Some use has the requirment that only some of columns will be update in
one load operation, and others will retain as original. However, Doris
can't handle this situation, because user must specify value for all
columns. Then if a column aggregation method is REPLACE, use must query
original value to overwrite it. This often needs some work for user to
do.
If this CL is applied, user can use REPLACE_IF_NOT_NULL instead of
REPLACE. Then when load data to table, if user don't intent to change
value of this column, user can specify NULL for this column. Doris will
retain original value for this column.
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