Adding IF NOT EXIST to a migration ALTER monitor

[damyanmp]

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Description

Fixes #(#5151)

Type of change

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• Bug fix (non-breaking change which fixes an issue)

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[damyanmp]

By using IF NOT EXISTS, you avoid the need to manually check whether the column exists, reducing the complexity of migration scripts and error-handling logic. @louislam

[CommanderStorm]

The collum should not exist beforehand
=> something else must have gone wrong such as a botched migration to a newer version or something else.

Given that thus this does not solve the issue (just the symptom in your specific case) I don't want to add this.
If the assumptions of the migration (f.ex. in this case that the collum does not exist) was broken, this should cause an error.