indexjoinscan.java

用java语言简单实现数据库的初步功能

package simpledb.index.query;

import simpledb.record.RID;
import simpledb.query.*;
import simpledb.index.Index;

/**
 * The scan class corresponding to the indexjoin relational
 * algebra operator.
 * The code is very similar to that of ProductScan, 
 * which makes sense because an index join is essentially
 * the product of each LHS record with the matching RHS index records.
 * @author Edward Sciore
 */
public class IndexJoinScan implements RidScan {
	private Scan s;
	private Index idx;
	private String joinfield;

	/**
	 * Creates an index join scan for the specified LHS scan and 
	 * RHS index.
	 * @param s the LHS scan
	 * @param idx the RHS index
	 * @param joinfield the LHS field used for joining
	 */
	public IndexJoinScan(Scan s, Index idx, String joinfield) {
		this.s = s;
		this.idx  = idx;
		this.joinfield = joinfield;
		beforeFirst();
	}

	/**
	 * Positions the scan before the first record.
	 * That is, the LHS scan will be positioned at its
	 * first record, and the index will be positioned
	 * before the first record for the join value.
	 * @see simpledb.query.Scan#beforeFirst()
	 */
	public void beforeFirst() {
		s.beforeFirst();
		s.next();
		resetIndex();
	}

	/**
	 * Moves the scan to the next record.
	 * The method moves to the next index record, if possible.
	 * Otherwise, it moves to the next LHS record and the
	 * first index record.
	 * If there are no more LHS records, the method returns false.
	 * @see simpledb.query.Scan#next()
	 */
	public boolean next() {
		while (true) {
			if (idx.next())
				return true;
			if (!s.next())
				return false;
			resetIndex();
		}
	}

	/**
	 * Closes the scan by closing its LHS scan and its RHS index.
	 * @see simpledb.query.Scan#close()
	 */
	public void close() {
		s.close();
		idx.close();
	}

	/**
	 * Returns the Constant value of the specified field.
	 * Since the index contains no interesting fields,
	 * the value will be obtained from the LHS scan.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public Constant getVal(String fldname) {
		return s.getVal(fldname);
	}

	/**
	 * Returns the integer value of the specified field.
	 * Since the index contains no interesting fields,
	 * the value will be obtained from the LHS scan.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public int getInt(String fldname) {
		return s.getInt(fldname);
	}

	/**
	 * Returns the string value of the specified field.
	 * Since the index contains no interesting fields,
	 * the value will be obtained from the LHS scan.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public String getString(String fldname) {
		return s.getString(fldname);
	}

	/** Returns true if the field is in the schema.
	 * Since the index has an empty schema, the method
	 * returns the value that the LHS returns.
	 * @see simpledb.query.Scan#hasField(java.lang.String)
	 */
	public boolean hasField(String fldname) {
		return s.hasField(fldname);
	}

	/**
	 * Returns the dataRid of the current index record.
	 * @see simpledb.index.query.RidScan#getDataRid()
	 */
	public RID getDataRid() {
		return idx.getDataRid();
	}

	private void resetIndex() {
		Constant searchkey = s.getVal(joinfield);
		idx.beforeFirst(searchkey);
	}
}