PostgreSQL: Drop column and recreate dependent views - postgresql

Question

I have below test table and dependent views created.
create table test_col_drp (col1 varchar(100), col2 varchar(100), col3 varchar(100));
create view test_col_drp_vw1 as select col1 col1_vw1, col2 col2_vw1, col3 col3_vw1 from test_col_drp;
create view test_col_drp_vw2 as select col1_vw1 col1_vw2, col2_vw1 col2_vw2, col3_vw1 col3_vw2 from test_col_drp_vw1;
I'm trying to drop a column from the table but getting below error:
alter table test_col_drp drop column col3;
ERROR: cannot drop table test_col_drp column col3 because other objects depend on it
DETAIL: view test_col_drp_vw1 depends on table test_col_drp column col3 view test_col_drp_vw2 depends on view test_col_drp_vw1
HINT: Use DROP ... CASCADE to drop the dependent objects too.
********** Error **********
What would be the best way to drop the column and recreate all the dependent views?
Working on:
PostgreSQL 9.6.6 on x86_64-pc-mingw64, compiled by gcc.exe (Rev5, Built by MSYS2 project) 4.9.2, 64-bit
Windows 10

Answer 1

First, you have to drop the views. Then, execute the alter table and finally create the views again. See:
-- This is what you have in your database
create table test_col_drp (col1 varchar(100), col2 varchar(100), col3 varchar(100));
create view test_col_drp_vw1 as select col1 col1_vw1, col2 col2_vw1, col3 col3_vw1 from test_col_drp;
create view test_col_drp_vw2 as select col1_vw1 col1_vw2, col2_vw1 col2_vw2, col3_vw1 col3_vw2 from test_col_drp_vw1;
-- drop views and alter table
drop view test_col_drp_vw2;
drop view test_col_drp_vw1;
alter table test_col_drp drop column col3;
-- creating the views again without col3
create view test_col_drp_vw1 as select col1 col1_vw1, col2 col2_vw1 from test_col_drp;
create view test_col_drp_vw2 as select col1_vw1 col1_vw2, col2_vw1 col2_vw2 from test_col_drp_vw1;
An alternative is to use the drop cascade. In this case, you do not need to drop each view individually but you still need to recreate them:
alter table test_col_drp drop column col3 cascade;
-- creating the views again without col3
create view test_col_drp_vw1 as select col1 col1_vw1, col2 col2_vw1 from test_col_drp;
create view test_col_drp_vw2 as select col1_vw1 col1_vw2, col2_vw1 col2_vw2 from test_col_drp_vw1;
Another option is to create a SQL command that generates your alter table using cascade and the create view:
with query_result as (
SELECT dependent_ns.nspname as dependent_schema
, dependent_view.relname as dependent_view
, source_ns.nspname as source_schema
, source_table.relname as source_table
, pg_attribute.attname as column_name
, row_number() over() as id
FROM pg_depend
JOIN pg_rewrite ON pg_depend.objid = pg_rewrite.oid
JOIN pg_class as dependent_view ON pg_rewrite.ev_class = dependent_view.oid
JOIN pg_class as source_table ON pg_depend.refobjid = source_table.oid
JOIN pg_attribute ON pg_depend.refobjid = pg_attribute.attrelid
AND pg_depend.refobjsubid = pg_attribute.attnum
JOIN pg_namespace dependent_ns ON dependent_ns.oid = dependent_view.relnamespace
JOIN pg_namespace source_ns ON source_ns.oid = source_table.relnamespace
WHERE 1=1
-- AND source_ns.nspname = 'public'
AND source_table.relname like 'test_col_drp%'
AND pg_attribute.attnum > 0
AND pg_attribute.attname = 'col3'
)
select concat('alter table ', source_table, ' drop column ' , column_name, ' cascade;' ) as sql_command from query_result where id = 1
union all
select concat('create or replace view ', dependent_view, ' as select * from ', source_table, ';') as sql_command from query_result
Then, the output will be:
alter table test_col_drp drop column col3 cascade;
create or replace view test_col_drp_vw1 as select * from test_col_drp;
create or replace view test_col_drp_vw2 as select * from test_col_drp_vw1;