fix : fail to alter existing boolean column from smallint with default value to boolean in postgres (#180)

[jeffry-luqman]

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

What did this pull request do?

drop existing default value to AlterColumn before change column type.
Without this, it will report error as I described in the issue #180

User Case Description

We used migrator to change column data type

[a631807682]

Is it also deleted when automatic transformation is allowed? like smallint -> bigint

[jeffry-luqman]

ok, I've fixed it with isUncastableDefaultValue method, If there are other types that have the same problem, we just need to fill in this method.

[a631807682]

ok, I've fixed it with isUncastableDefaultValue method, If there are other types that have the same problem, we just need to fill in this method.

Can we combine genUsingExpression and isUncastableDefaultValue? They will only work if you need to specify the conversion type.

pseudo code

if specifiedType, isAuto := autoConversion(...); !isAuto {
	// delete if has default value
	// use `USING column::xxx::xxx`
} else {
	// use `USING column::xxx`
}

[jeffry-luqman]

i have fixed it using ModifyColumn and DropDefaultValue method, what do you think?

[jeffry-luqman]

OK, I've fixed it to modifyColumn

[jinzhu]

Hi @jeffry-luqman

Can you fix the conflict? thank you.

[jeffry-luqman]

Hi @jeffry-luqman

Can you fix the conflict? thank you.

✅ Done