Get max value of same field in all tables in schema

Question

I have 10 tables in a schema, which all have a 'measurementdatetime' field. I'm trying to write a script which will return a row for each table showing the tablename and the max measurementdatetime for each.

I figure it should be coded something like this, but I can't figure out the exact syntax

    SELECT table_name AS table_full_name,
    MAX ( table_name || '.measurementdatetime'  ) AS max_timestamp
    FROM information_schema.tables
    WHERE table_schema = 'temp_work_w_roof'
    GROUP BY tables.table_name
    ORDER BY pg_total_relation_size('"' ||  table_name || '"') DESC

I get the 'Error relation my_tablename1 doesn't exist'

(Also: is it possible to compile this as a view? & if so how to code the preceding 'fieldnames' of the view if they're dynamic like this?)

Answer 1

You have to use the plpgsql language dynamic command, e.g.:

create or replace function get_max_measurementdatetime()
returns table (table_name text, max_value timestamp)
language plpgsql as $$
declare
    r record;
begin
    for r in
        select i.table_name, i.table_schema
        from information_schema.tables i
        where table_schema = 'temp_work_w_roof'
        and table_type = 'BASE TABLE'
    loop
        execute format (
            'select max(measurementdatetime) from %I.%I',
            r.table_schema, r.table_name)
        into max_value;
        table_name := r.table_name;
        return next;
    end loop;
end $$;

select *
from get_max_measurementdatetime();