sql/postgres: fixed inspection/migration for enums and indexes

[svstanev]

I'm evaluating Atlas for a project I'm working on. We're using Postgres and I stumbled across a couple of issues related to enums and indexes.

Enums:

1. When inspecting a database where 2 or more columns use the same enum type result contains enum definitions for each column which results in duplicates in the Atlas schema and later in the change plan/migrations;
2. schema.EnumType lacks schema information which is important for Pg as user types belong to a schema (namespace). This is fatal when performing NormalizeRealm: the schema name is changed to a temp one (atlas_dev_%s_%d) and then applied to the dev db. Since enums don't have a reference to the schema.Schema they belong to, the generated schema change statements (sql) are incorrect as enums are referenced by name only which usually means that they are created in the public schema and not the atlas_dev_xxx schema. Also enum types may be referenced in expressions like column default value:

create table if not exists table1 (
  ...
  state state_enum not null default 'on'::state_enum,
  ...
);

that should also reference the enum by the correct schema-qualified name.

Indexes:

1. I have an unique index like this:

```sql
create unique index if not exists u_entity_name on entity (
  entity_type,
  coalesce(parent_id,0),
  name
);

which when inspected become like this:

index "u_entity_name" {
    unique = true
    type   = BTREE
    on {
      expr = "COALESCE(parent_id, (0)::bigint)"
    }
    on {
      expr = "COALESCE(parent_id, (0)::bigint)"
    }
    on {
      expr = "COALESCE(parent_id, (0)::bigint)"
    }
  }

The reason is the query the index metadata is retrieved from Postgres.

Last but not least (as it is the first problem I hit): there is a problem if the DSN contains an username with _: the initial url is split in provider (postgres/...) and dns (the rest after the ://) and the later is parsed as an url (url.Parse). As this url already have the scheme stripped it becomes somewhat malformed as per the standard lib source code and results in an error: first path segment in URL cannot contain colon.

In any case Atlas is amazing and I'm looking forward to start using it as the main schema management tool for the project I'm currently working on.

[a8m]

Hey @svstanev 👋
Thanks for the kind words and for this amazing contribution.

Do you mind breaking this PR into 3 (URL fix, enums, and indexes)? It will make the review simpler and keep the history cleaner. 🙏

[svstanev]

@a8m I should have done it the first time but I discovered the index problem while debugging the enums and fixed both simultaneously. Anyway I'll submit the new PRs shortly.