Calcite - Relational Expression (RelNode, Algebra)

About

Relational Algebra in Calcite

A relational expression is represented by a tree of RelNode.

A RelNode can be considered as the same logic than the Spark dataframe.

List

  • TableScan
  • Project
  • Filter
  • Aggregate
  • Join
  • Union
  • Intersect
  • Sort

Type

Logical

Logical algebra has no implementation of the relational operator and therefore can't run.

Logical algebra is also known as the logical plan.

All logical operator starts with the prefix Logical

Example of a relation expression printed (explain):

4:LogicalProject(name=[$2])
  3:LogicalFilter(condition=[=($0, 100)])
    2:LogicalJoin(condition=[=($1, $5)], joinType=[inner])
      0:LogicalTableScan(table=[[HR, emps]])
      1:LogicalTableScan(table=[[HR, depts]])

This is equivalent to the below SQL:

select * 
from emps inner join depts on deptno
where empid = 100

Physical

Every logical operator needs to be transformed in a physical algebra to be executed.

Because the volcano planner (optimizer) is cost base, it's based on this physical operator to get a cost

Example of output for physical_plan (ie no logical Node) - The same plan as above but that say how to do perform the logical steps

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]])

Characteristic

  • A relational expression is immutable.

Management

Create

RelBuilder

To build a relational expression, use the algebra builder (RelBuilder)

The builder uses a stack to store the relational expression produced by one step and pass it as an input to the next step.

build() is a stack method to get the last relational expression, namely the root of the expression tree.

The builder methods perform various optimizations, including:

  • project returns its input if asked to project all columns in order
  • filter flattens the condition (so an AND and OR may have more than 2 children), simplifies (converting say x = 1 AND TRUE to x = 1)
  • If you apply sort then limit, the effect is as if you had called sortLimit

SqlNode

From a SqlNode, you get a logical plan. See Logical plan creation

Optimize

Visit

  • RelShuttle - Visitor that has methods for the common logical relational expressions.
final RelShuttle shuttle = new RelHomogeneousShuttle() {
      @Override public RelNode visit(TableScan scan) {
        final RelOptTable table = scan.getTable();
        if (scan instanceof LogicalTableScan
            && Bindables.BindableTableScan.canHandle(table)) {
          // Always replace the LogicalTableScan with BindableTableScan
          // because it's implementation does not require a "schema" as context.
          return Bindables.BindableTableScan.create(scan.getCluster(), table);
        }
        return super.visit(scan);
      }
    };
relNode = relNode.accept(shuttle);

Validation

Run

The RelRunner implementation runs a relational expression.

PreparedStatement run = RelRunners.run(relNode);
ResultSet resultSet = run.executeQuery();

More … see Calcite - Getting Started (from Sql to Resultset)

Print

See Calcite - Getting Started (from Sql to Resultset) for a full example.

  • with the RelOptUtil
RelOptUtil.toString(relNode)
  • with the RelWriter and the explain function
RelWriter rw = new RelWriterImpl(new PrintWriter(System.out, true));
relNode.explain(rw);

Example of output for a logical plan (ie only Logical Node)

4:LogicalProject(name=[$2])
  3:LogicalFilter(condition=[=($0, 100)])
    2:LogicalJoin(condition=[=($1, $5)], joinType=[inner])
      0:LogicalTableScan(table=[[HR, emps]])
      1:LogicalTableScan(table=[[HR, depts]])

Example for physical_plan (ie no logical Node) - The same plan as above but that say how to do perform the logical steps

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]])

Explain

See print

Read / Write

Documentation / Reference


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