Iterate on tables - sql

Question

I need to perform the same operation on several tables. Right now the query looks like this:
create view foobar
as
select this, that
from here
where this=1
union all
select this,that
from there
where this=1
..... .....
and so on, for several tables. All the results are union-ed.
Is there a way to, instead of writing this very long query where it's easy to get something wrong, to write something like
for table in here, there, upthere
do
select this, that from $table where this=1
and then take the union of them all.
I have the query working right now and it's going to take a while, but this is just curiosity and I didn't know how to search for this on Google!

Answer 1

Create a VIEW from UNION of all tables you need and then SELECT from this VIEW.
Additionally you can create a VIEW like this:
CREATE OR REPLACE VIEW table_set AS
SELECT 'table1' as table_name, field1, field2 ...
FROM table1
UNION ALL
SELECT 'table2' as table_name, field1, field2 ...
FROM table2
UNION ALL
SELECT 'table3' as table_name, field1, field2 ...
FROM table3
UNION ALL
...
and SELECT from this VIEW like:
SELECT field1, field3
FROM table_set
WHERE table_name IN ('table2','table4')
AND field5 = 'abc'

Answer 2

A view like #Igor desrcibes is one option.
If performance is of the essence and you don't have a lot of write operations, you could use that view to (re-)create a materialized view.
Or, for shorter syntax and dynamic selection of source tables, you could utilize a plpgsql function like this one:
CREATE OR REPLACE FUNCTION f_select_from_tbls(tables text[], cond int)
RETURNS TABLE (this int, that text) AS
$BODY$
DECLARE
tbl text; -- automatically avoids SQLi
BEGIN
FOREACH tbl IN ARRAY tables LOOP
RETURN QUERY EXECUTE format('
SELECT this, that
FROM %I
WHERE this = $1', tbl)
USING cond;
END LOOP;
END
$BODY$
LANGUAGE plpgsql;
Call:
SELECT * FROM f_select_from_tbls('{t1,t2,nonStandard tablename}', 4);
I assume a number of tables sharing columns of the same name and type (this int, that text) (You didn't define the data types in you question.) The function takes an array of text as tables names and an integer value as condition.
More about Looping Through Arrays in the manual.
More about Returning From a Function in the manual.
With dynamic SQL you have to be wary of SQL injection. That threat is neutralized here in two ways:
Table names in the array tables are injected with the function format() which properly quotes any non-standard table name
The value in parameter cond is passed in as is via USING, thereby making SQLi impossible.
Variant with VARIADIC parameter
The purpose is to make the function call simpler. You can hand in a list of text variables instead of building an array from them (array is built internally from the parameters). That's all.
CREATE OR REPLACE FUNCTION f_select_from_tbls2(cond int, VARIADIC tables text[])
RETURNS TABLE (this int, that text) AS
$BODY$
DECLARE
tbl text; -- automatically avoids SQLi
BEGIN
FOREACH tbl IN ARRAY tables LOOP
RETURN QUERY EXECUTE format('
SELECT this, that
FROM %I
WHERE this = $1', tbl)
USING cond;
END LOOP;
END
$BODY$
LANGUAGE plpgsql;
I placed tables last in this case to have an open end for the VARIADIC parameter.
Call:
SELECT * FROM f_select_from_tbls2(4, 't1','t2','iLLegal name')