indexjoinplan.java

用java语言简单实现数据库的初步功能

package simpledb.index.query;

import simpledb.tx.Transaction;
import simpledb.record.Schema;
import simpledb.query.*;
import simpledb.index.Index;
import simpledb.index.metadata.IndexInfo;

/** The Plan class corresponding to the <i>indexjoin</i>
 * relational algebra operator.
 * @author Edward Sciore
 */
public class IndexJoinPlan implements Plan {
	private Plan p;
	private IndexInfo ii;
	private String joinfield;
	
	/**
	 * Creates a new indexjoin node in the query tree,
	 * using the specified LHS subquery and RHS index.
	 * @param p the left-hand subquery
	 * @param ii information about the right-hand index
	 * @param joinfield the left-hand field used for joining
	 * @param tx the calling transaction
	 */
	public IndexJoinPlan(Plan p, IndexInfo ii, String joinfield, Transaction tx) {
		this.p = p;
		this.ii = ii;
		this.joinfield = joinfield;
	}

	/**
	 * Opens an indexjoin scan for this query
	 * @see simpledb.query.Plan#open()
	 */
	public Scan open() {
		Scan s = p.open();
		Index idx = ii.open();
		return new IndexJoinScan(s, idx, joinfield);
	}

	/**
	 * Estimates the number of block accesses to compute the join.
	 * The formula is:
	 * <pre> B(indexjoin(p,idx)) = B(p) + R(p)*B(idx) </pre>
	 * @see simpledb.query.Plan#blocksAccessed()
	 */
	public int blocksAccessed() {
		return p.blocksAccessed() + (p.recordsOutput() * ii.blocksAccessed());
	}

	/**
	 * Estimates the number of output records in the join.
	 * The formula is:
	 * <pre> R(indexjoin(p,idx)) = R(p)*R(idx) </pre>
	 * @see simpledb.query.Plan#recordsOutput()
	 */
	public int recordsOutput() {
		return p.recordsOutput() * ii.recordsOutput();
	}

	/**
	 * Estimates the number of distinct values for the 
	 * specified field. This value is the same as 
	 * in the LHS plan. 
	 * @see simpledb.query.Plan#distinctValues(java.lang.String)
	 */
	public int distinctValues(String fldname) {
		return p.distinctValues(fldname);
	}

	/**
	 * Returns the schema of the index join.
	 * Since the index has no interesting fields,
	 * the schema is the same as in the LHS plan.
	 * @see simpledb.query.Plan#schema()
	 */
	public Schema schema() {
		return p.schema();
	}
}