Getting referenced tables in Postgres - postgresql

Question

I have a list of foreign keys. I'd like to find out the tables where these FK's point to and the actual key the point to.
I've got a list of FK's like so:
columnName0, columnName1, columnName2
Foreign key references
columnName0 references table0.idTable0
columnName1 references table1.idTable1
columnName2 references table2.idTable2
Some sample tables:
Table0:
idTable0, PK
name
Table1:
idTable1, PK
age
Table2:
idTable2, PK
createdOn
A sample result:
| column | referenced_column | referenced_table |
|-------------|-------------------|------------------|
| columnName0 | idTable0 | table0 |
| columnName1 | idTable1 | table1 |
| columnName2 | idTable2 | table2 |
I'm trying to translate something I do in MySQL like this:
SELECT DISTINCT
COLUMN_NAME AS column,
REFERENCED_COLUMN_NAME AS referenced_column,
REFERENCED_TABLE_NAME AS referenced_table
FROM
INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE
COLUMN_NAME IN (?);
I'm going to have to use straight-up queries (unfortunately, no stored procedures).

Answer 1

You can query pg_constraint. For column names you should lookup pg_attribute. A foreign key may be based on multiple columns, so conkey and confkey of pg_constraint are arrays. You have to unnest the arrays to get a list of column names. Example:
select
conrelid::regclass table_name,
a1.attname column_name,
confrelid::regclass referenced_table,
a2.attname referenced_column,
conname constraint_name
from (
select conname, conrelid::regclass, confrelid::regclass, col, fcol
from pg_constraint c,
lateral unnest(conkey) col,
lateral unnest(confkey) fcol
where contype = 'f' -- foreign keys constraints
) s
join pg_attribute a1 on a1.attrelid = conrelid and a1.attnum = col
join pg_attribute a2 on a2.attrelid = confrelid and a2.attnum = fcol;
table_name | column_name | referenced_table | referenced_column | constraint_name
------------+-------------+------------------+-------------------+------------------------
products | image_id | images | id | products_image_id_fkey
(1 row)
In Postgres 9.4 or later the function unnest() may have multiple arguments and the inner query may look like this:
...
select conname, conrelid::regclass, confrelid::regclass, col, fcol
from pg_constraint c,
lateral unnest(conkey, confkey) u(col, fcol)
where contype = 'f' -- foreign keys constraints
...