When executing create table as select stmt,
the varchar/char/string type of column in created table will be unified to string type.
Because when select from external table (mysql/pg, etc), the length of varchar in external database
is calculated by "char" length, not "byte" length.
So if there is a column with varchar(10) in external table, then there will be a same varchar(10)
in created table. But the byte length of data in external table may be larger than 10, causing failure of CTAS.
Change to string will not impact performance of the capacity of disk storage.
And notice that if a string type column is the first column, it will be changed to varchar(65535),
because we do not allow string type column as sort key column.
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