TableAccessRel reads all the rows from a {@link RelOptTable}. If the table is a net.sf.saffron.ext.JdbcTable, then this is literally possible. But for other kinds of tables, there may be many ways to read the data from the table. For some kinds of table, it may not even be possible to read all of the rows unless some narrowing constraint is applied.
In the example of the net.sf.saffron.ext.ReflectSchema schema,
select from fields
cannot be implemented, but
select from fields as f where f.getClass().getName().equals("java.lang.String") can. It is the optimizer's responsibility to find these ways, by applying transformation rules.
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