Update the **create table** and **add column** operations so that they
support adding [Postgres
comments](https://www.postgresql.org/docs/current/sql-comment.html):
* **create table**: comments can be added to the table itself and to
each of its columns.
* **add column**: a comment can be added to the column.
A **create table** migration that includes a comment on the table itself
and on one of its columns looks like this:
```json
{
"name": "12_create_employees_table",
"operations": [
{
"create_table": {
"name": "employees",
"comment": "This is a comment for the employees table",
"columns": [
{
"name": "id",
"type": "serial",
"pk": true
},
{
"name": "role",
"type": "varchar(255)",
"comment": "This is a comment for the role column"
}
]
}
}
]
}
```
and an **add column** migration that includes a comment looks like this:
```json
{
"name": "30_add_column_simple_up",
"operations": [
{
"add_column": {
"table": "people",
"up": "'temporary-description'",
"column": {
"name": "description",
"type": "varchar(255)",
"nullable": false,
"comment": "This is a comment for the description column"
}
}
}
]
}
```
This allows new tables and columns to be created with comments.
Deletion and modification of comments should still be performed with a
raw SQL migration. Until we see a use case that requires versioned
modification/removal of comments, these operations are best performed
directly on the base table with raw SQL.
Add support for **rename column** migrations. A rename column migration
looks like:
```json
{
"name": "13_rename_column",
"operations": [
{
"rename_column": {
"table": "employees",
"from": "role",
"to": "jobTitle"
}
}
]
}
```
* On `Start`, the view in the new version schema aliases the renamed
column to its new name. The column in the underlying table is not
renamed.
* `Rollback` is a no-op.
* `Complete` renames the column in the underlying table.
