SELECT dynamic columns without functions in PostgreSQL - sql

Question

I need to select rows from two and more tables ("A", "B"). They have differences columns and I don't use inheritance for it.
So. For example:
SELECT * FROM "A" UNION SELECT * FROM "B"
ERROR: each UNION query must have the same number of columns
I can understand why.
I try get intersected columns from root schema in root table:
SELECT column_name FROM information_schema.columns
WHERE table_schema = 'client_root' AND table_name ='conditions'
It's ok! But I don't use query:
SELECT
(SELECT column_name FROM information_schema.columns
WHERE table_schema = 'client_root' AND table_name ='conditions')
FROM "client_123"."A"
So. How I can put sub select data in root select?

Answer 1

What you are trying to do is hardly possible in its entirety.
Create dynamic SQL
First, here is what you can do: a plpgsql function that creates the SQL for such a query:
CREATE OR REPLACE FUNCTION f_union_common_col_sql(text, text)
RETURNS text
AS $function$
DECLARE
_cols text;
BEGIN
_cols := string_agg(attname, ', ')
FROM (
SELECT a.attname
FROM pg_attribute a
WHERE a.attrelid = $1::regclass::oid
AND a.attnum >= 1
INTERSECT
SELECT a.attname
FROM pg_attribute a
WHERE a.attrelid = $2::regclass::oid
AND a.attnum >= 1
) x;
RETURN 'SELECT ' || _cols || '
FROM ' || quote_ident($1) || '
UNION
SELECT ' || _cols || '
FROM ' || quote_ident($1);
END;
$function$ LANGUAGE plpgsql;
COMMENT ON FUNCTION f_union_common_col_sql(text, text) IS 'Create SQL to query all visible columns that two tables have in common.
# Without duplicates. Use UNION ALL if you want to include duplicates.
# Depends on visibility dicatated by search_path
$1 .. table1: optionally schema-qualified, case sensitive!
$2 .. table2: optionally schema-qualified, case sensitive!';
Call:
SELECT f_union_common_col_sql('myschema1.tbl1', 'myschema2.tbl2');
Gives you the complete query. Execute it in a second call.
You can find most everything I used here in the manual on plpgsql functions.
The aggregate function string_agg() was introduced with PostgreSQL 9.0. In older versions you would: array_to_string(array_agg(attname), ', ').
Execute dynamic SQL?
Next, here is what you hardly can do:
CREATE OR REPLACE FUNCTION f_union_common_col(text, text)
RETURNS SETOF record AS
$BODY$
DECLARE
_cols text;
BEGIN
_cols := string_agg(attname, ', ')
FROM (
SELECT a.attname
FROM pg_attribute a
WHERE a.attrelid = $1::regclass::oid
AND a.attnum >= 1
INTERSECT
SELECT a.attname
FROM pg_attribute a
WHERE a.attrelid = $2::regclass::oid
AND a.attnum >= 1
) x;
RETURN QUERY EXECUTE '
SELECT ' || _cols || '
FROM quote_ident($1)
UNION
SELECT ' || _cols || '
FROM quote_ident($2)';
END;
$BODY$
LANGUAGE plpgsql VOLATILE;
COMMENT ON FUNCTION f_union_common_col(text, text) IS 'Query all visible columns that two tables have in common.
# Without duplicates. Use UNION ALL if you want to include duplicates.
# Depends on visibility dicatated by search_path
# !BUT! you need to specify a column definition list for every call. So, hardly useful.
$1 .. table1 (optionally schema-qualified)
$2 .. table1 (optionally schema-qualified)';
A function call requires you to specify the list of target columns. so this is hardly useful at all:
SELECT * from f_union_common_col('myschema1.tbl1', 'myschema2.tbl2')
ERROR: a column definition list is required for functions returning "record"
There is no easy way around this. You would have to dynamically create a function or at least a complex type. This is where I stop.