sql: fill information_schema.columns.is_identity correctly

[arulajmani]

Describe the problem

Currently, information_schema.columns.is_identity is hard-coded as DNull. Instead, we should fill it correctly with either "yes" or "no" .

[rafiss]

I think based on these docs, this column might specifically refer to identity columns as created by this syntax: #48532 (which we don't support yet). In which case, maybe we just always return false? We should confirm.

https://www.postgresql.org/docs/current/infoschema-columns.html

[rafiss]

Confirmed that in PostgreSQL, only GENERATED .. AS IDENTITY columns show true for this:

rafiss@127:postgres> create table ts (a serial primary key, b int);
CREATE TABLE

rafiss@127:postgres> create table t_identity (a int generated always as identity, b int);
CREATE TABLE

rafiss@127:postgres> select column_name, is_identity from information_schema.columns where table_name = 'ts';
+---------------+---------------+
| column_name   | is_identity   |
|---------------+---------------|
| a             | NO            |
| b             | NO            |
+---------------+---------------+

rafiss@127:postgres> select column_name, is_identity from information_schema.columns where table_name = 't_identity';
+---------------+---------------+
| column_name   | is_identity   |
|---------------+---------------|
| a             | YES           |
| b             | NO            |
+---------------+---------------+

So for now, we should always return false in CockroachDB. We should leave a TODO comment in the code with a link to #48532