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If Map by ordinal is deselected, the Defaults button looks for a source column of the same name as the target and uses Copy column to map the source to the target, if they are compatible. If they are not compatible, the target is left unmapped.
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If Map by ordinal is selected, the Defaults button maps each unmapped target column (if compatible) by the single source column of the same ordinal as the target column, regardless of name. The first target column is mapped by the first source column, if compatible, the second target column by the second source column, if compatible, and so on. The ordinal number of each column is assigned by the DBMS and captured by a refresh catalog.
Thus, the Map by ordinal checkbox determines how the target column is mapped, either by name or by ordinal number, when you use the Defaults.
You could have enabled Map by ordinal in the Distribute Tables dialog See Select source tables for distribution. These checkboxes function independently. The Distribute Tables dialog never unmaps an existing mapping.
The Mapping page or the Distribute Tables dialog produces a new mapping. The Mapping page could be used to modify an existing mapping. The Mapping page never unmaps or remaps an existing column mapping when Defaults is used.
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Sort by single clicking in the column header.
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Sort and resize the column by double-clicking in a column header.
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Resize the column without sorting by double-clicking in the separator bar between columns; the cursor changes to
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You must define a Distribution Key, that is, a column or a set of columns that uniquely identifies each row in a table, for each table involved in data distribution. The columns act as, but need not be defined as, a primary key in the data modeling sense.
A primary key or unique index is:
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Recommended for target-only, unidirectional tables for maximum Connect CDC performance
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Required by Connect CDC for bi-directional tables