lookupplan.java

用java语言简单实现数据库的初步功能

package simpledb.index.query;

import simpledb.record.Schema;
import simpledb.query.*;

/**
 * The Plan class for the <i>lookup</i> relational algebra operator.
 * @author Edward Sciore
 *
 */
public class LookupPlan implements Plan {
	private Plan rp, tp;
	private Schema sch = new Schema();

	/**
	 * Creates a new lookup node having the specified
	 * LHS rid-table and RHS data table.
	 * @param rp the LHS rid-table
	 * @param tp the RHS table
	 */
	public LookupPlan(Plan rp, Plan tp) {
		this.rp = rp;
		this.tp = tp;
		sch.addAll(rp.schema());
		sch.addAll(tp.schema());
	}

	/**
	 * Creates a new lookup scan for this query.
	 * @see simpledb.query.Plan#open()
	 */
	public Scan open() {
		RidScan rs = (RidScan) rp.open();
		TableScan ts = (TableScan) tp.open();
		return new LookupScan(rs, ts);
	}

	/**
	 * Estimates the number of block accesses to compute the 
	 * lookup, which is the sum of the costs of its
	 * LHS and RHS.
	 * @see simpledb.query.Plan#blocksAccessed()
	 */
	public int blocksAccessed() {
		return rp.blocksAccessed() + rp.recordsOutput();
	}

	/**
	 * Estimates the number of output records in the lookup,
	 * which is the same as in the LHS rid-table.
	 * @see simpledb.query.Plan#recordsOutput()
	 */
	public int recordsOutput() {
		return rp.recordsOutput();
	}

	/**
	 * Estimate the number of distinct values for the
	 * specified field. 
	 * This value is the same as in the LHS or RHS,
	 * depending on where the field came from.
	 * @see simpledb.query.Plan#distinctValues(java.lang.String)
	 */
	public int distinctValues(String fldname) {
		if (rp.schema().hasField(fldname))
			return rp.distinctValues(fldname);
		else
			return tp.distinctValues(fldname);
	}

	/**
	 * Returns the union of the LHS and RHS schemas.
	 * @see simpledb.query.Plan#schema()
	 */
	public Schema schema() {
		return sch;
	}
}