You'd do this anytime you are sure you don't need to worry about the table schema being out of sync with your package. Creating the table in the previous task is a great example.
There are cases when external metadata validation can be expensive (lots of columns, remote database, slow database server). In these cases, setting ValidateExternalMetadata can improve performance of the validation phase. However, when you do this you run
the risk of hitting some weird runtime errors if your package metadata is in fact out of sync with the underlying table. At
best you'll fail with an error, at worst the package continues to run with column data in the wrong columns!
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