Something wrong with execute in PostgreSQL - postgresql

Question

I make function that will change type of columns of any table. And for example I create a table test:
create table test(id text, name text, born text);
insert into test values ('1', 'Ivanov', '10-10-2012'), ('2', 'Petrov', '01-01-1999'),
('3', 'Sidorov', '03-12-1975');
And then make table with column name and new data type:
create table col_types(column_name text, data_type text);
insert into col_types values ('id', 'integer'), ('name', 'text'),
('born', 'date');
This my function:
create or replace function change_columns(my_table text, columns_types_table text) returns void as $$
declare
r text;
cur_type text;
begin
raise notice 'NOTICE smth: %', 1;
for r in (select column_name from information_schema.columns where table_name = my_table) loop
raise notice 'NOTICE r: %', 2;
execute 'select data_type from ' -- ERROR here
|| quote_ident(columns_types_table)
|| ' where column_name = '
|| quote_ident(r)
into strict cur_type;
raise notice 'NOTICE cur_type: %', 3;
execute 'alter table '
|| quote_ident(my_table)
|| ' alter column '
|| quote_ident(r)
||' type '
|| quote_ident(cur_type)
|| ' using '
|| r
|| '::'
|| cur_type;
end loop;
end
$$
language 'plpgsql';
My ERROR:
ERROR: column "id" does not exist
Where: PL/pgSQL function change_columns(text,text) line 9 at EXECUTE
Function request:
select change_columns('test', 'col_types');

Answer 1

The full error message (at least in Postgres 14) is
ERROR: column "id" does not exist
LINE 1: select data_type from col_types where column_name = id
^
QUERY: select data_type from col_types where column_name = id
CONTEXT: PL/pgSQL function change_columns(text,text) line 9 at EXECUTE
which makes it easy to see where you went wrong: id is treated as an identifier here. You actually wanted to run the query
select data_type from col_types where column_name = 'id'
-- ^ ^
To generate this, you need to use quote_literal instead of quote_identifier:
execute 'select data_type from '
|| quote_ident(columns_types_table)
|| ' where column_name = '
|| quote_literal(r)
into strict cur_type;
(online demo. Notice I also had to change the alter column statement not to escape the type name)
An even better approach would be to use a parameterised query with a USING clause:
execute 'select data_type from '
|| quote_ident(columns_types_table)
|| ' where column_name = $1'
into strict cur_type
using r;
(online demo)