Add db option to fix bug where tables with default values always trigger column migrations

[maggie-lou]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

There is a bug where if you use Clickhouse-Gorm to create a table with no default value, auto-migrate will always trigger a column migration even if there are no schema changes.

In order to stop doing these unnecessary column migrations, this code adds an option to consider empty string as an invalid default value.

User Case Description

If you create a Clickhouse table with Gorm auto-migrate and do not set a default value on a column, it will initialize the default value to the empty string. This is because Clickhouse columns are not nullable by default.

If you then run auto-migrate again, you would not expect a migration to take place, because you have not changed the schema. There is a bug because it will always run ALTER TABLE X MODIFY COLUMN

The problem is that the clickhouse operator reads the empty string as a valid default value, and sets DefaultValueValue.Valid = true. In gorm, this means that dvNotNull = true.

However because there's no default value set on the struct, gorm thinks there is not a default value and sets currentDefaultNotNull = false.

This causes us to always hit this case where Gorm thinks we are migrating from having no default value to empty string default value. It will then always set alterColumn=true and trigger unnecessary alter column calls.

[maggie-lou]

@jinzhu Could you please take a look at this small change? Thank you!

[maggie-lou]

Hey @jinzhu wanted to make sure you saw this. Do you have any feedback on this small change?