How to add data from table to biggest object query in PostgreSQL - sql

Question

Postgres 9.1+ database contains different schema for every company named firma and company number, like firma1, firma5, firma99, firma12.
Every schema contains a table with company name:
-- this table contains always exactly one row:
create table firma5.company ( company char(50) not null );
Following query lists biggest objects:
select
(n.nspname||'.'||relname)::char(45) as tablename
, pg_size_pretty(pg_total_relation_size(c.oid))::char(10) as totalsize
, case
when c.relkind='i' then 'index'
when c.relkind='t' then 'toast'
when c.relkind='r' then 'table'
when c.relkind='v' then 'view'
when c.relkind='c' then 'composite type'
when c.relkind='S' then 'sequence'
else c.relkind::text
end ::char(14) as "type"
from
pg_class c
left join pg_namespace n on n.oid = c.relnamespace
left join pg_tablespace t on t.oid = c.reltablespace
where
(pg_total_relation_size(c.oid)>>20)>0 and c.relkind!='t'
order by
pg_total_relation_size(c.oid) desc
This query shows company schemas like firma1, firma5 etc.
How to show company names ( firman.company.company ) also in this query result? Query may return also tables from schemas other than firmaN. In this case company name column should be empty or null.

Answer 1

This is not possible with plain SQL because you cannot specify beforehand the table name to join on, so you need to run a dynamic query. You can, however, create a simple function that returns the company name from a dynamic query, if the schema has company tables:
CREATE FUNCTION company_name (sch text) RETURNS text AS $$
DECLARE
comp text := NULL;
BEGIN
IF strpos(sch, 'firma') = 1 THEN
EXECUTE 'SELECT company FROM ' || sch || '.company' INTO comp;
END IF;
RETURN comp;
END; $$ LANGUAGE plpgsql STRICT STABLE;
And then use that function in your query:
select
(n.nspname||'.'||c.relname)::char(45) as tablename
, pg_size_pretty(pg_total_relation_size(c.oid))::char(10) as totalsize
, case
when c.relkind='i' then 'index'
-- when c.relkind='t' then 'toast' FILTERED OUT IN WHERE CLAUSE
when c.relkind='r' then 'table'
when c.relkind='v' then 'view'
when c.relkind='c' then 'composite type'
when c.relkind='S' then 'sequence'
else c.relkind::text
end ::char(14) as "type"
, company_name(n.nspname) as company -- <<<<<<<<<<<<<<<<<<<<<<<
from pg_class c
left join pg_namespace n on n.oid = c.relnamespace
--left join pg_tablespace t on t.oid = c.reltablespace NOT USED
where (pg_total_relation_size(c.oid)>>20)>0 and c.relkind!='t'
order by pg_total_relation_size(c.oid) desc;