Add Show Reconstructed Constraints For A Table as a Postgres TIL

postgres/show-reconstructed-constraints-for-a-table.md

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+# Show Reconstructed Constraints For A Table
+
+The [`pg_get_constraintdef`
+function](https://pgpedia.info/p/pg_get_constraintdef.html) can be used to
+reconstruct the command for creating a given constraint. This isn't necessarily
+the command (or commands) that originally created the constraint, but rather a
+reconstruction.
+
+We have to pass it an `oid` that corresponds to the constraint which we can get
+from the `pg_constraint` table. These results can be further narrowed down by
+the `conname` (constraint name) and `conrelid` (table name).
+
+Here is an example of listing the constraints on a `reading_statuses` table.
+
+```sql
+> select
+    conname,
+    pg_get_constraintdef(oid)
+  from pg_constraint
+  where conrelid = 'reading_statuses'::regclass;
+
+               conname               |                                                                               pg_get_constraintdef
+-------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ reading_statuses_pkey               | PRIMARY KEY (id)
+ fk_rails_17ee7cb2c4                 | FOREIGN KEY (user_id) REFERENCES users(id)
+ fk_rails_0d3729339f                 | FOREIGN KEY (book_id) REFERENCES books(id)
+ reading_statuses_valid_status_check | CHECK (((status)::text = ANY ((ARRAY['started'::character varying, 'completed'::character varying, 'abandoned'::character varying, 'already_read'::character varying])::text[])))
+(4 rows)
+```
+
+I came across this while experimenting with [an idea for a fail-fast Rails
+initializer
+check](https://gist.github.com/jbranchaud/12813a0558f9cd06bcc24b7d8706550c)
+that verifies the values of the `reading_statuses_valid_status_check` stay in
+sync with the Rails version of those values that live in a constant.