 9:56 AM, Berend Tober <btober@computer.org> wrote:

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

/messages/by-id/55C3F0B4.5010600@computer.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

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Another alternative is to simply extract all the view defs with the column
name you want to drop,
edit the viewdef to remove the columns and then use that to redefine the
views BEFORE
dropping the column from the table.

IE: in the following query, replace <COLUMN_NAME> with the name of the
column to be dropped.

1. Make a SQL pg_dump of the database before proceeding

2.
\o edit_views.sql

SELECT 'CREATE OR REPLACE VIEW '
|| n.nspname || '.' || c.relname || ' AS '
|| pg_get_viewdef(c.oid, true)
FROM pg_attribute a
JOIN pg_class c ON c.oid = a.attrelid
JOIN pg_namespace n ON (n.oid = c.relnamespace)
WHERE c.relkind = 'v'
AND n.nspname NOT LIKE 'pg_%'
AND n.nspname NOT LIKE 'sql_%'
AND n.nspname NOT LIKE 'information%'
AND a.attname = '<COLUMN_NAME>'
ORDER BY 1;

3. Edit edit_views.sql to remove all occurrences of the column being dropped

4.psql <your_db_name> < edit_views.sql

5. Then drop the column from the table.

*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.

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 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).

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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