Sql Engine - (Physical|Execution) Plan in calcite.
The physical plan is the relation algebra expression that describe how to perform the operation on the data. It's the output of the optimizer
Example of output of a physical_plan (There is no node that starts with the term logical, otherwise, it's a logical_plan)
1628:EnumerableProject(name=[$2]) 1627:EnumerableHashJoin(condition=[=($1, $5)], joinType=[inner]) 1626:EnumerableFilter(condition=[=($0, 100)]) 94:EnumerableTableScan(table=[[HR, emps]]) 92:EnumerableTableScan(table=[[HR, depts]])
As the physical plan is a relational expression, see relation expression print
Explain is just another term for printing.