Generate matrix of access in source-target table - sql

Question

I have two tables in PostgreSQL, class and inheritance.
Each row in inheritance has 2 class IDs source_id and target_id:
CREATE TABLE public.class (
id bigint NOT NULL DEFAULT nextval('class_id_seq'::regclass),
name character varying(500) COLLATE pg_catalog."default" NOT NULL,
CONSTRAINT class_pkey PRIMARY KEY (id)
)
CREATE TABLE public.inheritance (
id bigint NOT NULL DEFAULT nextval('inherited_id_seq'::regclass),
source_id bigint NOT NULL,
target_id bigint NOT NULL,
CONSTRAINT inherited_pkey PRIMARY KEY (id),
CONSTRAINT inherited_source_id_fkey FOREIGN KEY (source_id)
REFERENCES public.class (id),
CONSTRAINT inherited_target_id_fkey FOREIGN KEY (target_id)
REFERENCES public.class (id)
)
I want to create Access Matrix between all classes based in inheritance relationship in inheritance table.
I try this code:
select * , case when id in (select target_id from inheritance where source_id=1) then 1 else 0 end as "1"
, case when id in (select target_id from inheritance where source_id=2) then 1 else 0 end as "2"
, case when id in (select target_id from inheritance where source_id=3) then 1 else 0 end as "3"
, case when id in (select target_id from inheritance where source_id=4) then 1 else 0 end as "4"
, case when id in (select target_id from inheritance where source_id=5) then 1 else 0 end as "5"
, case when id in (select target_id from inheritance where source_id=6) then 1 else 0 end as "6"
, case when id in (select target_id from inheritance where source_id=7) then 1 else 0 end as "7"
, case when id in (select target_id from inheritance where source_id=8) then 1 else 0 end as "8"
, case when id in (select target_id from inheritance where source_id=9) then 1 else 0 end as "9"
from class
and get the right answer, but it's just for 9 static rows in class.
How can I get all number of rows in class using a dynamic SQL command?
If we can't do it with SQL, how can we do it with PL/pgSQL?

Answer 1

Static solution
SQL demands to know name and type of each result column (and consequently their number) at call time. You cannot derive result columns from data dynamically with plain SQL. You can use an array or a document type instead of separate columns:
SELECT *
FROM class c
LEFT JOIN (
SELECT target_id AS id, array_agg(source_id) AS sources
FROM (SELECT source_id, target_id FROM inheritance i ORDER BY 1,2) sub
GROUP BY 1
) i USING (id);
id
name
sources
1
c1
{2,3,4}
2
c2
{5}
3
c3
{5,6,7}
4
c4
{7}
5
c5
{8}
6
c6
{9}
7
c7
{9}
8
c8
null
9
c9
null
Dynamic solution
If that's not good enough you need dynamic SQL with two round-trips to the DB server: 1. Generate SQL. 2. Execute SQL. Using the crosstab() function from the additional module tablefunc. If you are unfamiliar, read this first:
PostgreSQL Crosstab Query
Generate SQL:
SELECT format(
$q$SELECT *
FROM class c
LEFT JOIN crosstab(
'SELECT target_id, source_id, 1 FROM inheritance ORDER BY 1,2'
, 'VALUES (%s)'
) AS ct (id int, %s int)
USING (id)
ORDER BY id;
$q$
, string_agg(c.id::text, '), (')
, string_agg('"' || c.id || '"', ' int, ')
)
FROM (SELECT id FROM class ORDER BY 1) c;
Returns a query of this form, which we ...
2. Execute:
SELECT *
FROM class c
LEFT JOIN crosstab(
'SELECT target_id, source_id, 1 FROM inheritance ORDER BY 1,2'
, 'VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9)'
) AS ct (id int, "1" int, "2" int, "3" int, "4" int, "5" int, "6" int, "7" int, "8" int, "9" int)
USING (id)
ORDER BY id;
... to get:
id | name | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
----+------+---+---+---+---+---+---+---+---+---
1 | c1 | | 1 | 1 | 1 | | | | |
2 | c2 | | | | | 1 | | | |
3 | c3 | | | | | 1 | 1 | 1 | |
4 | c4 | | | | | | | 1 | |
5 | c5 | | | | | | | | 1 |
6 | c6 | | | | | | | | | 1
7 | c7 | | | | | | | | | 1
8 | c8 | | | | | | | | |
9 | c9 | | | | | | | | |
db<>fiddle here
See:
Dynamic alternative to pivot with CASE and GROUP BY
Dynamically convert hstore keys into columns for an unknown set of keys
Dynamic execution with psql
Still two round-trips to the server, but only a single command.
Both of the following solutions use psql meta-commands and only work from within psql!
With \gexec
Using the standard interactive terminal, you can feed the generated SQL back to the Postgres server for execution directly with \gexec:
test=> SELECT format(
$q$SELECT *
FROM class c
LEFT JOIN crosstab(
'SELECT target_id, source_id, 1 FROM inheritance ORDER BY 1,2'
, 'VALUES (%s)'
) AS ct (id int, %s int)
USING (id)
ORDER BY id;
$q$
, string_agg(c.id::text, '), (')
, string_agg('c' || c.id, ' int, ')
)
FROM (SELECT id FROM class ORDER BY 1) c\gexec
Same result.
With \crosstabview
test=> SELECT *
test-> FROM class c
test-> LEFT JOIN (
test(> SELECT target_id AS id, source_id, 1 AS val
test(> FROM inheritance
test(> ) i USING (id)
test-> \crosstabview id source_id val
id | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
----+---+---+---+---+---+---+---+---+--
1 | 1 | 1 | 1 | | | | | |
2 | | | | 1 | | | | |
3 | | | | 1 | 1 | 1 | | |
4 | | | | | | 1 | | |
5 | | | | | | | 1 | |
6 | | | | | | | | 1 |
7 | | | | | | | | 1 |
8 | | | | | | | | |
9 | | | | | | | | |
(9 rows)
See (with related answers for both):
How do I generate a pivoted CROSS JOIN where the resulting table definition is unknown?
There are lots of subtleties in these solutions ...
Aside
Assuming there are some mechanisms in place to disallow duplicates and directly contradicting relationships. Like:
CREATE UNIQUE INDEX inheritance_uni_idx
ON inheritance (GREATEST(source_id, target_id), LEAST(source_id, target_id));
See:
How to create a Postgres table with unique combined primary key?