Union multiple tables - postgresql

Question

I tried to follow this answer
Dynamic UNION ALL query in Postgres
but I am getting
ERROR: syntax error at or near "record"
DECLARE
rec record;
strSQL text;
BEGIN
FOR
rec in
select table_name
from
information_schema.tables
where
table_name like 'history%'
loop
strSQL : = strSQL || 'Select * from' || rec.table_schema ||'.'|| rec.table_name || ' UNION ';
end loop;
-- remove last ' UNION ' from strSQL
--strSQL := 'Select row_number() over(order by rowid ) as row_num from (' || strSQL || ')';
execute strSQL;
anyone have any ideas?
background:
history table is moved every night to its own table with the date appended.
so history04242018 for each table name, any better way to get the data for multiple days?
edit: tables will always have the same amount of columns so the union should be fine
edit2: I only have read access.
update
with the suggestion of using an anonymous code block I am now using the following:
DO
$$
declare
strSQL text;
begin
select
string_agg(format('select * from %I.%I', table_schema, table_name), E' union\n')
into strSQL
from information_schema.tables
where table_name like 'history%';
execute strSQL ;
end $$;
however I now get the error
Describe Error: Failed to retrieve EXPLAIN plan(s): ERROR: syntax
error at or near "DO" Position: 58
0 record(s) affected

Answer 1

declare, for, loop, execute are the parts of the plpgsql, not plain sql (declare could be used in the plain sql but in different meaning). So you should to wrap your code into anonymous block or into function if you want to return some data from it:
create function get_history(p_day int)
returns table (<structure of history tables here>)
-- or
-- returns setof <history table name>
language plpgsql
as $$
declare
strSQL text;
begin
select
string_agg(format('select * from %I.%I', table_schema, table_name), E' union\n')
into strSQL
from information_schema.tables
where table_name like to_char(p_day, '"history__"FM09%');
return query execute strSQL;
end $$;
Also look at the Table partitioning (choose your PostgreSQL version at the top of article).
Update
However there are several ways to return query data from anonymous plpgsql block without changing DB schema: cursors and prepared statements.
IMO second one is simpler a bit, so:
do $$
declare
strSQL text;
begin
select
string_agg(format('select * from %I.%I', table_schema, table_name), E' union\n')
into strSQL
from information_schema.tables
where table_name like to_char(p_day, '"history__"FM09%');
-- Prepend "prepare", change the "foo" name as you wish
strSQL := 'prepare foo as ' || strSQL;
execute strSQL;
end $$;
-- Usage
execute foo;
-- And deallocate prepared statement when it does not need anymore:
deallocate foo;
Simple working example